57707

57707 - GPS Introduction to GPS Based on radio systems from...

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EE579: Annavaram & Krishnamachari © Introduction to GPS Slides based on materials from Slides based on materials from [1] TRIMBLE GPS Tutorial, online at http://www.trimble.com/gps/index.shtml [2] [2] Jean -Marie Marie Zogg , GPS Basics, online at GPS Basics, online at http://www.u - blox.cn/customersupport/docs/GPS blox.cn/customersupport/docs/GPS -X-02007.pdf 02007.pdf [3] Wikipedia Wikipedia entry on Global Positioning System GPS y Based on radio systems from 1940s, started in late 1970s. y Gives longitude, lattitude, height (with meter-scale accuracy), time (within 60 ns accuracy) y Based on a signals from a system of ~30 MEO satellites (~20000 km altitude) GPS Segments Extremely noisy Link: - 15dB SNR! Data rate: 50 bits per second Basic Idea y Estimating distance from 3 satellites can fix location y Distance estimation is based on timing, which itself has errors, so a 4 th satellite is needed to solve for four unknowns (x,y,z, and t) PN Sequences y Each satellite has a unique signature y Used to identify satellite and obtain time-difference estimate Transmitter
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EE579: Annavaram & Krishnamachari © Signal Modulation Navigation Message Format Accurate Timing y Satellites have atomic clocks y Receivers do not. Hence the use of an additional satellite to correct for timing errors. Satellite Position y Need to know where satellites are located y Because of predetermined orbits, their locations are obtained through an almanac program on the receivers y Slight errors in the precomputed orbits (e.g., due to the moon’s gravitational pull) – ephemeris measurements, which are measured from the ground, then repeated back form the satellites Error Sources Error Sources
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EE579: Annavaram & Krishnamachari © Differential GPS y Have a reference receiver with known location y The reference station works backwards on GPS signals. y “Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an ‘error correction’ factor.” [1] y The reference then transmits error information to mobile receivers over a radio link. Wide Area Augmentation System (WAAS) y An FAA system to provide the differential corrections y Ground stations to measure errors, geostationary satellites to broadcast these y Typically provides 1m accuracy A-GPS y Uses cellular network to obtain error correction from a server y Also provides faster fix Relativity matters! y Special Relativity: satellite clocks slow by 7us per day due to the time dilation effect. y General Relativity: clocks close to massive object are slower – so earth-bound clocks are slow by 45 us per day. y Without correction, would lose 10 kilometers each day!
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57707 - GPS Introduction to GPS Based on radio systems from...

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