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Course Number: TCS 3, Fall 2009

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JPL Horizons (Version 3.12) Jan 4, 2005 PURPOSE: The Horizons On-Line Ephemeris System provides access to key solar system data and dynamic production of highly accurate ephemerides for solar system objects. This includes 170000+ asteroids & comets, 128 natural satellites, 9 planets, the Sun, select spacecraft, and dynamical points such as Earth-Sun L1, L2, and system barycenters. Users may conduct...

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(Version JPL Horizons 3.12) Jan 4, 2005 PURPOSE: The Horizons On-Line Ephemeris System provides access to key solar system data and dynamic production of highly accurate ephemerides for solar system objects. This includes 170000+ asteroids & comets, 128 natural satellites, 9 planets, the Sun, select spacecraft, and dynamical points such as Earth-Sun L1, L2, and system barycenters. Users may conduct parameter searches of the comet/asteroid database, finding objects matching combinations of up to 42 different parameters. Users may define and integrate their own objects. Rise, transit and set may be identified to the nearest minute. When used with Sun and Moon sky-brightness data, observing windows can be identified. Close-approaches by asteroids and comets to the planets, Ceres, Pallas, and Vesta, can be rapidly identified along with the encounter uncertainties and impact probabilities. Orbital uncertainties can be computed for asteroids and comets. More than 100 different observational and physical aspect quantities can be requested as a function of time for both topocentric and geocentric observers, in one of 9 coordinate systems and 4 time scales (CT, TT, UT, Civil). 925 Earth station locations are available, along with several sites on other major bodies. Users may search for (or define) topocentric site coordinates on any planet or natural satellite with a known rotational model. Spacecraft-based observations are also supported. Output is suitable for observers, mission planners and other researchers, although this determination is ultimately the user's responsibility. The underlying planet/satellite ephemerides and small-body osculating elements are the same ones used at JPL for radar astronomy, mission planning and spacecraft navigation. In addition to parameter searches, object data summaries, and close-approach tables, four types of customizable ephemerides can be requested: 1) Observables (RA/DEC, Az/El, physical aspect, separation angles, uncertainty ellipses,etc.) 2) Osculating elements 3) Cartesian state vectors 4) SPK binaries (asteroids and comets only) The first three are ASCII tables; output is returned to the user via browser, e-mail, FTP or Kermit protocols and may be requested in a format suitable for spreadsheet import. SPK binary files allows user programs to reproduce the integrated target state at any instant and may be used as plug-ins to existing visualization and mission-design software. ACCESS METHODS: A) Telnet (full access via an interactive prompt-based interface): 1) Connect directly to system (telnet ssd.jpl.nasa.gov 6775, or telnet://ssd.jpl.nasa.gov:6775). 2) Specify an object to get a summary data screen. 3) Follow prompts. At any prompt, type ? or ?! for short and long explanations 4) Transmit results to your system by e-mail, FTP or Kermit B) E-mail (full access batch interface; allows up to 200 discrete times for 200 objects at once): 1) Send e-mail to "horizons@ssd.jpl.nasa.gov" with subject "BATCH-LONG". 2) An example command file will be mailed back to you. 3) Edit this text file, then mail it back with the subject header "JOB". 4) Results of your request are mailed back to you. C) Web (access a small subset of program functions with a passive-interactive GUI interface): Point your browser to http://ssd.jpl.nasa.gov/horizons.html Horizons was intended to be easy to use, with a step-function learning curve. The remainder of this documentation summarizes system capabilities, but is not necessary for successful use. While using the telnet system, type "?" or "?!" at any prompt for an explanation of options. See the "Acknowledgments" section for contact information. Complete documentation (this file) may also be retrieved at ftp://ssd.jpl.nasa.gov/pub/ssd/Horizons_doc.pdf ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ TABLE OF CONTENTS ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Introduction 1. 2. 3. Connecting to the System General Definitions Object Selection Major Body Selection Small Body Selection 4. Observing Site Specification Earth Sites Non-Earth Sites 5. 6. 7. 8. Other Main Prompt Commands Saving Program Settings Integrator Display Specification of Time Accepted Formats Gregorian and Julian Calendar Dates Ancient Dates 9. Reference Frames ICRF/J2000.0 FK4/B1950.0 10. Coordinate Systems 11. Searching for Small Bodies 12. User-Specified Small Bodies 13. Customizing Requested Ephemeris 14. Definition of Observer Table Quantities 15. Close-Approach Tables 16. Understanding Rise, Transit and Set Indicators 17. Constellation Identification 18. SPK File Production 19. Statement of Ephemeris Limitations 20. Long-Term Ephemeris Solar System Model, Precession Model, Nutation Model Universal Time (CT to UT Conversion), Greenwich Mean Sidereal Time Body Rotations 21. Background 22. List of Major-Body Ephemerides On-Line 23. Sources and References 24. Acknowledgements Appendices 1. CONNECTING TO THE SYSTEM TELNET: The Horizons on-line ephemeris and data system is available as a telnet service. This is suitable for people who want full access to program features in an interactive prompt-based way. From a telnet-capable machine running a "VT100" type terminal emulation, telnet to "ssd.jpl.nasa.gov 6775", where 6775 is a port number. From within a web-browser, such as Netscape, enter location "telnet://ssd.jpl.nasa.gov:6775". The system will start a terminal session automatically. No user-ID or password is required. If a user-name/password is requested, the port number was not specified. A few PC-type telnet programs do not fully implement the telnet protocol and may not pass the port number to the network, or may need to be reconfigured to function properly, or may have a different syntax for specifying port numbers. Consult your system userss guide if there is a problem. Horizons will also attempt to determine your window size. If it cannot, it will default to a 24 row by 79 column screen display. If this is inappropriate, and your display paging is choppy, manually set your screen size by using the command "TTY {rows} {columns}", where {rows} and {columns} are replaced by appropriate integers. Window sizes less than 79 columns arent recommended since data-screen displays are formatted with that minimum size in mind and may be difficult to read on something smaller. WEB: Point your browser to http://ssd.jpl.nasa.gov/horizons.html. This graphical interface is intended for the more casual user, or general public, and offers access to a subset of program features using pull-down menus, fill-in boxes and buttons to click. E-MAIL: The program can also be controlled by sending e-mail messages to the program at the address "horizons@ssd.jpl.nasa.gov". Response is determined by the subject of the message you send. This method is for those who want access to program features without the overhead of answering prompts or manipulating graphical interfaces; generally those already familiar with what the program does and who know what they want. It has the additional capability of allowing users to specify up to 200 discrete times (to aid astrometric reduction) and up to 200 objects at once. It does not allow SPK file production available via telnet. To get started, send e-mail to the above address with the subject "BATCH-LONG". The latest, fullycommented example run-stream will be mailed back. Edit this file to produce the results you want, then mail back with the subject "JOB". Acceptable e-mail subject commands are: SUBJECT HEADER JOB BATCH-LONG BATCH-BRIEF DOC-TEXT DOC-PS QUESTION MEANING Execute the following Horizons run-stream Retrieve latest fully commented example batch file Retrieve latest example batch file without comments Retrieve ASCII version of current documentation Retrieve PostScript version of current documentation Message forwarded to cognizant engineer 2. GENERAL DEFINITIONS RA Right ascension; the angular distance on the celestial sphere eastward along the celestial equator from the reference equinox to the meridian of the object. RA is analogous to longitude, with the plane containing the equinox defining zero RA much as the Greenwich meridian defines zero longitude. Expressed in units of hours, minutes and seconds or degrees, as requested. DEC Declination; the angular distance on the celestial sphere north (positive) or south (negative) of the celestial equator. It is analogous to latitude. Usually expressed in degrees. AZ Azimuth; the angle measured eastward along the "horizon" (the plane perpendicular to the local zenith) from the North to the point where the meridian passing through local zenith and the object intersects the horizon plane. EL Elevation; the angular distance above or below the plane perpendicular to the local zenith.This plane is not necessarily the visible horizon, due to station elevation ("horizon dip" effect). Geometric coordinates Referred to the mean equator and equinox of a particular reference frame (ICRF/J2000.0 or FK4/B1950.0). Geometric coordinates are the true, or instantaneous states of a body at a particular ephemeris time. Astrometric coordinates Accounts for the finite but varying amount of time it takes light to travel from the target to the observer and is expressed with respect to the equinox and mean equator of a particular reference frame (ICRF/J2000.0 or FK4/ B1950.0). Apparent coordinates Takes into account factors which appear to change target position with respect to the background stars and inertial coordinate system: light-time, stellar aberration, the relativistic deflection of light. Usually, a final rotation to some "of-date" coordinate system is performed, such as precession-nutation to true-equator and equinox-of-date. Refracted coordinates Apparent coordinates approximately corrected for atmospheric refraction. Available for Earth-based sites only. Small body Refers to a comet or asteroid for which the trajectory is integrated from orbital elements. Typically no cartographic coordinate system is available, with the exceptions, so far, being Gaspra and Ida. Major body Refers to a planet, natural satellite or the Sun. In special cases, a comet or asteroid can be redefined as a major body. Only major bodies can be coordinate centers (observing sites). State vectors are interpolated from previously defined ephemerides, such as DE-405 (or later), which are stored as Chebyshev coefficients. Interpolation recovers the integrator state to the mm level. Target body Refers to the object of interest, selected by the user. It can be a major-body or small-body. Primary body Refers to closest body about which a target body orbits. For natural satellites, this