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Understanding GPS and Its Functioning

Learn about the Global Positioning System (GPS) and how it works. Explore the objectives, receiver requirements, and spread spectrum radio techniques used in GPS. Discover the predecessor of GPS, TRANSIT, and the development of the basic navigation satellite concept.

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Understanding GPS and Its Functioning

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  1. Part II WHAT IS GPS AND HOW IT WORKS GPS ERROR SOURCES GS608 Reference materials can be found at: www.gmat.unsw.edu.au/snap/gps/about_gps.htm More GPS links are provided on the course web page

  2. Global Positioning System (GPS) • The NAVSTAR Global Positioning System (GPS) is a satellite-based radio-positioning and time-transfer system, designed, financed, deployed and operated by the US Department of Defense. • However, the system has currently significantly larger number of civilian users as compared to the military users.

  3. Global Positioning System (GPS) • The NAVSTAR Global Positioning System (GPS) program was initiated in 1973 through the combined efforts of the US Army, the US Navy, and the US Air Force. • The new system, designed as an all-weather, continuous, global radio-navigation system was developed to replace the old satellite navigation system, TRANSIT, which was not capable of providing continuous navigation data in real time on a global basis.

  4. GPS – Objectives 1/2 • Suitable for all classes of platform: aircraft, ship, land-based and space (missiles and satellites), • Able to handle a wide variety of dynamics, • Real-time positioning, velocity and time determination capability to an appropriate accuracy, • The positioning results were to be available on a single global geodetic datum, • Highest accuracy to be restricted to a certain class of user, • Resistant to jamming (intentional and unintentional), • Redundancy provisions to ensure the survivability of the system,

  5. GPS – Objectives 2/2 • Passive positioning system that does not require the transmission of signals from the user to the satellite(s), • Able to provide the service to an unlimited number of users, • World-wide coverage • Low cost, low power, therefore as much complexity as possible should be built into the satellite segment, and • Total replacement of the Transit 1 satellite and other terrestrial navaid systems.

  6. GPS Receiver Requirements GPS user hardware must have the ability to track and obtain any selected GPS satellite signal (a receiver will be required to track a number of satellites at the same time), in the presence of considerable ambient noise This is now possible using spread-spectrum and pseudo-random-noise coding techniques

  7. Spread Spectrum Radio (SSR) Technique 1/2 • Spread Spectrum Radio (SSR) was almost exclusively used by military until 1985, when FCC allowed spread spectrum’s unlicensed commercial use in three frequency bands: 902-928 MHz, 2.4-2.4835 GHz and 5.725-5.850 GHz. • SSR differs from other commercial radio technologies because it spreads, rather than concentrates, its signal over a wide frequency range within its assigned bands. • A key characteristic of spread spectrum radios is that they increase the bandwidth of the transmitted signal by a significantly large ratio to the original signal bandwidth. • The main signal-spreading techniques are direct sequencing and frequency-hopping

  8. Spread Spectrum Radio (SSR) Technique 2/2 • Direct sequencing continuously distributes the data signal across a broad portion of the frequency band; it modulates a carrier by a digital code with a bit rate much higher than the information signal bandwidth (used by GPS). • Alternatively, frequency-hopping radios move a radio signal from frequency to frequency in a fraction of a second. • The spread spectrum receiver has to reconstruct the original modulating signal from the spread-bandwidth signal by a process called correlation (or de-spreading). The fact that the interference remains spread across a large bandwidth allows the receiver to filter out most of their signal energy, by selectively allowing through only the bandwidth needed for the de-spread wanted signal. • Thus, the interference is reduced by SSR processing. Transmitting and receiving SSR radios must use the same spreading code, so only they can decode the true signal.

  9. Spread Spectrum Radio (SSR) Technique 3/3 A Spectrum Analyzer Photo of a Direct Sequence (DS) Spread Spectrum signal. A Spectrum Analyzer Photo of a Frequency Hop (FH) Spread Spectrum signal.

