1. GPS 101 HOW GPS WORKS University of Idaho GIS Day November 19, 2014
Dr. Lawrence R. Weill

2. PRE-SATELLITE RADIONAVIGATION SYSTEMS
LF Radio Direction Finding (~1900 to present): Angle-based position fixing and homing.
Radar ( to present): Angle- and range-based positioning relative to surrounding objects.
Omega (1971 to 1997): Hyperbolic phase-difference positioning on a global scale.
DECCA ( ): European hyperbolic phase difference positioning.
Loran A (1942 to 1980), Loran C (1974 to present): Hyperbolic TOA difference positioning.
VOR/DME (1946 to present): Aviation angle- and range-based position fixing and homing using VHF and UHF frequencies.
TACAN (~1960 to present): Military version of VOR/DME.

3. LF RADIO DIRECTION FINDING

5. LORAN & DECCA POSITIONING (HYPERBOLIC)

6. ACCURACY OF PRE-SATELLITE
RADIONAVIGATION SYSTEMS

7. 1957: SPUTNIK, WORLD’S FIRST SATELLITE

8. DOPPLER-BASED SATELLITE NAVIGATION
Sputnik gave rise to satellite radionavigation using Doppler.
1957: Scientists at MIT noted frequency variation of Sputnik’s signal due to Doppler, and realized this can be used to track the satellite’s location. In reverse, the received Doppler frequency variation can be used to find the position of the receiver.
1959: The Doppler principle for finding location was used in TRANSIT, the first operational satellite-based navigation system. It was developed by Johns Hopkins Applied Physics Laboratory (APL) to support the U.S. Navy submarine fleet, and eventually used 10 low-earth orbit satellites.
Transit positioning accuracy was roughly 400 meters. Ships had to wait for satellite passes and resolve a cross-track ambiguity. Ship movement complicated the process of establishing a position.

9. TRANSIT POSITIONING (DOPPLER CURVE)

10. EVOLUTION OF GPS (1)
1960: Raytheon Corporation suggests the first 3-D TDOA satellite navigation system (MOSAIC) for the Mobile Minuteman ICBM system. MOSAIC was dropped in 1961 when the Mobile Minuteman program was cancelled.
1963: Aerospace Corporation develops its concept of a 3-D satellite navigation system. The Air Force supports this effort and designates it System 621B, which later demonstrated a new pseudorandom noise (PRN) ranging signal.
1964: The Naval Research Laboratory (NRL) develops the Navy Timation satellite system for advancing the development of high-stability clocks in space, an important foundation for GPS. The first Timation satellite is launched in 1967.
: Various groups, including the Army, Navy, and Air Force, are engaged in satellite navigation concept debates.

11. EVOLUTION OF GPS (2)
December 17, 1973: The NAVSTAR GPS concept is solidified.
1973 to 1979: GPS concept validation takes place.
July 14, 1974: The very first NAVSTAR satellite is launched, carrying an atomic clock.
1978 to 1985: 10 Block I GPS satellites, used for full-scale development and testing of the GPS system, are launched.
1989 to 1994: 24 Block II operational satellites are launched.
December 1990: GPS system becomes operational.
April 1995: GPS reaches full operational capability.

12. GPS Constellation
6 Orbits
55o Inclination
20,200 km Altitude
~ 32 Sats
11 hr 56 min Orbital Period

13. 2D POSITIONING

19. 2D POSITIONING EQUATIONS
GPS also provides very accurate time!

21. TRANSMISSION OF REPEATING CODE PERIODS

22. ONE PERIOD OF C/A CODE
Each satellite has its own unique C/A code

23. WHY USE A PSEUDORANDOM CODE?
● A pseudorandom code can have much greater energy than a short pulse while still providing high-resolution measurement of range.
● The high resolution is accomplished by a correlation process.
● Correlation also provides protection from any interfering signal that does not match the pseudorandom code being used.
● By using a unique pseudorandom code for each satellite, all satellites can transmit on the same frequency without mutual interference.