would be a planet, although they orbit the Sun as well. For planets and small-bodies, the primary body is the Sun. Interfering body The largest other body in the system. Such a body can visually complicate observations at the site due to its brightness or by covering up the target. On the Earth, the "interfering body" is the Moon. On Io, it would be Jupiter. On Mars, it would be Phobos (largest body, though unlikely to genuinely interfere). Mercury and Venus have no interfering bodies. Deflecting body This is the Sun plus the most massive object in the planet/satellite system (e.g. the system barycenter). These two masses are used to compute the relativistic deflection of light that can change the apparent position of the target body. 3. OBJECT SELECTION How to select objects is the principal thing that must be learned to use Horizons effectively with the telnet or e-mail access methods. The two classes of objects, accessed slightly differently, are the major bodies (planets, satellites, spacecraft) and small bodies (comets and asteroids). Accessing the different object types is described in the sections below. MAJOR BODIES: Type 'MB' to get a general list of all major-body strings that can be used to search on. To select a major body, enter one of the following ... (1) String to search on ("Mars" or "Trit") (2) JPL ID integer code or fragment (3) An IAU code Examples (at the main prompt): Horizons> mars Horizons> 501 Horizons> N* (uniquely select Mars center; '499' does same) (uniquely select Io) (list all major bodies with 'n' in an ID field) Major bodies may have two integer ID's. Those >100, ending in 99 (such as 199, 299, 399, etc.) refer to planet CENTERS. To select planet SYSTEM BARYCENTERS, use the codes less than 10 (1, 2, 3). For example, "399" is the Earths center, "3" is Earth-Moon Barycenter and "301" is the center of the Moon. For Mercury, Venus and Mars, there is no significant difference between planet-center and system barycenter (1=199, 2=299, 4=499). If a planet name is entered, it may not be considered unique if a distinct system barycenter is defined. For example, if "Saturn" is entered, a list containing "Saturn" and the "Saturn Barycenter" will be returned. To specify Saturn (the planet-center), you must use its unique ID code, "699". System barycenters are available over longer time-spans than planet-centers. SMALL BODIES (ASTEROIDS & COMETS): To select an asteroid or comet, enter a list of parameters to search on SEPARATED BY A SEMI-COLON (;). Type 'SB' for a list of the 40 field keywords that can be matched or consult the list later in this document. Match symbols are from the set { >, <, <>, = }. For example, from the main prompt: Horizons> A < 2.5; IN > 7.8; STYP = S; GM <> 0; Horizons> Vesta; Horizons> DES = 1993*; Horizons> 1; Horizons> ; (match parameters) (or "ASTNAM = Vesta;" for faster search) (Objects with designations containing 1993) (Object in file position #1) (Enter your own elements) For example, "A < 2.5; IN > 7.8; STYP = S; GM <> 0; " searches for all S-type small-bodies with semi-major axis less than 2.5 AU and inclination greater than 7.8 degrees with a known (non-zero) GM. Spaces in the command are not considered, nor are upper/lower-case distinctions. Exceptions are object names and designations. Name searches consider spaces. Designation searches consider spaces AND upper/lower-case. If you want to match a fragment of a name or designation, end it with a '*' (e.g. DES = 1993*;). Otherwise, it is assumed a complete name or designation is specified and the search must match exactly and completely. For example: NAME = CERES; (matches only if object name is "Ceres") NAME = CERES*; (match "Ceres", "Monoceres", etc) The same keyword can be used more than once in a search command. For example, "IN > 10; IN < 20;" will list those objects possessing an inclination between 10 and 20 degrees. If the directive "LIST;" is in the search request, the matched parameters will be displayed. For example, "IN > 150; LIST" will display the inclination of each object with inclination greater than 150 degrees. If LIST is input on the command-line by itself, the default is toggled and matching quantities will be output without the need to specify the keyword on each subsequent search. Once a small-body is uniquely identified, a screen of data will be displayed. If more than one small-body matches given parameters, a list of matching objects is displayed. Individual objects from the matched list can then be requested by giving the displayed record number followed by a semi-colon. The command-line semi-colon is used to indicate a small-body request and resolve number ambiguities. For example, if you enter '1' you will select Mercury Barycenter. Enter '1;' to retrieve the small-body in record #1 (Ceres). Small-body record numbers are assigned as follows: Record # range -----------------1->100000 100001->400000 400001->500000 Object type ------------------------------------------------------------------------Reserved for NUMBERED asteroids (record # = asteroid #) Reserved for UNNUMBERED asteroids Reserved for COMET APPARITIONS Elements for more than one comet apparition may be listed ("apparition" refers to a particular perihelion passage), since out-gassing near perihelion can alter the orbit for each passage. Select an apparition from the list with the closest epoch prior to the date of interest for the ephemeris. The record (or file) number of unnumbered asteroids and comet apparitions should NOT be considered constants; they may change as the database is updated, until an updated small-body storage system is implemented. To enter your own heliocentric ecliptic elements, type ";" at the main prompt. This capability is described in more detail in a later section. 4. COORDINATE CENTER (OBSERVING SITE) SELECTION While osculating element tables may be generated with respect to a major body center only, vector and observer tables may additionally be produced with respect to an arbitrary observing site, defined with respect to a major body center. EARTH SITES For the Earth, a list of 925+ sites is predefined. The list generally matches that of the Minor Planet Center, expanding on radar sites (which have negative ID numbers on this system) as necessary. Station "500" is the geocenter. NON-EARTH SITES For non-Earth major bodies, station 500 also represents the body center. For those major bodies with IAU rotational models, additional topocentric sites may be defined. Spacecraft landing sites are typically predefined on non-Earth bodies. USING A PREDEFINED SITE There are several equivalent ways of specifying a location. The most general form is ... site @ body ... where "site" is a numeric code or name fragment to match, and "body" is a numeric major body code or name fragment to match. A list of major body codes follows later in this document. Here are four equivalent ways of searching for the same Earth location: Code 675@399 palomar@399 675@ Palomar Meaning Site #675 on Earth (Palomar Mountain) " " " (observer table only) If an observer table has been requested, the @ may be dropped; the Earth will be assumed if an integer like "675" or a name fragment like "Palom" is input. For a vector table, the DIFFERENT assumption is made that a coordinate center request lacking a "@" symbol is a major body. For example, '10' would mean the Caussols site for an observer table, but "Sun" for a vector table. '10@' or '10@399' would mean the Caussols site for both table types. If your specification returns more than one possible match, the list of matched sites is returned. Refine your site request to be more specific, by using numeric codes for example, and try again. While one can spell out the names of the bodies and sites, it is possible unique matches won't be returned. Thus, use the unique ID numbers when known. For example, "675@Earth" will first look for the body, find both the Earth & Earth-Moon barycenter, thus have to quit before finding specific Palomar site coordinates. "675@399" is unique and avoids this problem. Spaces & upper/lower case are ignored. Here are examples for sites on bodies other than the Earth: Code Viking@499 Viking 1@499 1 @301 500 @ 501 3 @ 499 Meaning List all defined Viking lander sites on Mars Select Viking 1 landing site on Mars Site #1 on the Moon Io body center Site #3 on Mars The asterisk ('*') can be used to generate lists: Code *@301 *@Phobos *@399 *@ * * Meaning List all predefined sites on the Moon List all predefined sites on the Martian moon Phobos List all predefined sites on Earth List all predefined sites on Earth (observer/vector table) List all predefined sites on Earth (observer/vector table) List all major bodies (element table only) There are a several ways to request a body-centered site for a major body. For example: Code 500@601 geo@601 g@601 g@Mimas 500@Deimos geo g@399 Meaning Mimas body center " " " Deimos body center Earth Geocenter Earth Geocenter INPUT OF TOPOCENTRIC SITE COORDINATES For sites with IAU rotation models, topocentric sites may be input by the user as follows: Code c @ Europa coord @ 502 Meaning Request prompting for user location on satellite Europa (same) The following natural satellites DO NOT have rotation models, thus do not support topocentric site definition. Only body-centered observers may be defined: Jupiter: Himalia (506), Elara (507), Pasiphae (508), Sinope (509), Lysithea (510), Carme (511), Ananke (512), Leda (513), 517-528. Saturn: Hyperion (607), 619-630 Uranus: Caliban (716), Sycorax(717), 718-720 Neptune: Nereid (802) After coordinate input is requested, the site location may be entered as either geodetic or cylindrical coordinate triplets, separated by commas: GEODETIC (generally this means map coordinates) E-long - Geodetic east longitude (DEGREES) lat - Geodetic latitude (DEGREES) h - Altitude above reference ellipsoid (km) CYLINDRICAL E-long - Angle eastward from XZ plane (DEGREES) DXY - Distance from Z axis (KM) DZ - Height above XY equator plane (KM) This system always uses planetographic/geodetic coordinates. This is typically the one used on maps, such as those by the USGS, unless the map says otherwise. In these coordinates, the rotational pole of the body that lies on the positive (north) side of the invariable plane of the solar system (the plane perpendicular to the solar system's angular momentum vector) is called the "north pole". Northern latitudes are positive, southern are negative. The planetographic latitude takes into account body oblateness and, for a point on the surface, is the angle between the body equatorial plane and the normal to the reference surface at that point. For a point not on the reference surface, the geodetic latitude is the latitude of the point on the reference surface where the normal passes through the point at some altitude (h) above the reference surface. Prograde (or direct) rotation of a body is rotation eastward, or counter-clockwise, as seen from the north pole. For such bodies, east longitude is measured negatively to the east (0 to -360 degrees) from the prime meridian. Retrograde rotation is rotation clockwise (westward) as seen from the north pole. East longitude is measured positively to the