  10. TRANSIT as GPS Predecessor • Researchers at Johns Hopkins observed Sputnik in 1957. • Noted that the Doppler shift provided closest approach to earth. • Developed a satellite system that achieved accurate positioning • Called TRANSIT and provided basic ideas behind GPS

  11. Development of Basic Navigation Satellite Concept 1964-1967 • SYSTEMATIC STUDY OF EVERY WILD IDEA IMAGINABLE • CONVERGED ON “PSEUDORANGING” IN 1967 • MAJOR STUDY CONTRACTS LET IN 1968 TO TUNE THE CONCEPT

  12. Motto Adopted by the Joint Program Office on GPS Program The mission of this Program is to: 1. Drop 5 bombs in the same hole, and 2. Build a cheap set that navigates (<$10,000), and don’t you forget it!

  13. CHOICE OF CARRIER FREQUENCY • L-Band • C-Band should be studied • DESIGN OF SIGNAL STRUCTURE • Military and civilian use included • ORBIT/CONSTELLATION SELECTION Major Issues Identified in 1968 Studies

  14. Managed Concept Debates 1969-1972 • EXPANDED TRANSIT • Insisted on worldwide overage • 153 satellites in 400 mile polar orbits • Transit carrier frequency • EXPANDED TIMATION • Initially only a Time Transfer System • Insisted on worldwide coverage • Expanded concept to intermediate altitude circular • orbit constellation of 30 to 40 satellites

  15. Convergence on Final System 1973-1974 • SWITCHED CONCEPT TO 12-HOUR CIRCULAR ORBITS • 3 planes, 8 satellites each • i = 63° • RETAINED DIRECT-SHIFT KEYED SPREAD SPECTRUM PN SEQUENCE • DUAL FREQUENCY SIGNAL ON L-BAND • PICKED INITIAL DEPLOYMENT OF 4+2 ‘BLOCK I” SATELLITES

  16. PHASE I DESIGN 1974-1980 • BLOCK I SATELLITE CONTRACTS WITH ROCKWELL INTERNATIONAL • 6 satellites followed by 6 more • All satellite performance projections achieved. 3dB more transmitted power • then required • Exceptional (1x ) on-orbit Rubidium clock performance achieved. • DETAILS OF SIGNAL STRUCTURE & NAV MESSAGE DEFINED • C/A code designed with civil sector in mind • “P-Code” designed by Magnavox • Navigation message identical on both signals

  17. PHASE II DESIGN 1981-1989 • BLOCK II SATELLITES • Rockwell International • Selective Availability and Anti-Spoof (Y-Code) Implemented • Constellation downsized to 21 satellites (6 planes) • Nav message slightly modified • OPERATIONAL CONTROL SEGMENT • Monitors at Ascension, Diego Garcia, Guam, Hawaii, and Colorado Springs • 24-satellite ephemeris (orbit) determination • PHASE II/PHASE III USER EQUIPMENT • Rockwell Collins, Magnavox and Teledyne Systems • Rockwell Collins and Magnavox • Rockwell Collins

  18. GPS Satellite System – Final Design 1/2 • 24 satellites • altitude ~20,000 km • 12-hour period • 6 orbital planes, inclination 55o • Applications: practically unlimited! • Positioning and timing • Navigation • Mapping and GIS data collection • Engineering and communication • Agriculture • ITS

  19. GPS Satellite System – Final Design 2/2 • continuous signal transmit • fundamental frequency 10.23 MHz • almost circular orbit (e = 0.02) • at least 4 satellites visible at all times from any point on the Earth’s surface (5-7 most of the time)

  20. GPS Policy Board* • Department of Agriculture • Department of Commerce • Department of Defense • Department of Interior • Department of State • Department of Transportation • NASA *created to give larger voice to civilian applications of GPS.

  21. GPS Constellation • Block I (not operational) • Block II/IIA/IIR/IIR-M • Currently (as of January 4, 2006) • 29 satellites Block II/IIA/IIR/IIR-M (the most recent Block IIR-M satellite was launched on September 26, 2005 ) • Oldest satellite: PRN 15 from Block II-9, Oct. 1, 1990 • AS1/SA capability (to limit the access to the system by unauthorized users) • multiple clocks onboard 1 The process of encrypting the P-code by modulo-2 addition of the P-code and a secret encryption W-code. The resulting code is called the Y-code. AS prevents an encryption-keyed GPS receiver from being “spoofed” by a bogus, enemy-generated GPS P-code signal. Y-code is not available to the civilian users. 2The Department of Defense policy and procedure of denying to most non-military GPS users the full accuracy of the system. SA is achieved by dithering the satellite clock and degrading the navigation message ephemeris. Turned to zero on May 2, 2000.

