An Overview of the Global Positioning Satellite System (GPS) Edward J. Delp N9YTE October 25, 2000

Overview n The Navigation Problem n Earlier Approaches n GPS Description n How does GPS Work?

Acknowledgement Many of the graphical images used in this talk are courtesy of Many of the graphical images used in this talk are courtesy of Peter H. Dana of the Department of Geography, University of Texas at Austin notes/gps/gps.html notes/gps/gps.html The best web site for GPS

The Navigation Problem n The ancient question: Where am I? n Earth coordinates: latitude and longitude Lafayette: N40/W86 Lafayette: N40/W86 n Latitude can be determined by Sun angle n What about longitude?

Latitude and Longitude

Longitude Problem n No easy way to determine longitude n On July 8, 1714 the Longitude Act was established in England to solve the “longitude problem” n Two solutions were proposed -- use of stars and moons -- use of stars and moons -- the “time” solution -- the “time” solution

Longitude Longitude : The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time Dava Sobel

Longitude Problem: References n

The “Time” Solution n Where am I? What time is it in Greenwich, England? n The ability to find one’s position is based on how well one can tell what time is it? n The development of the chronometer n To find longitude to within 0.5 degree requires a clock that loses or gains no more than 3 seconds/day

Longitude n How does this work? n The earth turns 360 degrees in 24 hours: 15 degrees = 1 hour 15 degrees = 1 hour n If you know the time in Greenwich when it is local noon at your location one can find your longitude relative to Greenwich n Must know “datum” reference to use maps

Datum Reference Lone Pine Cemetery - N ’’ E ”

Satellite Navigation n US Department of Defense has need for very precise navigation n In 1973, the US Air Force proposed a new system for navigation using satellites n The system is known as: Navigation System with Timing and Ranging: Global Positioning System or NAVSTAR GPS

NAVSTAR GPS Goals n What time is it? n What is my position (including attitude)? n What is my velocity? n Other Goals: - What is the local time? - What is the local time? - When is sunrise and sunset? - When is sunrise and sunset? - What is the distance between two points? - What is the distance between two points? - What is my estimated time arrival? - What is my estimated time arrival?

GSP System Simply stated: The GPS satellites are nothing more than a set of clocks in the sky more than a set of clocks in the sky

GPS Segments n Space Segment: the constellation of satellites n Control Segment: control the satellites n User Segment: users with receivers

Space Segment

n System consists of 24 satellites in the operational mode: 21 in use and 3 spares 3 other satellites are used for testing 3 other satellites are used for testing n Altitude: 20,200 Km with periods of 12 hr. n Current Satellites: Block IIR- $25,000, KG 2000 KG n Hydrogen Maser Atomic Clocks

Hydrogen Maser Clock These clocks lose one second every 2,739,000 million years

GPS Orbits

Control Segment Master Control Station is located at the Consolidated Space Operations Center (CSOC) at Flacon Air Force Station near Colorado Springs

Control Segment

CSOC n Track the satellites for orbit and clock determination n Time synchronization n Upload the Navigation Message n Manage DOA

Operational Capabilities Initial Operational Capability - December 8, 1993 Full Operational Capability declared by the Secretary of Defense at 00:01 hours on July 17, 1995

GPS Transmitted Signal n Two signals are transmitted on carriers: L1 = MHz L1 = MHz L2 = MHz L2 = MHz These are derived from the system clock of MHz (phase quadrature) n Modulation used is Direct Sequence Spread Spectrum (code division multiple access - CDMA) (code division multiple access - CDMA)

GPS Signals

GPS Clock Signals n Two types of clock signals are transmitted n C/A Code - Coarse/Acquisition Code available for civilian use on L1 provides 300 m resolution n P Code - Precise Code on L1 and L2 used by the military provides 3m resolution

Spread Spectrum n Spread Spectrum is used because - resistance to jamming - resistance to jamming - masks the transmissions - masks the transmissions - resist multipath effects - resist multipath effects - multiple access - multiple access n All 24 GPS satellites transmit on the same two frequencies BUT use a different ID sequence

GPS Signals n The satellites transmit as part of their unique Spread Spectrum signal a clock or timing signal n The range or distance to the satellite is obtained by measuring how long it takes for the transmitted signal to reach the receiver n This is not the “true” range due to clock errors - what is obtained is know as the “pseudo-range”

GPS Position n By knowing how far one is from three satellites one can ideally find their 3D coordinates n To correct for clock errors one needs to receive four satellites

GPS: How does it work? n Typical receiver: one channel C/A code on L1 n During the “acquisition” time you are receiving the navigation message also on L1 n The receiver then reads the timing information and computes the “pseudo- ranges” n The pseudo-ranges are then corrected

GPS: How does it work? n Corrected ranges are used to compute the position n This is a very complicated iterative nonlinear equation

Navigation Message n To compute your position one must know the position of the satellite n Navigation Message - transmitted on both L1 and L2 at 50 bits/s for 30 s n Navigation message consists of two parts: - satellite almanac - satellite almanac - clock bias - clock bias

Why Do I Need To See 4 Satellites? n The problem is that the clock signal from the satellite is corrupted by atmospheric refraction n Another major problem is that the receiver’s clock is not very accurate n For a 2D fix 3 satellites

Why Do I Need To See 4 Satellites?

Denial of Accuracy (DOA) n The US military uses two approaches to prohibit use of the full resolution of the system n Selective Availability (SA) - noise is added to the clock signal and the navigation message has “lies” in it n Anti-Spoofing (AS) - P-code is encrypted n The military sometimes turns off both DOA techniques

Differential GPS n Used to improve accuracy n Put a “satellite” on the ground at a precise position n Differential signal is not “transmitted” on standard satellite frequencies

Uses of GPS n Airplane and Boat Navigation n Continental Drift n Surveying n Precise Timing n Iceberg Tracking n Archaeological Expeditions n Mobile Multimedia

GPS Clock Rollover n n GPS System Time rolled over at midnight August 1999, 132 days before the Year 2000 n n On 22 August 1999, unless repaired, many GPS receivers claimed that it is 6 January 1980 n y2k/gpsweek.htm

Conclusion n GPS will find more civilian uses n DOD has promised to eliminate SA n Russia has a system known as GLONASS n The EU is discussing deploying its own system

References n n B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins, GPS: Theory and Practice, Third Edition, Springer-Verlag, n n T. Logsdon, The Navstar Global Positioning System, Van Nostrand, n n A. Leick, GPS Satellite Surveying, Second edition, Wiley, 1995.

References n n T. A. Herring, "The Global Positioning System," Scientific American, pp , February n n N. J. Hotchkiss, A Comprehensive Guide to Land Navigation with GPS, Alexis, n n Special Edition on the Global Positioning System, Satellite Times, March/April n n D. Sobel, Longitude, Walker, 1995.

Web Sites n GPS Program Office: n US Coast Guard Navaigation Center n GPS Precise Orbits n GPS World Magazine