east (0 to 360 degrees) from the prime meridian. Exceptions are the Earth, Moon and Sun where longitude has historically been measured both east and west of the prime meridian 0 to 180 degrees. Though these bodies are direct rotators, longitude is nonetheless measured positively to the east on this system, 0 to 360 degrees, due to historical precedence. If the positive west longitude of a site on these 3 bodies is given, it should be input here as positive east longitude, which would be (360 - West Longitude). If the negative east longitude is given instead, input (360 + East Longitude). The following major bodies are either retrograde or exceptions and require site input with positive east longitude: Retrograde (+ east longitude): Venus (299), Arial (701), Umbriel (702), Titania (703), Oberon (704), Miranda (705), Cordelia (706), Ophelia (707), Bianca (708), Cressida (709), Desdemona (710), Juliet (711), Portia (712), Rosalind (713), Belinda (714), Puck (715), Uranus (799), Pluto (999), Charon (901) Also + east longitude (prograde exceptions): Sun (10), Earth (399), Moon (301) All others are prograde and must be input with negative longitude east of the adopted prime meridian. Since such sites are usually expressed in terms of positive west longitude on maps, negative east longitude would be ... (West longitude - 360) INTERPRETING NON-EARTH OBSERVER TABLES When placing a site on a body other than the Earth, some definitions become useful: Interfering body: The largest other body in the system. Such a body can visually complicate observations at the site due to its brightness or by covering up the target. On the Earth, the "interfering body" is the Moon. On Io, it would be Jupiter. On Mars, it would be Phobos (largest body, though unlikely to genuinely interfere). Mercury and Venus have no interfering bodies. Observer tables provide some optional quantities that can be used to characterize the effect of the interfering body (or IB): how far is the target from the IB in the plane-of-sky, is it obscured by the IB, what fraction of the IB is lit by the Sun as seen from the observing site, and so on. Deflecting body: This is the Sun PLUS the most massive object in the planet/satellite system (e.g. the system barycenter). These two masses are used to compute the relativistic deflection of light that can change the apparent position of the target body. Other changes: REFRACTION There are no refraction effects modeled for non-Earth sites. Any request for refraction is ignored and the refraction angle will be zero. This affects rise-set determination on non-Earth bodies as well. AIRMASS There is no airmass model or airmass cut-off available for non-Earth sites. Any request for airmass computation is ignored. APPARENT RA & DEC The origin of Right Ascension for apparent coordinates on NON-EARTH sites with rotational models is the meridian containing the Earth equinox of J2000.0. Apparent declination is with respect to the particular body's true equator-of-date. This allows an observer to align axes with the pole and use the local apparent sidereal time output by this system to set the RA origin and acquire the target. For objects lacking a pole & prime meridian rotational model (spacecraft and certain asteroids that may have been redefined as "major bodies"), the reference frame (ICRF/J2000.0 or FK4/B1950.0) coordinate system is used to compute apparent places. That is, apparent RA and DEC are defined with respect to the Earth mean-equator and equinox of the frame epoch. TIME The print-time output by this system for observer tables (UT or TT) is the instantaneous time on Earth. For non-Earth sites, it is unrelated to the rotation of the body. Local apparent solar time at the observing site can be requested, as can the instantaneous light time from Earth to the non-Earth site. LIMITATIONS OF NON-EARTH/MOON ROTATION MODELS For bodies outside the Earth-Moon system, precession and nutation effects are usually not known to high accuracy. Thus, the non-Earth/Moon IAU rotation models, used by this system to determine topocentric site motion relative to the inertial frame as a function of time, are good to about 0.1 degree in the present era. For many satellites and the planet Mercury, the official IAU pole direction was simply assumed perpendicular to the body's mean orbit plane, lacking better information. For many satellites in the IAU model, the rotation rate was assumed equal to the mean orbital period. Some small satellite rotational models are strictly valid only at the time of the Voyager spacecraft flyby; extrapolation to other times is hazardous. Topocentric results for such bodies (610-614, for example) should be used cautiously if at all. Results in these cases reflect only the best available model, which is a suspect one. As rotation models are refined through observation of surface features by visiting spacecraft (Cassini, etc.), Horizons will be updated to use the best officially sanctioned models available. 5. OTHER COMMANDS Program information: MB ................ SB ................. NEWS ........... ?! ................... Program controls: LIST ............... PAGE ............. EMAIL {X}.... TTY {R} {C}.. X ..................... - ....................... Show planet/natural-satellite (major-body) ID fields. Show small-body search-field names & meanings. Display program news (new capabilities, updates, etc.). Extended help ('?' for brief help). Toggle display of small-body match-parameter values. Toggle screen paging (scrolling) on or off. Set your email address to {X} for output delivery. Check or reset screen size; "tty" or "tty 24 79" to set. Exit JPL on-line system (also "QUIT" or "EXIT"). Return to the previous prompt (back-up!). Storing format default settings: LOAD {macro} .... Load previously SAVED output-format {macro}. SAVE {macro} ...... Save/replace output-format macro with current settings. DELETE {macro} .. Delete previously saved output-format macro. Useful short-cuts: 1) 2) 3) 4) Move backward through the prompts by typing "-". Quit from any prompt by entering 'q'. To use a default, or previously entered value, press return. After selecting an object, enter "e+" to produce an ephemeris format like the last one, without additional prompting. 6. SAVING PROGRAM SETTINGS Telnet (interactive) users may select program options once, then save all settings for recall during future sessions. This can save time, by reducing the the time devoted to changing the same defaults or routinely defining the same output format each time you connect. Others in your organization may load and use the same pre-defined format settings by name. To save program settings, go through the prompts and define the settings as you require. Then return to the main "Horizons>" prompt. #1) Type "SAVE {NAME}", where {NAME} contains 1-12 characters. #2) Next time you telnet to Horizons, type "LOAD {NAME}". Your output preferences will then be loaded in as the new defaults. If a mistake is made, or it is desirable to change a setting later, two commands are relevant: DELETE and SAVE DELETE a macro with command "DELETE {NAME}". Alternatively, change specific settings manually, then replace the stored macro with a SAVE to an existing name. Delete and replace operations require input of a confirming password. LOAD does not. Thus, anyone can use your settings if they know the macro name. Only those who know the password can change or delete a macro. Start/stop dates are also saved in the macro, as is observing location. You need only load the macro and select the target. Remaining defaults will be as defined in the format macro. If the macro is for an individual (personal use), you may want to set the e-mail address prior to saving. Otherwise don't, so users of the macro will be prompted for it in the future. A macro may be loaded, then specific settings overruled by responding to the program prompts. For example, if your last table prior to saving the macro was a "vector" table, that table type will be saved as the default. Settings for the other table types are saved as well so, to access them, manually respond to the prompt requesting table type, over-riding the macro's "vector" default on that issue. Start and stop times are also macro settings that may commonly be overruled as necessary. Ideally, macro names would be something clean and logical: "OBS670-1" for macro #1 for Observatory Code 670, etc. ... but the name is up to you. The use of macros may make it less likely to stumble upon new capabilities as they are added, though they will described here and in the system news, as necessary. 7. INTEGRATOR DISPLAY Comet and asteroid ephemerides are integrated from initial conditions called "osculating elements". These describe the 3-dimensional position and velocity of the body at a specific time. The integrator starts with this state and takes small time steps, summing the perturbing forces at each step before taking another step. A variable order, variable step-size integrator is used to control error growth. In this way, the gravitational attraction of other major solar system bodies on the target body trajectory is taken into account. The integrator will thus start at the epoch, or time, of the osculating elements. It then integrates forward or backward, as necessary, to the start of the requested table. Once it reaches the table start time, it may have to reverse direction and go forward in time to generate the table. Every 50th step will be displayed so one can get some sense of the progress of the ephemeris. Direction reversals are also displayed. If you request output at small time intervals, the integrator may proceed rapidly to the start of your table. There may then be long (apparent) pauses, as numerous interpolations within a given integration step are performed to compute states at closely spaced print times. The last number on the integrator display line is the most recent step size in days. 8. SPECIFICATION OF TIME ACCEPTED FORMATS: Time may be specified many ways, in addition to the primary form "YYYY-MMM-DD HH:MM". Of particular note are Julian day number and day-of-year forms. Input start times may be specified to 1/1000th of a second. Examples are shown below. Generally, if the input start time has more digits of precision specified than the output format selected, start time will be truncated to the appropriate level. For example, if a start time of 23:45:12.4 is specified, but the output format is set to minutes, start time will automatically be changed to 23:45(:00.000). User Input 1997-May-5 12:30:23.3348 1/9/96 3 12 59.2 1 9 96 3,12,59.2 2 jan 91 3:00 12.2 91 MAR 10 12:00:00 29 February 1975 3:00 10 October 29 3:58 dec 31 86 12 86-365 // 12 JUL 98 JD 2451545. JD2451545. 