  22. GPS Constellation • Block I • vehicle numbers (SVN) 1 through 11 • launched between 1978 and 1985 • concept validation satellites • developed by Rockwell International • circular orbits • inclination 63 deg • one Cesium and two Rubidium clocks • design life of 5 years (majority performed well beyond their life expectancy)

  23. GPS Constellation • Block II • vehicle numbers (SVN) 13 through 21 • launched between 1989 and 1990 • full scale operational satellites • developed by Rockwell International • nearly circular orbits • inclination 55 deg • two Cesium and two Rubidium clocks • design life of 7.3 years • AS/SA capabilities

  24. GPS Constellation • Block IIA • vehicle numbers (SVN) 22 through 40 • launched since 1990 (18 out of 19) • second series of operational satellites • developed by Rockwell International • nearly circular orbits • inclination 55 deg • two Cesium and two Rubidium clocks • design life of 7.3 years • AS/SA capabilities

  25. GPS Constellation • Block IIR • vehicle numbers (SVN) 41 through 62 • total of 7 launched (1 unsuccessful) • operational replenishment satellites • developed by Lockheed Martin • nearly circular orbits • inclination 55 deg • one Cesium and two Rubidium clocks • design life of 7.8 years • AS/SA capabilities

  26. GPS Constellation • Block IIM • first launch Sept. 26, 2005 • multiple Rb clocks • first step of GPS modernization program • nearly circular orbits, inclination 55 deg • Adding a second civil signal • C/A-type code at L2 frequency (1227.60 MHz) • Low power signal, not intended for precision navigation • Adding another military (M) code • Future: third civil signal • P-type codes (precise) at L5 frequency (1176.45 MHz) • Higher power signal, intended for precision navigation

  27. GPS Constellation • Block IIF • planned for launch between 2007 and 2010 ? • operational follow on satellites • nearly circular orbits • inclination 55 deg • design life of 12.7 years • will carry an inertial navigation system • will have an augmented signal structure (third frequency)

  28. GPS Constellation • Block III • In November 2000, Lockheed Martin and Boeing were each awarded a $16-million, 12-month study contract by the Air Force to conceptualize the next generation GPS satellite, which will be known as GPS Block-3.

  29. LAUNCH LAUNCH FREQ ORDER PRN SVN DATE STD PLANE --------------------------------------------------------------- IIA-16 01 32 22 NOV 92 Cs F1 IIA-17 29 29 18 DEC 92 Rb F4 IIA-18 22 22 03 FEB 93 Rb B1 IIA-19 31 31 30 MAR 93 Cs C3 IIA-20 07 37 13 MAY 93 Rb C4 IIA-21 09 39 26 JUN 93 Cs A1 IIA-22 05 35 30 AUG 93 Cs B4 IIA-23 04 34 26 OCT 93 Rb D4 IIA-24 06 36 10 MAR 94 Cs C1 IIA-25 03 33 28 MAR 96 Cs C2 IIA-26 10 40 16 JUL 96 Cs E3 IIA-27 30 30 12 SEP 96 Cs B2 IIA-28 08 38 06 NOV 97 Rb A5 **IIR-1 42 17 JAN 97 IIR-2 13 43 23 JUL 97 Rb F5 IIR-3 11 46 07 OCT 99 Rb D2 IIR-4 20 51 11 MAY 00 Rb E1 IIR-5 28 44 10 JUL 00 Rb B5 IIR-6 14 41 10 NOV 00 Rb F1 IIR-7 18 54 30 JAN 01 Rb E4 GPS Constellation LAUNCH LAUNCH FREQ ORDER PRN SVN DATE STD PLANE --------------------------------------------------------------- ^II-1 14 14 FEB 89 Cs E1 II-2 02 13 10 JUN 89 Cs B3 ^ II-3 16 16 18 AUG 89 Cs E5 ^ II-4 19 19 21 OCT 89 Cs A4 II-5 17 17 11 DEC 89 Cs D3 ^II-6 18 24 JAN 90 Cs F3 *II-7 20 26 MAR 90 II-8 21 21 02 AUG 90 Cs E2 II-9 15 15 01 OCT 90 Cs D2 IIA-10 23 23 26 NOV 90 Cs E4 IIA-11 24 24 04 JUL 91 Rb D1 IIA-12 25 25 23 FEB 92 Cs A2 *IIA-13 28 10 APR 92 IIA-14 26 26 07 JUL 92 Rb F2 IIA-15 27 27 09 SEP 92 Cs A3 * Satellite is no longer in service. ** Unsuccessful launch. TOTAL: 28 as of January 6, 2003 ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txt