24. CORRELATION

25. -3 chip misalignment
Sum of chip products = -4
Alignment
Sum of chip products = 35
+3 chip misalignment
Sum of chip products = -4

26. CORRELATION FUNCTION

27. CORRELATOR REJECTION CAPABILITY

28. EARLY-LATE
CODE TRACKING

29. GPS CODE TRACKING LOOP

31. 50 BPS DATA BIT STRUCTURE

33. GPS DATA FRAME

34. GPS RECEIVER OPERATIONS AT TURNON
● Determine visible satellites and approximate Dopplers: Uses approximate receiver position (if known) and stored almanac data.
● Signal acquisition: Search for each visible satellite, both in frequency and code shift.
● Signal tracking: Track the code and carrier of each satellite.
● Bit synchronization: Determine start times of bits in the 50 bps navigation data message.
● Read navigation data message: Get timing and ephemeris data to put transmit time tags on received code epochs and establish positions of satellites. Ephemeris data may take up to 30 seconds.
● Make code and/or carrier pseudorange measurements.
● Solve positioning equations.

35. PSEUDORANGE ERROR SOURCES
Ionospheric Error: Up to meters. Reduced by dual-frequency operation, reference station corrections, and differential GPS (DGPS).
Tropospheric Error: Up to 20 meters. Reduced by tropospheric model corrections, using higher-elevation satellites, and DGPS.
Multipath: Up to meters. Reduced by avoiding nearby reflectors, using receiver multipath mitigation technology, and using higher bandwidth signals. Not reducible by DGPS.
Satellite Clock and Ephemeris Errors: Generally below 1 meter. Almost completely eliminated by DGPS.
Receiver Errors (Thermal Noise, Processing): Generally below 1-3 meters. Reduced by improved signal processing in HW or SW.

36. POSITION DILUTION OF PRECISION (PDOP)
PDOP is a number indicating the ratio of position accuracy to pseudorange accuracy. It depends on satellite position geometry.
Poor Geometry (PDOP = ~18)
Good Geometry (PDOP = ~3)

37. CODE-BASED DIFFERENTIAL GPS
(Sub-meter Positioning Accuracy)

38. CODE-BASED DIFFERENTIAL GPS METHODS

39. GPS RECEIVER CARRIER TRACKING

40. DIFFERENTIAL CARRIER PHASE

41. DIFFERENTIAL CARRIER PHASE PRINCIPLE

42. SECOND-DIFFERENCE CARRIER PHASE PRINCIPLE

43. DIFFERENTIAL CARRIER PHASE POSITIONING
(Centimeter-level Accuracy)

44. GPS MODERNIZATION: NEW CIVILIAN SIGNALS
L1 C/A: The legacy civil signal, which will continue to be
broadcast on L1 frequency MHz.
L2C: ● Signal on L2 frequency MHz, currently broadcast by seven satellites.
● Enables L1/L2 ionospheric error corrections.
● Higher transmitted power level for faster signal acquisition and better positioning accuracy.
L5: ● Signal on L5 frequency MHz.
● Technically advanced signal with higher power and greater bandwidth
L1C: ● Signal at L1 frequency designed to enable interoperability betweeen GPS and international GNSS systems.

45. MAGELLAN SYSTEMS ● Ad in Los Angeles Times, 1986
● Technical Founders of Magellan Systems: Don Rea (hardware), Norm Hunt (software), Larry Weill (algorithms)
● Seed money from Ed Tuck
● Design challenges with steep learning curve
● Difficulty finding investors willing to commit
● Don’s breadboard

47. Magellan NAV 1000: World’s first handheld GPS receiver, first sold in 1989.
Single-channel slow sequencing design.
Approximately the size of a brick.
Weight approximately 1 ½ lbs.
Rotatable quadrifilar antenna.
Dot-matrix LCD display.
Powered by 6 AA alkaline cells. Battery life approximately 4 hrs.
TTFF from cold start 1 to 3 min.

49. MEMORABLE EVENTS ● Discovery of map error ● Communicating with aliens
● Saving lives
● NAV 1000 in the Gulf War—Iraq invades Kuwait in 1990
● Two new products designed under the radar
● BTG vs. Magellan
● Magellan in the Time and Navigation exhibit at the Smithsonian National Air and Space Museum