278bc-jan-12 12:34 AD 99-Aug-12 12:34 bc 278-Jan-12 12:34 Program Interpretation ( 5 MAY 1997 12:30:23.334 ) ( 9 JAN 1996 03:13 ) ( 9 JAN 1996 03:13 ) ( 2 JAN 1991 03:00 ) (10 MAR 1991 12:00 ) ( 1 MAR 1975 03:00 ) (29 OCT 2010 03:58 ) (31 DEC 1986 12:00 ) (31 DEC 1986 12:00 ) ( 1 JUL 1998 00:00 ) ( 1 JAN 2000 12:00 ) ( 1 JAN 2000 12:00 ) (B.C. 12 JAN 278 12:34) (A.D.12 AUG 99 12:34) (B.C. 12 JAN 278 12:34) Recommended: Acceptable: The program will interpret other forms as well, but if you get too casual, you may end up with a surprise interpretation. The program's time-span prompts indicate the earliest & latest dates that may be used for the selected target/ center combination, as well as the type of time assumed being input (UT, CT, or TT). For cartesian coordinates or osculating elements tables, only CT may be used. For "observer tables", output may be either UT or TT. To change the UT default for observer tables, append a "TT" when entering the START time. To switch back, append a "UT" to the start time. The three time systems are described as follows: CT: Coordinate Time. The uniform time scale, or independent variable, of the ephemerides. Used for cartesian and osculating element tables. TT : Terrestrial (Dynamic) Time. Called TDT prior to 1991. Used for observer quantity tables. This is proper time as measured by an Earth-bound observer and is directly related to atomic time, TAI. TT periodically differs from CT by, at most, 0.002 seconds. UT: Universal Time. This can mean one of two non-uniform time-scales based on the rotation of the Earth. For this program, prior to 1972, UT means UT1. After 1972, UT means UTC or "Coordinated Universal Time". Future UTC leap-seconds are not known yet, so the closest known leap-second correction is used over future timespans. TIME ZONE CORRECTIONS Output time-tags may also be in local civil time. When specifying start time, enter your time-zone correction in the format: YYYY-Mon-Dy HH:MM UT{s}HH{:MM} ... where {s} .......... optional sign (+ or -). If unspecified, it is assumed "+". HH ......... integer hours time-zone difference from UT {:MM} ....... optional minutes offset (usually 0) North American standard time (winter) zone corrections are as follows: Atlantic Standard Time (AST) Eastern Standard Time (EST) Central Standard Time (CST) Mountain Standard Time (MST) Pacific Standard Time (PST) = = = = = UT-4 hours UT-5 hours UT-6 hours UT-7 hours UT-8 hours If daylight savings is in effect (summer), add one hour to above offsets. For example, "1999-jun-2 12:30 UT-8" produces a table in Pacific Standard Time. A "-7" would provide Pacific Daylight Time (or MST, if it is winter). GREGORIAN AND JULIAN CALENDAR DATES Input calendar dates 1582-Oct-15 and after are taken to be expressed in the extended Gregorian calendar system. Prior dates are assumed to be in the Julian proleptic calendar. Historically, not all regions switched calendars at the same time (or even in the same century). Thus, the user must be aware of which calendar was in effect for a particular historical record. It should NOT be assumed this system's calendar automatically correlates with a date from an arbitrary historical document. Here is the progression near the calendar switch point. Note that Julian calendar dates are different than (and unrelated to) Julian day numbers.: Calendar Type Julian Julian (skipped) (skipped) (skipped) (skipped) (skipped) (skipped) (skipped) (skipped) (skipped) (skipped) Gregorian Gregorian Gregorian Calendar Date 1582-Oct-03 1582-Oct-04 "1582-Oct-05" "1582-Oct-06" "1582-Oct-07" "1582-Oct-08" "1582-Oct-09" "1582-Oct-10" "1582-Oct-11" "1582-Oct-12" "1582-Oct-13" "1582-Oct-14" 1582-Oct-15 1582-Oct-16 1582-Oct-17 Julian Day Number 2299158.5 2299159.5 ----------> 2299160.5 | 2299151.5 | 2299152.5 | 2299153.5 | 2299154.5 | 2299155.5 | 2299156.5 | 2299157.5 | 2299158.5 | 2299159.5 | 2299160.5 <--------2299161.5 2299162.5 Examination of this table shows that the date labels from Oct 5, 1582 through Oct 14, 1582 don't exist. Of course, the days themselves do, as is shown in the continuous Julian day number column; it's just a matter of what one calls them. If a non-existant calendar date label is specified, this program will automatically use a day number, as shown above, that maps into the previous Julian calendar system. For example, requesting a date of 1582-Oct-14 (skipped) is the same as requesting the Julian calendar date 1582-Oct-04. ANCIENT DATES Objects 0-10, 199, 299, 301, 399 and 499 (planet barycenters, their equivalents and the Sun & Moon) are available over a 3000 B.C. to A.D. 3000 interval. When specifying ancient calendar dates, this system requires input in the "BC/AD" scheme. If no "BC" marker is input with a calendar date, it is assumed to be "AD". Exceptions are AD years less than 100 which must have an AD symbol in the date in order to be recognized as a valid year. For example, "66ad-jan-27" will be accepted, but "66-Jan-27" cannot be parsed. On output, observer-table lines begin with a 'b' in column 1, to indicate B.C. dates, and a space (" ") to indicate A.D. dates. In this system, there are no negative years. The progression is as follows: Julian Day Number (Jan 1 00:00) 1720327.5 1720692.5 1721057.5 1721423.5 1721788.5 Labeling-convention BC/AD Arithmetical 3bc 2bc 1bc 1ad 2ad -2 -1 0 1 2 From this, one can see that no days (in the arithmetical year "0", for example) are skipped in the BC/AD scheme, but they do have a different label than in the corresponding arithmetical system. OUTPUT STEPPING: Fixed time steps: Output time steps are specified as integers with some associated units from the set {days, hours, minutes}. Example responses to the prompt include"30 days", "1 day", "10 min", and so on. To get half day steps, specify "12 hour". It is possible to obtain output at less than 1 minute intervals (telnet & e-mail interfaces only). After specifying a start and stop time, give a positive integer as the "time-step" without giving units, such as"10". This will divide the time span into 10 parts. For example, if start and stop times are one hour (3600 seconds) apart, specifying a step of "240" will produce output every 15 seconds (3600/15 = 240 intervals). "3600" will produce output every second. Rise/set and satellite eclipse circumstances may not be accurate to less than a minute since factors such as the primary's oblateness and atmosphere are not currently modelled. Time-varying steps: Output is typically at fixed time intervals. However, observer tables may additionally be requested at timevarying steps based on an angular shift specification. That is, "output only if the object has moved at least X arcseconds in the plane-of-sky". When specifying the step-size, with the telnet or e-mail interfaces, respond with something like "VAR ####", where '####' is an integer from 60 to 3600 arcseconds. This will trigger output whenever the object's position is predicted to be '####' arcseconds different from the current output step in the observer's plane-of-sky. To preserve system performance, the time-varying output mode uses a simple linear extrapolation to predict the time when the object should have moved the requested distance. Due to non-linearities in the object's actual motion in the plane-of-sky, this projection can be off by .1 to 5 (or more) arcsecs. Thus the angular-motion print criteria you give should be considered approximate. Computed quantities will be exact for the given time in the output, but the particular output time may not be exactly that required for the requested angular change. 9. REFERENCE FRAMES It is necessary to adopt a commonly agreed-upon coordinate system for describing the position and velocity of an object in three-dimensional space. This program has two basic frames available; the default is ICRF/J2000.0 which can be changed to FK4/B1950.0, if desired, at the appropriate prompt. "J2000" selects an Earth Mean-Equator and dynamical Equinox of Epoch J2000.0 inertial reference frame, where the Epoch of J2000.0 is the Julian date 2451545.0. "Mean" indicates nutation effects are ignored in the frame definition. The system is aligned with the IAU-sponsored J2000 frame of the Radio Source Catalog of the International Earth Rotational Service (ICRF). The ICRF is thought to differ from FK5 by at most 0.01 arcsec. J2000.0 reference vectors have the following properties: +Z is normal to ICRF Mean Earth Equator of Epoch J2000.0 +X is parallel to ICRF Mean Earth Dynamical Equinox of Epoch J2000.0 +Y completes the right-handed system "B1950" selects an inertial reference frame based on Earth Mean-Equator and FK4 catalog Equinox of Epoch B1950.0 (FK4/B1950.0), where the Epoch of B1950.0 is the Julian date at the start of the Besselian year B1950.0 (2433282.42345905). The Fricke equinox correction at Epoch is applied. 10. Coordinate Systems Cartesian vectors and osculating elements may be requested in one of three available coordinates systems derived from the selected basic reference frame. These systems are defined with respect to the reference frames (above) as follows: Earth mean equator and equinox of reference epoch ("frame"): Reference epoch: J2000.0 or B1950.0 xy-plane : plane of the Earth's mean equator at the reference epoch x-axis : out along ascending node of the instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch z-axis : along the Earth mean north pole at the reference epoch Ecliptic and mean equinox of reference epoch ("ecliptic") Reference epoch: J2000.0 or B1950.0 xy-plane : plane of the Earth's orbit at the reference epoch x-axis : out along ascending node of instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch z-axis : perpendicular to the xy-plane in the directional (+ or -) sense of Earth's north pole at the reference epoch. Body mean equator and node of date ("body") Reference epoch: "of date" Reference plane: ICRF/J2000.0 or FK4/B1950.0 xy-plane : central-body mean equator plane at reference epoch x-axis : out along the ascending node of the central-body mean equator plane on the reference plane at the reference epoch z-axis : along the central-body mean north pole at the reference epoch Observer table coordinates, such as RA and DEC, may be with respect to two possible coordinate systems: Earth mean equator and equinox of reference epoch (astrometric coordinates): Reference epoch: J2000.0 or B1950.0 xy-plane : plane of the Earth's mean equator at the reference epoch x-axis : out along ascending node of the instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch z-axis : along the Earth mean north pole at the reference epoch Earth true equator and equinox of date (apparent coordinates) Reference epoch: "of date" xy-plane: plane of the Earth's true equator at the reference epoch x-axis : out along ascending node of instantaneous plane of the Earth's orbit and the Earths true equator plane at the reference epoch z-axis : along the Earth's true north pole at the reference epoch 11. SEARCHING FOR SMALL-BODIES Search for small-bodies with following keywords (Type R=real, I=integer, C=char). Use comparisons from the set { <, >, <>, = }. Separate each field with a semi-colon. Example search formulation at main prompt: A < 2.5; IN > 7.8; STYP = S; GM <> 0; The first group of keywords are common to asteroids AND comets: Type C C R R R R R R R R R R R R R R R R R R R R R I Keyword NAME ........... DES ............... EPOCH ......... CALEPO ...... A ................... EC ................. IN .................. OM ................ W .................. TP ................. CALTP ......... MA ............... PER .............. RAD ............. GM ............... QR ................ ADIST .......... ANGMOM ... N ................... DAN ............. DDN ............. L ................... B ................... NOBS ........... Description Asteroid OR comet name fragment Object designation Julian Date of osculating elements Calendar date of osc. elements; YYYYMMDD.ffff Semi-major axis (AU) Eccentricity Inclination of orbit plane (DEG) wrt ecliptic Longitude of Ascending Node (DEG) wrt ecliptic/equinox Argument of Perihelion (DEG) wrt ecliptic/equinox Perihelion Julian Date Perihelion calendar date; YYYYMMDD.ffff Mean anomaly (DEG) Orbital period (YRS) Object radius (KM) Object GM (KM^3/S^2), only a few are known Perihelion distance (AU) Aphelion distance (AU) Specific angular momentum (AU^2/DAY) Mean motion (DEG/DAY) Heliocentric dist. (AU) of ascending node Heliocentric dist. (AU) of descending node Ecliptic longitude of perihelion (DEG) Ecliptic latitude of perihelion (DEG) Number of astrometric determinations in solution The next parameters are ASTEROID SPECIFIC. If one or more is used, the search will conclude faster by examining asteroids only. For example, including something like "H > -10;" will limit search to asteroids only: C R R R R R C ASTNAM ..... B-V ............... H ................... G ................... ROTPER ...... ALBEDO ..... STYP ............ Asteroid name fragment (designation if unnamed) B-V color (asteroid) Absolute magnitude parameter (asteroid) Magnitude slope parameter; can be < 0 (asteroid) Rotational period, hrs (asteroid) Geometric albedo (asteroid) Spectral type, Tholen scheme (asteroid) The next parameters are COMET SPECIFIC. If one or more is used, the search will conclude faster by examining comets only. For example, including something like "M1 > -10;' will limit search to comets only: C I R R R COMNAM .... COMNUM .... M1 ................. M2 ................. K1 .................. Comet name fragment (designation if unnamed) Comet number Total absolute magnitude (comet) Nuclear absolute magnitude (comet) Total magnitude scaling factor (comet) R R R R R R K2 ................... PHCOF .......... A1 ................... A2 ................... A3 ................... DT .................. Nuclear magnitude scaling factor (comet) Phase coefficient for k2=5 (comet) Radial non-grav accel (comet), 10^-8 AU/DAY^2 Transverse non-grav accel (comet), 10^-8 AU/DAY^2 Normal non-grav accel (comet), AU/d^2 Non-grav lag/delay parameter (comet), days If one of the keywords 'ASTNAM', 'COMNAM', 'NAME' or DES is used, no other parameter may be specified for that search. Directives: There are 3 directives that may be used to limit or control searches: Directive COM ............... AST ................. LIST ................ Description Limit search to comets only Limit search to asteroids only Display parameter values for matched objects. (This may be set as a default for all subsequent searches by typing "LIST" at the main system prompt "Horizons>".) For example, "A < 2.5; IN > 10; AST;" "A < 2.5; IN > 10; AST; LIST;" match parameters against asteroids ONLY. match AND display values of the parameters. Contents of the Small-body Database: Excluded from the database are single opposition asteroids with observational data arcs less than 30 days. Exceptions are NEOs, PHAs, TNOs, spacecraft targets, radar targets and periodic comets which are included immediately and updated on a daily basis as new discoveries and observations are made and reported. Users can also input their own objects, as described in the next section. 12. USER-SPECIFIED SMALL-BODIES It is possible to define an object not in the database by inputting its HELIOCENTRIC ECLIPTIC elements and some other parameters. Type ';' at the main prompt. It is also possible to display a database object, then "cut-andpaste" elements back into the program, varying parameters (such as magnitude), as needed. Cut-and-paste is a function of your local terminal capability. PRESS <return> ON A BLANK LINE WHEN DONE. Input format is: LABEL= VALUE LABEL= VALUE ... LABEL= VALUE ... . . ... where acceptable label strings are defined as follows: EPOCH ... Julian ephemeris date (CT) of osculating elements EC ........... Eccentricity QR .......... Perihelion distance in (AU) TP ........... Perihelion Julian date OM .......... Longitude of ascending node (DEGREES) wrt ecliptic W ............ Argument of perihelion (DEGREES) wrt ecliptic IN ........... Inclination (DEGREES) wrt ecliptic Instead of {TP, QR}, {MA, A} or {MA,N} may be specified (not both): MA ........ Mean anomaly (DEGREES) A ............ Semi-major axis (AU) N ............ Mean motion (DEG/DAY) Note that if you specify elements with MA, {TP, QR} will be computed from them. The program always uses TP and QR. OPTIONAL INPUTS RAD ...... Object radius (KM) AMRAT .... Area-to-mass ratio (m^2/kg). Total absorption is assumed, so scale the value to account for reflectivity. For example, if 15% of light is reflected, specify a value for AMRAT in which the actual value is multiplied by 1.15. For asteroids, additional OPTIONAL parameters can be given: H ............. Absolute magnitude parameter (asteroid) G ............. Magnitude slope parameter; can be < 0 (asteroid) For comets, additional OPTIONAL parameters can be given: M1 ........... Total absolute magnitude (comet) M2 ........... Nuclear absolute magnitude (comet) K1 ............ Total magnitude scaling factor (comet) K2 ............ Nuclear magnitude scaling factor (comet) PHCOF ... Phase coefficient for k2=5 (comet) A1 ............ Radial non-grav accel (comet), AU/DAY^2 A2 ............ Transverse non-grav accel (comet), AU/DAY^2 A3 ............ Normal non-grav accel (comet), AU/d^2 DT ............ Non-grav lag/delay parameter (comet), days You may enter each value on a separate line or several on one line. If you make a mistake, re-entering the label on another line will over-ride the previously specified value. To erase a value, enter something like "H=", where no value is given. To cancel all input, enter "-" as the first character on a line. To log-out, enter a "q" or "x" as first character on a line. When done, after having pressed <return> on a blank line, you will be asked whether the reference frame of the elements is FK5/J2000.0 or FK4/B1950.0. You will also be asked the object name. Here is an example of input to define a new object (in this case, a comet with out-gassing): EPOCH= 2450200.5 EC= .8241907231263196 QR= .532013766859137 TP= 2450077.480966184235 OM= 89.14262290335057 W = 326.0591239257098 IN= 4.247821264821585 A1= -5.113711376907895D-10 A2= -6.288085687976327D-10 13. CUSTOMIZING REQUESTED EPHEMERIDES Keys are embedded in output ephemerides to assist with automated reading of the output by user's own software. The keys are defined as follows: $$SOE $$EOE Start of ephemeris End of ephemeris Ephemerides may be customized by changing output default flags. The '*' symbols below denote login defaults. All tables may be optionally output in a "comma-separated-value" format for import into spreadsheets. 1. Cartesian state vector table (Any object with respect to any major body site): Reference frame: J2000 (ICRF/J2000.0) B1950 (FK4/B1950.0) * Coordinate system: Earth mean equator and equinox of reference system (J2000, B1950) * Ecliptic and mean equinox of reference system (J2000 or B1950) Central body mean equator and node of date Aberration corrections: NONE (geometric state vectors) LT (light-time) LT+S (light-time & stellar aberration) Units: KM and seconds KM and days AU and days Quantities Output: Format 1 2 * 3 4 5 6 Output Position components {x,y,z} only State vector {x,y,z,vx,vy,vz} State vector + 1-way light-time + range + range-rate Position + 1-way light-time + range + range-rate Velocity components {vx, vy, vz} only 1-way light-time + range + range-rate * 2. Osculating elements table (any object with respect to any planet, barycenter or the Sun): Note: In general satellite osculating elements output by Horizons should NOT be used to initialize a separate integration or extrapolation outside of Horizons. Such elements assume Keplerian motion (two point masses, etc.) which usually DOES NOT MATCH any kinematic model of the satellite orbit Horizons uses (which may be a precessing ellipse, for example). Extrapolation for satellites is better done using mean orbital elements at http://ssd.jpl.nasa.gov.sat_elem.html. Reference frame: J2000 (ICRF/J2000.0) B1950 (FK4/B1950.0 ) * Coordinate system: Earth mean equator and equinox of reference system (J2000, B1950) * Ecliptic and mean equinox of reference system (J2000 or B1950) Central body mean equator and node of date Units: KM and seconds KM and days AU and days * Output quantities (fixed): JDCT Epoch Julian Date (Coordinate Time) EC Eccentricity QR Periapsis distance IN Inclination w.r.t. xy-plane (degrees) OM Longitude of Ascending Node (degrees) W Argument of Perifocus (degrees) Tp Time of periapsis (users selects periapse-relative or absolute date) N Mean motion (degrees/DU) MA Mean anomaly (degrees) TA True anomaly (degrees) A Semi-major axis AD Apoapsis distance PER Orbital period 3. Observer table (any object with respect to major body center or observing site on any major body): Default quantities that are output without request, as appropriate: Time Solar-presence Lunar-presence (or rise/transit/set marker) Selectable quantities. Output in order requested. No initial default exists. You will be prompted at least once. A detailed definition of these values follows, with the '*' symbols marking those quantities affected by user selection of airless or refraction-corrected apparent quantities. Quantities preceded by a '>' are statistical uncertainties that can be computed for asteroids and comets if a covariance is available, either in the database or supplied by the user. Numbers could change if new quantities are added: 1. Astrometric RA & DEC * 2. Apparent RA & DEC 3. Rates; RA & DEC * 4. Apparent AZ & EL 5. Rates; AZ & EL 6. Sat. X & Y, pos. ang 7. Local app. sid. time 8. Airmass 9. Vis mag. & Surf Brt 10. Illuminated fraction 11. Defect of illumin. 