Unformatted text preview: nal Doppler. Doppler errors are driven by
predicted state errors in the transmitted satellite velocity vector, and the slow drift of the on-board
satellite frequency standard. The best indicator of steady state range rate error due to predicted
satellite ephemeris and clock states is the rate-of-change of SIS UREs. Typically, the range rate
error associated with steady state SIS UREs is less than one millimeter/second RMS. Page A-38 October 2001 GPS SPS Performance Standard APPENDIX B
Key Terms, Definitions, and Acronyms
SECTION B-1 Key Terms and Definitions
Almanac Longitude of the Ascending Node (Ωo): Equatorial angle from the Prime Meridian
(Greenwich) at the weekly epoch to the ascending node at the ephemeris reference epoch.
Coarse/Acquisition (C/A) Code: A PRN code sequence used to modulate the GPS L1 carrier.
Corrected Longitude of the Ascending Node (Ωk) and Geographic Longitude of the
Ascending Node (GLAN): Equatorial angle from the Prime Meridian (Greenwich) to the
ascending node, both at arbitrary time tk.
Dilution of Precision (DOP): The magnifying effect on GPS position error induced by mapping
GPS ranging errors into position within the specified coordinate system through the geometry of
the position solution. The DOP varies as a function of satellite positions relative to user position.
The DOP may be represented in any user local coordinate desired. Examples are HDOP for local
horizontal, VDOP for local vertical, PDOP for all three coordinates, and TDOP for time.
Equatorial Angle: An angle along the equator in the direction of Earth rotation.
Geometric Range: The difference between the estimated locations of a GPS satellite and an
Groundtrack Equatorial Crossing (GEC, λ, 2 SOPS GLAN): Equatorial angle from the Prime
Meridian (Greenwich) to the location a groundtrack intersects the equator when crossing from the
Southern to the Northern hemisphere. GEC is equal to Ωk when the argument of latitude (Φ) is
Instantaneous User Range Error (URE): The difference between the pseudo range measured
at a given location and the expected pseudo range, as derived from the navigation message and
the true user position, neglecting the bias in receiver clock relative to GPS time. A signal-in-space
(SIS) URE includes residual orbit, satellite clock, and group delay errors. A system URE
(sometimes known as a User Equivalent Range Error, or UERE) contains all line-of-sight error
sources, to include SIS, single-frequency ionosphere model error, troposphere model error,
multipath and receiver noise.
Longitude of Ascending Node (LAN): A general term for the location of the ascending node –
the point that an orbit intersects the equator when crossing from the Southern to the Northern
Longitude of the Groundtrack Equatorial Crossing (GEC, λ, 2 SOPS GLAN): Equatorial
angle from the Prime Meridian (Greenwich) to the location a groundtrack intersects the equator
when crossing from the Southern to the Northern hemisphere....
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