GPS Global Positioning System Dr Volkan Nalbantolu AE

GPS Global Positioning System Dr. Volkan Nalbantoğlu AE 484 Inertial Navigation Systems May 2008 1 / 55

Contents Radio Navigation n GPS History n Parts of GPS n GPS Signals n How Does GPS Work? n GPS Error Sources n 2 / 55

Radio Navigation Systems Radio navigation consists of finding position and heading by using electromagnetic wave propogation. n Examples: n ¨ Radar ¨ VHF Omnidirectional Range (VOR, VOR/DME) ¨ Long Range Navigation (LORAN-C) ¨ Global Positioning System (GPS) 3 / 55

What is GPS ? GPS is a satellite based navigation system. n It is developed and financed by the U. S. Department of Defense. n It provides position velocity and timing information anywhere in the world under any weather condition. n 4 / 55

Initially it was developed as a military system (1970 s) n In 1980 s it became available for civilian use. n Today it is being used in land, air and marine applications by millions of people. n 5 / 55

GPS History First radio navigation research started in 1920 s n During the 2 nd World War LORAN became operational and it was possible to find latitude and longitude by using the time of arrival information of the radio signals sent from ground stations. n 6 / 55

GPS History n During 1959 -1964: USA developed the TRANSIT satellite system to determine the location of nuclear submarines. ¨ 7 low orbit sattelites ¨ Latitude-Longitude information ¨ Long measurement time ¨ Only for slow dynamic vehicles 7 / 55

GPS History n 1978 -1985 ¨ n Total of 11 block I Satellites sent 1989 first block II Satellite sent 8 / 55

Current Configuration Nominally there are 24 satellites (4 on 6 orbital planes) n Currently there are 29 operational Block II/IIA/IIR-M satellites n 9 / 55

GPS Segments n There are 3 segments ¨ Space ¨ Control ¨ User 10 / 55

GPS Space Segment n n Consitsts of the space vehicles (satellites) and the radio signals sent by these satellites. GPS satellites ¨ ¨ ¨ Height ~20200 km 6 orbits with at least 4 satellites on each orbit Period ~ 1 revolution / 12 hour Weight ~950 kg Size 1, 6 x 6 m 11 / 55

GPS Space Segment At least 5 satellites are visible from anywhere on the earth n There are solar panels and 12 navigation antennas on each satellite. n Block II/IIA Block IIR ve IIR-M 12 / 55

GPS Control Segment n n Monitors and Controls the GPS satellites. One Master Control Station (MCS), Five Monitor Stations (MS) 13 / 55

GPS Control Segment 14 / 55

GPS Control Segment n Functions of the Control Segment ¨ Detection and determination of Satellite orbits ¨ Correction of satellite clocks ¨ Updating the satellite messages ¨ Monitoring the status of each satellite and performing the maintanence tasks 15 / 55

GPS User Segment n n Consists of receivers that can decode the satellite signals GPS receivers transform the satellite signals into position, velocity and time information. 16 / 55

GPS Services n GPS has two levels of information ¨ Precise Positioning Service - PPS ¨ Standard Positioning Service - SPS 17 / 55

GPS Services – PPS n Precise Positioning Service (PPS) ¨ Can be used by authorized users only ¨ Planned for military purposes 18 / 55

GPS Services – PPS n Access to PPS is controlled by two methods ¨ SA (Selective Availability), GPS accuracy is degraded intentionally by adding pseudorandom errors on the signals. ¨ A-S (Anti-Spoofing), Encrypted code 19 / 55

GPS Services – PPS n ¨ 16 m SEP (3 D - %50) Position accuracy ¨ 100 ns (1 σ) timing accuracy 20 / 55

GPS Services – SPS n Standard Positioning Service (SPS) ¨ Open to all users but less accurate ¨ With Selective Availability 100 m SEP (3 D - %50) position accuracy n 337 ns (1 σ) time accuracy n 21 / 55

GPS Services – SPS ¨ SA has been removed on May 2000 ¨ SPS users have accuracies close to PPS 22 / 55

GPS Signals GPS satellites send very weak radio signals on two L – band frequencies (L 1 and L 2) n L 1 and L 2 are carrier frequencies. These are sinusoidal signals n 23 / 55

GPS Signals All GPS satellites use the same frequency carriers (L 1 and L 2) n But each satellite has its own identification code n These are two types of codes modulating the L 1 and L 2 carriers. n ¨ C/A – Code ¨ P – Code 24 / 55

GPS Signals n L 1: 1575. 42 MHz ¨ ¨ n Modulated by. C/A-code & P-code Signal Power: -160 d. BW L 2: 1227. 6 MHz ¨ ¨ Modulated by P-code only Signal Power : -166 d. BW L 1 C/A P(Y) 1575 MHz L 2 P(Y) 1227 MHz 25 / 55

GPS Signals Carriers (L 1/L 2) Bipolar Phase Shift Keying (BPSK) Modulation C/A - Code (L 1) P - Code (L 1/L 2) Nav Data (L 1/L 2) A-S Encryption P – P(Y) Phase Quadrature O SA Degredation 26 / 55

GPS Signals 27 / 55

GPS Signals n GPS receivers generate the equivalent of these codes internally and compares with the ones coming from the satellites. n GPS receiver shifts the internally generated code until it matches with the received one (cross-correlation) 28 / 55

GPS Signals n n Another Message on the L 1 ve L 2 carrier frequency is the “Navigation Message” Navigation Message 50 Hz Clock rate ¨ Has information specific for each satellite ¨ Has the satellite position and time delay information ¨ 29 / 55

