Start Looking at How GPS Works-Slide Show

GPS Original Design • Started development in the late 1960s as NAVY/USAF project to replace Doppler positioning system • Aim: Real-time positioning to < 10 meters, capable of being used on fast moving vehicles. • Limit civilian (“non-authorized”) users to 100 meter positioning. 2 GPS Design • Innovations: –Use multiple satellites (originally 21, now ~28) –All satellites transmit at same frequency –Signals encoded with unique “bi-phase, quadrature code” generated by pseudo-random sequence (designated by PRN, PR number): Spread-spectrum transmission. –Dual frequency band transmission: •L1 ~1.5 GHz, L2 ~1.25 GHz 3 Measurements • Measurements: – Time difference between signal transmission from satellite and its arrival at ground station (called “pseudo-range”, precise to 0.1–10 m) – Carrier phase difference between transmitter and receiver (precise to a few millimeters) – Doppler shift of received signal • All measurements relative to “clocks” in ground receiver and satellites (potentially poses problems). 5 Positioning • For pseudo-range to be used for “point-positioning” we need: – Knowledge of errors in satellite clocks – Knowledge of positions of satellites • This information is transmitted by satellite in “broadcast ephemeris” • “Differential” positioning (DGPS) eliminates need for accurate satellite clock knowledge by differencing the satellite between GPS receivers (needs multiple ground receivers). 6 Satellite constellation • Since multiple satellites need to be seen at same time (four or more): – Many satellites (original 21 but now 28) – High altitude so that large portion of Earth can be seen (20,000 km altitude —MEO) 7 Current constellation • Relative sizes correct (inertial space view) • “Fuzzy” lines not due to orbit perturbations, but due to satellites being in 6-planes at 55o inclination. 10/31/2005 12S56 8 Ground Track Paths followed by satellite along surface of Earth. 9 Pseudo-range accuracy • Original intent was to position using pseudo-range: Accuracy better than planned • C/A code (open to all users) 10 cm-10 meters • P(Y) code (restricted access since 1992) 5 cm-5 meters • Value depends on quality of receiver electronics and antenna environment (little dependence on code bandwidth). 10 GPS Antennas (for precise positioning) Nearly all antennas are patch antennas (conducting patch mounted in insulating ceramic). • Rings are called chokerings (used to suppress multi-path) 11 Positioning accuracy • Best position accuracy with pseudo-range is about 20 cm (differential) and about 5 meters point positioning. Differential positioning requires communication with another receiver. Point positioning is “stand-alone” • Wide-area-augmentation systems (WAAS) and CDMA cell-phone modems are becoming common differential systems. • For Earth science applications we want better accuracy • For this we use “carrier phase” where “range” measurement noise is a few millimeters (strictly range change or range differences between sites) 12 Carrier phase positioning • To use carrier phase, need to make differential measurements between ground receivers. • Simultaneous measurements allow phase errors in clocks to be removed i.e. the clock phase error is the same for two ground receivers observing a satellite at the same time (interferometric measurement). • The precision of the phase measurements is a few millimeters. To take advantage of this precision, measurements at 2 frequencies L1 and L2 are needed. Access to L2 codes in restricted (anti-spoofing or AS) but techniques have been developed to allow civilian tracking of L2. These methods make civilian receivers more sensitive to radio frequency interference (RFI) • Next generation of GPS satellites (Block IIF) will have civilian codes on L2. Following generation (Block III) will have another civilian frequency (L5). 13 Phase positioning • Use of carrier phase measurements allows positioning with millimeter level accuracy and sub-millimeter if measurements are averaged for 24-hours. • Examples: – The International GPS Service (IGS) tracking network. Loose international collaboration that now supports several hundred, globally distributed, high accuracy GPS receivers. – Applications in California: Southern California integrated GPS network 14 IGS network • Stations in the IGS network continuously track GPS satellites and send their data to international data centers at least once per day. All data are publicly available. • A large number of stations transmit data hourly with a few minutes latency (useful in meteorological applications of GPS). • Some stations transmit high-rate data (1-second sampling) in real-time. (One system allows ±20 cm global positioning in real-time with CDMA modem connection). 16 Uses of IGS data • Initial aim was to provide data to allow accurate determination of the GPS satellite orbits: Since IGS started in 1994, orbit accuracy has improved from the 30 cm to now 2-3 cm • From these data, global plate motions can be observed in “realtime” (compared to geologic rates) • Sites in the IGS network are affected by earthquakes and the deformations that continue after earthquakes. The understanding of the physical processes that generate post-seismic deformation could lead to pre-seismic indicators: – Stress transfer after earthquakes that made rupture more/less likely on nearby faults – Material properties that in the laboratory show pre-seismic signals. • Meteorological applications that require near real-time results 17 Orbit Improvement 10/31/2005 12S56 18 Motions in California Red vectors relative to North America; Blue vectors relative to Pacific Motion across the plate boundary is ~50 mm/yr. In 100-years this is 5 meters of motion which is released in large earthquakes 10/31/2005 12S56 20 CONCLUSIONS • GPS, used with millimeter precision, is revealing the complex nature and temporal spectrum of deformations in the Earth. • Programs such as Earthscope plan to exploit this technology to gain a better understanding about why earthquakes and volcanic eruptions occur. • GPS is probably the most successful dual-use (civilian and military) system developed by the US • In addition to the scientific applications, many commercial applications are also being developed. 21