  30. BLOCK I BLOCK II/IIA

  31. BLOCK IIR BLOCK IIF

  32. GPS Receiver Manufacturers NovAtel Inc. http://www.novatel.ca Trimble http://www.trimble.com Topcon http://www.topconps.com Thales Navigation http://www.thalesnavigation.com/en/ Garmin http://www.garmin.com Leica http://www.leica-gps.com Over 67 GPS manufacturers and over 467 types of receivers, 106 antennas ! (GPS World, January 2000)

  33. Who are GPS largest customers? • Survey & Mapping ~ 54% • Navigation ~ 20% • Tracking & Comm ~18% • Military ~ 6% • Car Navigation ~ 2%

  34. GPS Applications • military • civilian aircraft, land mobile, and marine vessel navigation • time transfer between clocks • spacecraft orbit determination • geodesy (precise positioning) • attitude determination with multiple antennas • geophysics (ionosphere, crustal motion monitoring, etc.) • surveying (static and kinematic, also real-time) • Intelligent Transportation Systems • GIS, Mobile Mapping Systems

  35. THE DEPLOYED CONSTELLATION

  36. GPS Antenna Coverage Antenna has ~28° field of view

  37. First GPS satellite Block I was launched in 1978 Air Force-launched Delta II carried the 18th GPS satellite into orbit in February 1993. 38

  38. Source: http://www.nasm.edu

  39. Before GPS, pilots relied only on navigational beacons located across the country • Now, with GPS fully operational, aircraft can fly the most direct routes between distant airports.

  40. How accurate is GPS? • Depending on the design of the GPS receiver and the measurement techniques employed, the accuracy is from 100 meters under Selective Availability (SA) policy (below 10 m with SA turned to zero) to better than 1 centimeter. • In order to obtain better than 100 (10 with SA turned to zero) meter accuracy, differential GPS must be used (two simultaneously tracking receivers or differential services). 41

  41. Why is GPS so accurate ? • The key to GPS accuracy is the fact that the signal is precisely controlled by the highly accurate atomic clock • Atomic clock’s stability is 10-13 – 10-14 per day (this means that the clock can loose 1 sec in 3,000,000 years!) • This highly accurate frequency standard produces the fundamental GPS frequency, 10.23 MHz, which is a basis for derived frequencies L1 (1575.42 MHz = =154*10.23) and L2 (1227.60 MHz = 120*10.23)

  42. The basis of GPS is • "triangulation" from satellites. • To "triangulate," a GPS receiver measures distance using the travel time of radio signals. • To measure travel time, GPS needs very accurate timing, which it achieves with some tricks • The primary unknowns are three coordinates of the receiver antenna (user)

  43. Mathematically we need four satellite ranges to determine exact position. • Three ranges are enough if we reject ridiculous answers or use other tricks.

  44. Source: http://www.nasm.edu

  45. How distance measurements from three satellites can pinpoint you in space 1/3 Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles. 47

  46. How distance measurements from three satellites can pinpoint you in space 2/3 Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away. That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersect. 48

  47. How distance measurements from three satellites can pinpoint you in space 3/3 If we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even farther, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres. 49

  48. Finally: In order to find the correct location (out of two points determined by the observation of three ranges to three satellites) we may need to make a fourth observation to the fourth satellite – this way we get the unique answer to our positioning problem. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement. However, a fourth measurement becomes very handy for another reason…

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