12. Sat. angle separ/vis 13. Target angular diam. 14. Obs sub-lng & sub-lat 15. Sun sub-long & sub-lat 16. Sub Sun Pos. Ang & Dis 17. N. Pole Pos. Ang & Dis 18. Helio eclip. lon & lat 19. Helio range & rng rate 20. Obsrv range & rng rate 21. One-Way Light-Time 22. Speed wrt Sun & obsrvr 23. Sun-Obsrvr-Target angl 24. Sun-Target-Obsrvr angl 25. Targ-Obsrv-IB/Illum% 26. Obsr-Primary-Targ angl 27. Pos. Ang;radius & -vel 28. Orbit plane angle 29. Constellation ID 30. Delta-T (CT - UT) 31. Obs eclip. lon & lat 32. North pole RA & DEC 33. Galactic latitude 34. Local app. SOLAR time 35. Earth->Site lt-time 36. RA & DEC uncertainty 37. POS error ellipse 38. POS uncertainty (RSS) 39. Range & Rng-rate sig. 40. Doppler/delay sigmas * > > > > > ... or select one of the pre-defined alphabetic formats below: A = All quantities B = Geocentric only D = Small-body topocentric E = Spacecraft geocentric The alphabetic assignments specifically mean: A = 1-40 D = 1-5,8-10,11,13,18-29,33-34,36-40 C = Small-body geocentric F = Spacecraft topocentric B 1-3, = 6, 9-33 E = 1-3, 8,10,18-25,29 C = 1-3,9-11,13,18-29,33, 36-40 F = 1-5, 8,10,18-25,29 ... with the small-body cases primarily skipping cartographic dependent quantities. Note that Ida and Gaspra are exceptions, having IAU-defined mapping grids, so that C & D options won't provide all available data for such objects. Below, a * indicates program initial defaults: Reference frame: J2000 (ICRF/J2000.0) B1950 (FK4/B1950.0 ) Time scale: UT (Universal Time) TT (Terrestrial Time) Time zone correction (used for UT-based output only); default = +00:00 Time format Calendar * * * * JD (Julian date) Both Time output precision (calendar format only) MINUTES (HH:MM) SECONDS (HH:MM:SS) FRACSEC (HH:MM:SS.fff) Right-ascension format Hours, minutes, seconds of arc (DEC degrees, minutes, seconds) Decimal degrees High-precision RA/DEC output No (~ 10^-2 arcsec; HH MM SS.ff DD MM SS.f) Yes (~ 10^-4 arcsec; HH MM SS.ffff DD MM SS.fff) Apparent coordinate corrections Airless apparent Refracted apparent Minimum elevation (integer values only); default = -90 degrees (no cut-off) Maximum airmass (real value); default = -38.0 (no cut-off) Rise/Transit/Set (RTS) ONLY print; default= NO Skip Daylight Print No Yes Solar Elongation Angle Cut-off; default= 0, 180.0 degrees (no cut-off) Comma-separated-value spreadsheet output No Yes * * * * * * * * * * 14. DEFINITION OF OBSERVER TABLE QUANTITIES The menu of observer table output quantities was shown above. The format of the table is as follows. "Labels" refers to possible column headings: TIME One output line for each step. The line begins with a 'b' if the date is BC, a blank (" ") if AD. This is followed by the date and time, which is either UT or TT, in calendar or JD format (or both), depending on user defaults. SOLAR PRESENCE Time tag is followed by a blank, then a solar-presence symbol: '*' 'C' 'N' 'A' '' Daylight (refracted solar upper-limb on or above apparent horizon) Civil twilight/dawn Nautical twilight/dawn Astronomical twilight/dawn Night OR geocentric ephemeris INTERFERING BODY/LUNAR PRESENCE The solar presence symbol is immediately followed by another marker symbol: 'm' '' 'r' 't' 's' Refracted upper-limb of Moon/IB on or above apparent horizon Refracted upper-limb of Moon/IB below apparent horizon OR geocentric ephemeris Rise (target body on or above cut-off RTS elevation) Transit (target body at or past local maximum RTS elevation) Set (target body on or below cut-off RTS elevation) The 'rts' codes will be displayed under two conditions only: if the print interval is less than or equal to 30 minutes or the RTS-only print option has been selected. For non-Earth observing sites, no twilight/dawn codes (C, N, or A) are output, refraction is not modelled and the interfering body marker is 'x' instead of the 'm' reserved for Earth's Moon. NOTE: "n.a." is output if a requested quantity is not available for selected object. For example, azimuth and elevation for a geocentric ephemeris. STATISTICAL UNCERTAINTIES Asteroid and comet output can include formal +/- 3 standard-deviation statistical orbit uncertainty quantities if a covariance is available. Such output indicates there is a 99.7% chance the actual value is within given bounds. These statistical calculations assume observational data errors are normally (randomly) distributed. If there are systematic biases (such as timing, reduction or star-catalog errors), results can be optimistic. Because the epoch covariance is mapped using linearized variational partial derivatives, results can also be optimistic for times far from the solution epoch, particularly for objects having close planetary encounters. QUANTITY DEFINITIONS 1. Astrometric RA & DEC Corrected for light-time only. With respect to the Earth mean equator and equinox of the reference Epoch. If FK4/B1950.0 frame output is selected, elliptic aberration terms are added. Labels: R.A._(ICRF/J2000.0)_DEC R.A._( FK4/B1950.0)_DEC R.A._(J2000.0)_DEC. R.A._(B1950.0)_DEC. (HMS/DMS format) (HMS/DMS format) (degree format) (degree format) 2. Apparent RA & DEC Apparent right ascension and declination of the target with respect to the center/site body's true-equator and Earth equinox of-date. For non-Earth sites with rotational models, the origin of RA is the meridian containing the Earth equinox of J2000.0. For non-Earth sites without rotational models, RA and DEC are with respect to the REFERENCE FRAME (FK4/B1950 or ICRF/J2000.0) coordinate system. Corrected for light-time, the gravitational deflection of light, stellar aberration, precession and nutation. There is an optional (approximate) correction for atmospheric refraction (Earth only). Labels: R.A._(a-apparent)__DEC. R.A._(r-apparent)__DEC. R.A._(a-appar)_DEC. R.A._(r-appar)_DEC. (airless, HMS/DMS format) (refracted, HMS/DMS format) (airless, degrees format) (refracted, degrees format) 3. Rates; RA & DEC The rate of change of apparent RA and DEC (airless). d(RA)/dt is multiplied by the cosine of the declination. Units are ARCSECONDS PER HOUR. Labels: dRA*cosD d(DEC)/dt 4. Apparent AZ & EL Apparent azimuth and elevation of target. Corrected for light-time, the gravitational deflection of light, stellar aberration, precession and nutation. There is an optional (approximate) correction for atmospheric refraction (Earth only). Azimuth measured North(0) -> East(90) -> South(180) -> West(270). Elevation is with respect to plane perpendicular to local zenith direction. TOPOCENTRIC ONLY. Units: DEGREES Labels: Azi_(a-appr)_Elev Azi_(r-appr)_Elev (airless) (refracted) 5. Rates; AZ & EL The rate of change of apparent azimuth and elevation (airless). d(AZ)/dt is multiplied by the cosine of the elevation angle. TOPOCENTRIC ONLY. Units are ARCSECONDS PER MINUTE. Labels: dAZ*cosE d(ELV)/dt 6. X & Y satellite offset & position angle Satellite differential coordinates WRT the central body along with the satellite position angle. Differential coordinates are defined in RA as X=[(RA_sat - RA_primary)*COS(DEC_primary)], and in DEC as Y=(DEC_sat-DEC_primary). Non-Lunar satellites only. "SatPANG" is CCW angle from the North Celestial Pole to a line from planet center to satellite center. Units: ARCSECONDS (X & Y) and DEGREES (position angle) Labels: X_(sat-primary)_Y SatPANG 7. Local Apparent Sidereal Time The angle measured westward in the body true equator-of-date plane from the meridian containing the bodyfixed observer to the meridian containing the true Earth equinox (defined by intersection of the true Earth equator of date with the ecliptic of date). For non-Earth sites, a somewhat different definition is used. The value returned is measured from the observer meridian to the meridian containing the Earth equinox of the J2000.0 system. TOPOCENTRIC ONLY. Units are HH MM SS.ffff or decimal hours (HH.ffffffffff) Labels: L_Ap_Sid_Time 8. Airmass Relative optical airmass; a measure of extinction. The ratio between the absolute optical airmass at target refracted elevation to the absolute optical airmass at zenith. Based on work of Kasten and Young (Applied Optics, vol. 28 no. 22, 15-Nov-1989). TOPOCENTRIC, ABOVE HORIZON ONLY. Unitless. Labels: a-mass 9. Visual magnitude & Surface Brightness Approximate (apparent) visual magnitude & surface brightness. Value for Pluto includes Charon. The Sun's altitude above the Saturn ring-plane is not considered for Saturn. When the Moon is at phase angles < 7 deg (within 1 day of full), the computed magnitude tends to be ~ 0.12 too small. Surface brightness is returned for asteroids only if a radius is known. It is the average visual magnitude of a square-arcsecond of the illuminated portion of the apparent disk. For observing sites not on the Earth or Moon, planet and satellite values are not available. Sun, comet and asteroid values are. Units are (none) and VISUAL MAGNITUDES PER SQUARE ARCSECOND. Magnitude laws: Sun: Asteroids: Comets: APmag= M - 5 + 5*log10(d), where M=4.83, d=distance from Sun (parsecs) APmag= H + 5*log10(delta) + 5*log10(r) -2.5*log10((1-G)*phi1 + G*phi2) T-mag=M1 + 5*log10(delta) + k1*log10(r) N-mag=M2 + 5*log10(delta) + k2*log10(r) + phcof*beta Surface brightness: S-brt= V + 2.5*log10(k*PI*a*b') Labels: APmag S-brt APmag T-mag N-mag (Non-comet with known dimensions) (Non-comet with unknown dimensions) (Comets; total & nuclear magnitudes) 10. Illuminated fraction Percent of target object circular disk illuminated by Sun (phase), as seen by observer. Units are PERCENT. Labels: Illu% 11. Defect of illumination Angular width of target circular disk diameter not illuminated by Sun. Available only if target radius is known. Units are ARCSECONDS. Labels: Def_illu 12. Angular separation/visibility The angle between the center of a non-lunar target body and the center of the primary body it revolves around, as seen by the observer. Units are ARCSECONDS. Non-lunar natural satellite visibility codes (limb-to-limb): /t = Transitting primary body disk, /O /p = Partial umbral eclipse, /P /u = Total umbral eclipse, /U /- = Target is the primary body, /* = = = = Occulted by primary body disk, Occulted partial umbral eclipse, Occulted total umbral eclipse, None of above ("free and clear") ... the radius of major bodies is taken to be the equatorial value (max) defined by the IAU2000 system. Atmospheric effects and oblateness aspect are not currently considered in these computations. Light-time is. Labels: ang-sep/v 13. Target angular diameter The angle subtended by the disk of the target seen by the observer, if it was fully illuminated. The target diameter is taken to be the IAU2000 equatorial diameter. Oblateness aspect is not currently included. Units are ARCSECONDS. Labels: Ang-diam 14. Observer sub-longitude & sub-latitude The planetographic (geodetic) longitude and latitude of the center of the target disk seen by the observer. Uses the IAU2000 rotation models. For the gas giants only (Jupiter, Saturn, Uranus and Neptune), these longitudes are based on the Set III prime meridian angle, referred to the planet's rotating magnetic field. Latitude is always referred to the body dynamical equator. Note there can be an offset between the dynamical pole and the magnetic pole. Units are DEGREES. Labels: Ob-lon Ob-lat 15. Solar sub-longitude & sub-latitude The planetographic (geodetic) longitude and latitude of the center of the target disk seen by an observer at the center of the Sun. Uses the IAU2000 rotation models. For the gas giants only (Jupiter, Saturn, Uranus and Neptune), these longitudes are based on the Set III prime meridian angle, referred to the planet's rotating magnetic field. Latitude is always referred to the body dynamical equator. Note there can be an offset between the dynamical pole and the magnetic pole. Units are DEGREES. Labels: Sl-lon Sl-lat 16. Sub-solar position angle & angular distance from disk center Target sub-solar point position angle (CCW with respect to direction of true-of-date Celestial North Pole) and angular distance from the sub-observer point (center of disk) at print time. Negative distance indicates the subsolar point is on the hemisphere hidden from the observer. Units: DEGREES and ARCSECONDS Labels: SN.ang SN.ds 17. North pole position angle & distance Target's North Pole position angle (CCW with respect to direction of true-of-date celestial North) and angular distance from the sub-observer point (center of