GPS Signals ¨ 50 Hz 6 s for one subframe n 30 s for one frame n 12, 5 min for the whole set n 30 / 55

How Does GPS Work? n Based on a geometric principle “Position of a point can be calculated if the distances between this point and three objects with known positions can be measured ” 31 / 55

How Does GPS Work? n If the distance to one object is known: ¨ Then I am on a sphere with the object at the center 32 / 55

How Does GPS Work? n If I know the distance to a second object: ¨ Then I am on a circle which is the intersection of two spheres 33 / 55

How Does GPS Work? n If I know the distance to a thrid object: ¨ Then I am on one of the two points which are at the intersection of three spheres 34 / 55

How Does GPS Work? n To find the distance to a satellite “Signal Time of Transmission” is used n How is Signal Time of Transmission calculated? 35 / 55

How Does GPS Work? n GPS receiver generates the same signal that is coming from the satellite (C/A Code) starting at the same time. Satellite Receiver n But the code coming from the satellite is delayed because it travels the distance between the satellite and the receiver. 36 / 55

How Does GPS Work? n GPS receiver shifts the internally generated code until it matches with the received one and finds ΔT, Signal Time of Transmission T Code generatede by the receiver Code generatede by the sattelite 37 / 55

How Does GPS Work? Signal Time of Transmission is actually an indication of the distance between the receiver and the satellite n Signal travels with the speed of light and in Δt time travels a distance of n Pr= C. ΔT (C = Speed of light) 38 / 55

How Does GPS Work? Pr is the “Pseudo-Range” n It is called Pseudo-Range because it is not the real range between the receiver and the satellite due to uncertainties such as: n ¨ Synchronisation error between the receiver and satellite clocks ¨ Change in the medium in which the signal travels 39 / 55

How Does GPS Work? 40 / 55

How Does GPS Work? The dominant source of error in Pseudo. Range calculation is the synchronisation between the receiver and the satellite n Satellites have very accurate and very expensive atomic clocks n It is not practical to use atomic clocks in the receivers. Standard crystal oscillators are used instead n 41 / 55

How Does GPS Work? This syncrhronisation error is called Clock Bias n To eliminate clock bias a forth satellite is used n ¨ 4 unknowns (3 dimensional position + Clock Bias) ¨ 4 equations 42 / 55

How Does GPS Work? Pi = Pseudo-Range to satellites Xi , Yi , Zi = 3 Dimensional satellite cartesian coordinates X , Y , Z = 3 Dimensional satellite cartesian coordinates b = Receiver clock bias (in terms of distance) 43 / 55

How Does GPS Work? These 4 non-linear equations are solved and receiver coordinates and clock bisa are obtained n These equations are in ECEF (Cartesian) Coordinates n Latitiude, Longitude and hight values can be obtained by a transformation n 44 / 55

How Does GPS Work? ECEF and Latitude / Longitude LOCAL Meridian Z GREENWICH Meridian User position h Pz R Y O Px Px: ECEF Pos x (M) Py: ECEF Pos y (M) Pz: ECEF Pos z (M) Py X Equator 45 / 55

GPS Error Sources ¨ ¨ ¨ ¨ SA (Selective Availability) Satellite clock errors Satellite orbit errors Atmospheric effects Receiver noise Multipath Number of satellites in range Satellite geometric configuration 46 / 55

GPS Error Sources n n n DOP (Dilution of Precision) GDOP - Geometric DOP It is a metric to define the effect of the satellite geometry on the accuracy of the solution: ¨ PDOP – Position DOP (3 D Position) ¨ HDOP – Horizontal DOP (Horizontal position) ¨ TDOP – Time DOP (Time) 47 / 55

GPS Error Sources n Satellites close to each other have larger uncertainty 48 / 55

GPS Error Sources n Satellites far away from each other have less uncertainty 49 / 55

GPS Error Sources n 1 signifies the ideal situation n Satellites grouped on the same side cause larger DOP – Bad accuracy n Well distributed, smaller DOP – better accuracy 50 / 55

GPS Error Sources GPS Pseudo-Range Error Budget Segment Space Control User Error Source Frequency stability D-Band Delay Satellite acceleration uncertainty Other Ephemeris Estimation Other Ionospheric Delay compensation Troposphere Delay compensation Receiver noise Multipath Other Total System Error (m, %95) Reference: Navstar GPS User Equipment Introduction Error contribution (m, %95) P-Code C/A-Code 6. 5 1. 0 2. 0 1. 0 8. 2 1. 8 4. 5 9. 8 – 19. 6 3. 9 2. 4 1. 0 13. 0 15. 7 - 23. 1 51 / 55

Differential GPS - DGPS ¨ Used for applications where GPS accuracy is not enough ¨ In a typical DGPS application There is a reference receiver (base receiver) at an exactly known location n And there are other receivers (rover receivers) that can receive the correction signals sent by the base receiver. n 52 / 55

Differential GPS - DGPS Correction Signals GPS Referance Station DGPS Transmitter GPS & DGPS Receiver 53 / 55

Differential GPS - DGPS n n Since the exact location of the reference station is known it can calculate the distances to satellites accurately It compares these distances with its own solutions as a GPS Calculates corrections from these measurements Sends these corrections to the rover receivers from a different frequency than the GPS frequencies. 54 / 55

Differential GPS - DGPS Transmission is usually over a FM channel n The rover receivers are able to receive these corrections and they use them to correct their solutions n Corrections are valid within a certain range n Referance and rover receivers must have the same satellites in view n 55 / 55