disk) at print time. Negative distance indicates N.P. on hidden hemisphere. Units: DEGREES and ARCSECONDS Labels: NP.ang NP.ds 18. Heliocentric ecliptic longitude & latitude Geometric heliocentric (J2000 or B1950) ecliptic longitude and latitude of target at the instant light leaves it to be observed at print time(at print time minus 1-way light-time). Units: DEGREES Labels: hEcl-Lon hEcl-Lat 19. Heliocentric range & range-rate Target apparent heliocentric range ("r") and range-rate ("rdot") as seen by the observer. Units are AU and KM/S. Labels: r rdot 20. Observer range & range rate Target apparent range ("delta") & range-rate ("delta-dot") relative to observer. Units are AU and KM/S. Labels: delta deldot 21. One-Way Light-time Target 1-way down-leg light-time, as seen by observer. The elapsed time since light (observed at print-time) left or reflected off the target. Units: MINUTES Labels: 1-way_LT 22. Speed wrt Sun & observer Magnitude of velocity of target with respect to the Sun center and the observer at the time light left the target to be observed. Units are KM/S. Labels: VmagSn VmagOb 23. Sun-Observer-Target angle Target's apparent solar elongation seen from observer location at print-time. If negative, the target center is behind the Sun. Units are DEGREES. For observing centers with defined rotation models, an additional marker is output under the column labelled '/r' (for relative position). If there is no rotation model associated with the observing center, no /r column will be present. Under this column, /T indicates target trails Sun (evening sky) /L indicates target leads Sun (morning sky) NOTE: The S-O-T solar elongation angle is the total separation in any direction. It does not indicate the angle of Sun leading or trailing. Labels: S-O-T /r 24. Sun-Target-Observer angle Target's apparent PHASE ANGLE as seen from observer location at print time. Units are DEGREES. Labels: S-T-O 25. Target-Observer-Interfering_Body/Illum% Apparent angle between the target body and a potential visually interfering body (the largest body in the system (e.g. Moon), except for the one the observer is on), seen from the observer's location, along with fraction of the IB disk that is illuminated by the Sun. A negative interference (T-O-I) angle indicates the target center is behind the interfering body. Labels: T-O-I/Illu% 26. Observer-Primary-Target angle Apparent angle between a target, its primary's center and an observer at print time. Units: DEGREES. Labels: O-P-T 27. Position angle; radius & -velocity vector The position angles of the extended Sun->target radius vector ("PsAng") and the negative of the target's heliocentric velocity vector ("PsAMV"), as seen in the plane-of-sky of the observer, measured CCW from reference frame North Celestial Pole. Small-bodies only. Units are DEGREES. Labels: PsAng PsAMV 28. Orbit plane angle Angle between observer and target orbital plane, measured from center of target at the moment light seen at observation time leaves the target. Positive values indicate observer is above the objects orbital plane, in the direction of reference frame +z axis. Small-bodies only. Units: DEGREES. Labels: PlAng 29. Constellation ID The 3-letter abbreviation for the constellation name of target's astrometric position, as defined by the IAU (1930) boundary delineation. Labels: Cnst 30. CT-UT Difference between uniform Coordinate Time scale ("ephemeris time") and Earth-rotation dependent Universal Time. Prior to 1972, the difference is with respect to UT1 (CT-UT1). For 1972 and later, the delta is with respect to UTC (CT-UTC). Values beyond the next July or January 1st may change if a leap-second is introduced at later date. Units: SECONDS Labels: CT-UT 31. Observer Ecliptic Longitude & Latitude Observer-centered ecliptic-of-date longitude and latitude of the target's apparent position (corrected for lighttime, stellar aberration, precession, nutation and the deflection of light due to the Sun and Earth). Units: DEGREES Labels: ObsEcLon ObsEcLat 32. Target North Pole RA & DEC Right Ascension and Declination (IAU2000 rotation model) of target body's North Pole direction at the time light left the body to be observed at print time. Consistent with requested reference frame; ICRF/J2000.0 or FK4/ B1950.0 RA and DEC. Units: DEGREES. Labels: N.Pole-RA N.Pole-DC 33. Galactic Latitude Observer-centered Galactic System II (post WW II) latitude of the target's apparent position (corrected for light-time, the deflection of light due to the Sun and Earth and stellar aberration). Units: DEGREES. Labels: GlxLat 34. Local Apparent Solar Time Local Apparent SOLAR Time at observing site. TOPOCENTRIC ONLY. Units are HH.fffffffffff (decimal hours) or HH MM SS.ffff 35. Earth to Site Light-time Instantaneous light-time of the station with respect to Earth center at print-time. The geometric (or "true") separation of site and Earth center, divided by the speed of light. Units: MINUTES Labels: 399_ins_LT 36. Plane-of-sky RA and DEC pointing uncertainty Uncertainty in Right-Ascension and Declination. Output values are the formal +/- 3 standard-deviations (sigmas) around nominal position. Units: ARCSECONDS Labels: RA_3sigma DEC_3sigma 37. Plane-of-sky error ellipse Plane-of-sky (POS) error ellipse data. These quantities summarize the target's 3-dimensional 3-standarddeviation formal uncertainty volume projected into a reference plane perpendicular to the observer's line-of-sight. Labels: SMAA_3sig = Angular width of the 3-sigma error ellipse semi-major axis in POS. Units: ARCSECONDS. SMIA_3sig = Angular width of the 3-sigma error ellipse semi-minor axis in POS. Units: ARCSECONDS. Theta = Orientation angle of the error ellipse in POS; the clockwise angle from the direction of increasing RA to the semi-major axis of the error ellipse, in the direction of increasing DEC. Units: DEGREES. = Area of sky enclosed by the 3-sigma error ellipse. Units: ARCSECONDS ^ 2. Area_3sig 38. Plane-of-sky ellipse RSS pointing uncertainty The Root-Sum-of-Squares (RSS) of the 3-standard deviation plane-of-sky error ellipse major and minor axes. This single pointing uncertainty number gives an angular distance (a circular radius) from the target's nominal position in the sky that encompasses the error-ellipse. Units: ARCSECONDS. Labels: POS_3sigma 39. Uncertainties in plane-of-sky radial direction Range and range rate (radial velocity) formal 3-standard-deviation uncertainties. Units: KM, KM/S Labels: RNG_3sigma RNGRT_3sig 40. Radar uncertainties (plane-of-sky radial direction) Doppler radar uncertainties at S-band (2380 MHz) and X-band (8560 MHz) frequencies, along with the round-trip (total) delay to first-order. Units: HERTZ and SECONDS Labels: DOP_S-sig DOP_X-sig RT_delay-sig 15. CLOSE-APPROACH TABLES For asteroids and comets, a close-approach table may be requested. Output is produced only when the selected object reaches a minimum distance within a set spherical radius from a planet, Ceres, Pallas, or Vesta. User-specifications for this table can include the time-span to check, the radius of detection for planets and asteroids, the maximum uncertainty in time-of-close-approach before the table is automatically cut-off, and whether to output optional error ellipse information projected into the B-plane The B-plane mentioned above is defined by the three orthogonal unit vectors T, R, and S (the origin being the body center). T lies in the B-plane, pointing in the direction of decreasing celestial longitude. R lies in the B-plane, pointing in the direction of decreasing celestial latitude (south). S is directed along the relative velocity vector at body encounter, perpendicular to the B-plane, and thus R and T. The B vector is the vector in the plane from the body to the point where the incoming object's velocity asymptote pierces the R-T plane. Note the B-plane is defined only when the incoming object is hyperbolic with respect to the body. For objects with covariances, statistical quantities are output for each close-approach. All tabulated statistical quantities (MinDist, MaxDist, TCA3Sg, SMaA, SMiA, Gamma, Nsigs and P_i/p) are based on a linearized covariance mapping in which higher-order (small) terms in the variational partial derivatives of the equations of motion are dropped. Due to possible non-linearities in any given object's actual dynamics, this can result in significant errors at epochs distant in time from the solution epoch. Consequently, long linearized mappings (hundreds, dozens, or sometimes just several years from the present time) should be considered useful approximations, pending additional analysis, especially in these cases: A) objects with several close planetary encounters, B) objects with very close planetary encounters (< 0.01 AU), C) objects with very short data arcs (days or weeks). While linearized projections will tend to indicate such cases with obviously rapid uncertainty growth, the specific numbers output can inadequately represent orbit uncertainty knowledge. Possible output quantities are described below. "Nominal" effectively means "highest-probability for the given orbit solution", although there can be other possible orbits of equal probability. If there is no covariance available, no statistical quantities are returned. Date (CT) Nominal close-approach date (Coordinate Time). Calendar dates prior to 1582-Oct-15 are in the Julian calendar system. Later calendar dates are in the Gregorian system. Body Name or abbreviation of the planetary body or major asteroid beingclosely approached by the selected small-body. CA Dist Nominal close-approach distance at the close-approach time. Units: AU MinDist Minimum close-approach distance possible (formal 3 standard-deviations with linearized covariance mapping). Units: AU MaxDist Maximum close-approach distance possible (formal 3 standard-deviations with linearized covariance mapping). Units: AU Vrel Relative velocity of the object and the body it is approaching at the nominal time of close-approach. Units: KM/S TCA3Sg Close-approach-time 3-standard deviation uncertainty. Units: MINUTES SMaA 3-sigma error ellipse semi-major axis projected into the B-plane at nominal time of closest-approach. Units: KM SMiA 3-sigma error ellipse semi-minor axis projected into the B-plane at nominal time of closest-approach. Units: KM Gamma Orientation angle of error ellipse in the B-plane. Counter-clockwise angle from the B vector to the semi-major axis of the error ellipse. Units: DEGREES Nsigs The number of standard deviations (sigmas) required for the error ellipse to intersect the body being closely approached. Units: STANDARD DEVIATIONS P_i/p Linearized probability of the object impacting the body. 16. UNDERSTANDING RISE, TRANSIT AND SET INDICATORS There are 2 ways the system can be used to mark rise, transit and set (RTS) conditions: activate the RTS-only print option OR produce a general observer table with step-size less than 30 minutes. NORMAL-TABLE RTS-MARKER MODE RTS is indicated automatically during normal observer table generation, when the step-size is less than 30 minutes. Markers are placed to indicate the event occurred at some point in the previous step. Thus, precision of the indicator depends on the step-size selected. For this mode, rise and set are always with respect to the true-visualhorizon reference plane (TVH), described below. RTS-ONLY PRINT MODE The advantage of this mode is it allows production of a more compact RTS table over a longer time-span than does the "normal" table generation mode. When RTS-only print is selected, the program will search for the events at a user-specified resolution, from 1 to 9 minutes. Output will be generated ONLY for these three events. The marker symbols in the table indicate that the event took place sometime in the previous step interval. This RTS-only mode can be turned on at two different points in the program: #1) Preferably, when specifying the ephemeris/search step-size #2) ... but also in the "change defaults" prompt structure Three types of criteria are available for the rise and set conditions, relative to an input elevation angle (nominally 0 degrees). Select by specifying, when prompted at #1 or #2, one of these symbols: TVH ... True visual horizon plane. The horizon seen by an observer on the reference ellipsoid. Allows for horizon dip effect and refraction, but not local topography. GEO ... Geometric horizon plane. The horizon is defined by the plane perpendicular to the reference ellipsoid local zenith (no horizon dip). Refraction is allowed for. RAD ... Radar case. Geometric horizon plane, no refraction. For example, when prompted for the step-size, one could enter "5 min GEO' to search, at five-minute steps, for the refracted rise/set relative to the geometric horizon plane. BACKGROUND DESCRIPTION Rise and set elevations are taken to be the maximum of 0 or the input elevation cut-off value [0-90 deg], set in the "change defaults" prompt section. Thus, if there are local hills, one could set the cut-off at 10 degrees and get RTS relative to that elevation. At low elevations, these rise/set times should be viewed as approximations, realistically good to perhaps only 1-2 minutes at the horizon due to local atmospheric variation and topography. To speed RTS-only searches, use the largest step-size compatible with the required accuracy. For example, considering the inherent atmospheric instability at the horizon, one should rarely need to identify rise/set to better than 5 minute accuracy. Setting a search-step of 5 minutes will then produce a table 5 times faster than 1 minute searching. The program computes approximate refraction angles assuming yellow-light observations at 10 deg C sea- level with pressure of 1010 millibars. Corrected coordinates should be accurate to < 10 arcsec, but errors may be much larger near the horizon (+- 0.3 deg) or fluctuate unpredictably with local weather. Both Moon and Sun rise/set are based on when the refracted upper limb of the object reaches the specified elevation. Transit is based on the center of the target body. 17. CONSTELLATION IDENTIFICATION The observed background constellation of a target (corrected for light-time) may be requested for an observer table. The output field will contain a three letter abbreviation of the constellation name, from the list shown below. Constellation boundaries are those delineated by Gould (1877) and Delporte (1930) under the auspices of the International Astronomical Union. __________________________________________________________________________ | Abbrev. | Constellation Name || Abbrev. | Constellation Name | | | || | | | And | Andromeda || Leo | Leo | | Ant | Antila || LMi | Leo Minor | | Aps | Apus || Lep | Lepus | | Aqr | Aquarius || Lib | Libra | | Aql | Aquila || Lup | Lupus | | Ara | Ara || Lyn | Lynx | | Ari | Aries || Lyr | Lyra | | Aur | Auriga || Men | Mensa | | Boo | Bootes || Mic | Microscopium | | Cae | Caelum || Mon | Monoceros | | Cam | Camelopardis || Mus | Musc | | Cnc | Cancer || Nor | Norma | | CVn | Canes Venatici || Oct | Octans | | CMa | Canis Major || Oph | Ophiuchus | | CMi | Canis Minor || Ori | Orion | | Cap | Capricornus || Pav | Pavo | | Car | Carina || Peg | Pegasus | | Cas | Cassiopeia || Per | Perseus | | Cen | Centaurus || Phe | Phoenix | | Cep | Cepheus || Pic | Pictor | | Cet | Cetus || Psc | Pisces | | Cha | Chamaeleon || PsA | Pisces Austrinus | | Cir | Circinus || Pup | Puppis | | Col | Columba || Pyx | Pyxis | | Com | Coma Berenices || Ret | Reticulum | | CrA | Corona Australis || Sge | Sagitta | | CrB | Corona Borealis || Sgr | Sagittarius | | Crv | Corvus || Sco | Scorpius | | Crt | Crater || Scl | Sculptor | | Cru | Crux || Sct | Scutum | | Cyg | Cygnus || Ser | Serpens | | Del | Delphinus || Sex | Sextans | | Dor | Dorado || Tau | Taurus | | Dra | Draco || Tel | Telescopium | | Equ | Equuleus || Tri | Triangulum | | Eri | Eridanus || TrA | Triangulum Australe | | For | Fornax || Tuc | Tucana | | Gem | Gemini || UMa | Ursa Major | | Gru | Grus || UMi | Ursa Minor | | Her | Hercules || Vel | Vela | | Hor | Horologium || Vir | Virgo | | Hya | Hydra || Vol | Volans | | Hyi | Hydrus || Vul | Vulpecula | | Ind | Indus || | | | Lac | Lacerta || | | |___________||________________________||_____________|_______________________|| 18. SPK FILE PRODUCTION Introduction: An SPK file is a binary file which may be smoothly interpolated to retrieve an object's position and velocity at any instant within the file time-span. Such files may be used as input to visualization and mission design programs, allowing them to quickly retrieve accurate target body observation and data analysis ephemerides without having to repeatedly integrate equations of motion. An SPK file could be considered a "recording" of the integrator. SPK stands for "Spacecraft and Planet Kernel". It is a file element of the SPICE system devised and maintained by the NAIF (Navigation and Ancillary Information Facility) team at JPL. SPK files may hold ephemerides for any kind of spacecraft or solar system body, but Horizons produces SPK files only for comets and asteroids. Potential users are advised that programming and science/math skills at an advanced college level are needed to utilize these files. Users must have a computer with 25-50 Mbytes of disk space, 8 Mbytes of available RAM and a FORTRAN or C compiler. The user's own code must be capable of calling FORTRAN or C modules. Internet FTP capability is needed to obtain the necessary SPICE components as well as the SPK files generated by Horizons. For information on SPK files in general, contact Charles.H.Acton-Jr@jpl.nasa.gov (NAIF team leader) or see web site "http://pds-naif.jpl.nasa.gov/". Horizons Implementation: IMPORTANT: These informal file releases should not be used for "category A" flight project purposes (involving the safety and success of spacecraft hardware and mission) without first contacting ... Donald.K.Yeomans@jpl.nasa.gov Supervisor, Solar System Dynamics Group, 818-354-2127 A particular object's orbit may be insufficiently well-determined, over the chosen time-span, to be suitable for some high-precision purposes. Background: SPK files can be produced only with the telnet interface. Horizons allows a maxmimum of 20 small-bodies per SPK file. To construct an SPK for a comet or asteroid, Horizons integrates the object's trajectory over a userspecified time span greater than 32 days, but less than 25 years. The position components, at discrete steps, over some interval, are fit to a series of Chebyshev polynomials. When a users' application program reads the SPK file, the polynomials are accessed and interpolated to retrieve the requested state. SPK files are capable of storing trajectory data with a fidelity greater than 1 millimeter (more accurately than should ever be required). In practice, it is the Chebyshev fit that determines how closely the SPK interpolation matches the integrator. The typical trade-off is that higher fidelity SPK files are obtained by fitting higher degree polynomials to smaller time intervals. The cost for increased accuracy is larger file size. File Fidelity: Choosing the best way to represent a trajectory in a file is complicated by the wide range of small-body orbits and anomalies such as close-approaches to major planets. Horizons seeks to strike a rough balance between file size and file fidelity, valuing fidelity more than file size. Prior to the integration, a default mesh (state vector interval) is selected for polynomial fits. There is the "loose" mesh for main-belt objects (eccentricity less than 0.35, semi-major axis greater than 2.3 AU). This covers the majority of objects. Integrator states are preserved to the meter level (1-sigma) or less for most objects. There is a "standard" mesh that will fit all but a few objects well; close-approaches are described accurately to the 10-50 meter range and < 1 meter at other times. File sizes are 4 times larger than "loose" mesh objects. Finally, for a few objects, a "tight" mesh will be necessary. File sizes are 4x larger than "standard", 16x larger than "loose". Mesh assignment is automatic, but not all cases requiring a tight mesh can be detected in advance (which is why this is being discussed). At the end of an integration, a summary of polynomial fit maximum errors is displayed: ++++++++++++++++++++++++++++++++++++++++++++++++++ A-posteriori SPK fidelity estimate (rel. to integrator): Max. error (3 std. dev) Time --------------------------------------------------------X: 0.7104212997280315D-03 m 1998-May-09 12:00:00.000 Y: 0.1287005692494599D-02 m 1998-May-09 12:00:00.000 Z: 0.7502616895491441D-03 m 1998-May-09 12:00:00.000 RSS: 0.1650446811753079D-02 m 1998-May-09 12:00:00.000 ++++++++++++++++++++++++++++++++++++++++++++++++++ This data, along with other summary information, is also stored in the SPK file comment area. It can be read using the "spacit" or "commnt" utility in the SPICE Toolkit distribution. The table shows the maximum three standard deviation error detected in the Chebyshev fit to the integrator position vector components. The maximum root-sum-square (RSS) of component error is also shown. If the error from the default mesh selection is too large for your application, contact Jon.D.Giorgini@jpl.nasa.gov for instructions on forcing Horizons to a tighter mesh and improving fidelity. Transferring SPK files: Within the Horizons system, SPK files are created as binary files on a Sun Sparc/UNIX platform. These files can be used on several popular platforms, but may be unreadable on others. Reasons for this include: 1) Data-type representation (machine word-size) 2) Floating point representations (IEEE or not) 3) Byte order (least significant byte first vs. last) If you are using a verion of the SPICE Toolkit higher than 52, you will be able to directly read Horizons binary files on any platform. If not, the machine you intend to use the SPK file on falls into one of two possible categories: Compatible systems: If your system has 32-bit words, IEEE floating-point, and is "big-endian" (stores highest order byte first) like the Sun Sparc, you will be able to use the Horizons-generated binary SPK files directly; respond "no" to the transfer format prompt and use the binary mode of FTP to retrieve the file. Known compatible machines are the HP 9000 series, Motorola 68K series (MacInt...
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