1 Dr. Guoqing Zhou GPS Overview CET 318 Book: p. 11-24 1. Basic Knowledge of GPS In 1973, DOD organized a Joint Program Office (JPO) located at the U.S. Air Force Systems Command's Space Division, Los Angeles Air Force Base (AFB) to What? to establish, develop, test, acquire, and deploy a spaceborne positioning system, called Navigation System with Timing and Ranging (NAVSTAR) Global Positioning System (GPS) is the result of this initial directive. Purpose: Military Functions: All-weather, 24hrs Principle: ranging system Other Application: Technical Problem: design, Potential Users: Cost: 30 billions 1. GPS was conceived as a ranging system from known positions of satellites in space to unknown positions on land, sea, in air and space. 2. The satellite signal continually --- be measured with a synchronized receiver. 3. The original objectives of GPS were the instantaneous determination of Position Velocity (i.e., navigation) Precise coordination of time (i.e., time transfer). 4. Goals: – All-weather, all-day, and space-based navigation system – To satisfy the requirements for the military forces – To accurately determine their position, velocity, and time in a common reference system, anywhere and anytime. Three Segments: – Space segment consisting of satellites which broadcast signals, – Control segment steering the whole system, – User segment including the many types of receivers. 2. Basic Principle 3 known points, 1 unknown point, 3 circles to determine the unknown point by radius, the fourth point is for verification a 2 3 known known known 1 Un-known a a Four unknowns: the three point coordinates + the clock error. Thus, four satellites are necessary to solve for the four unknowns. 2 GPS Satellite GPS Receiver Time System Coordinate System s= v t (pseudorange) s: range v: light velocity t: time s= t Σa(i) b(i) cos(?) (carrier phase) s: range ? : phase of electronic magnetic wave t: time Frequencies: L1, L2 Single frequencies receiver Dule frequency receive Carrier Phase receiver Pseudorange receiver 3. Space Segment – Nearly circular orbits – Altitude of about 20200 km – 24-hour worldwide coverage. – 21 + 3 satellites in six orbital planes (A to F) – An inclination of 55° – Four satellites per plane. Furthermore, four active spare satellites for replenishment will be operational. This constellation provides global coverage with four to eight simultaneously observable satellites above 15° elevation at any time of day. 3.1 Constellation –10°, occasionally up to 10 satellites visible; –5°, occasionally 12 satellites visible. 1. General Remarks The GPS satellites, essentially, provide a platform for radio transceivers, atomic clocks, computers, and various ancillary equipment used to operate the system. The auxiliary equipment of each satellite 1. Solar panels for power supply 2. A propulsion system for orbit adjustments and stability control. 3.2 GPS Satellites The satellites have various systems of identification: – Launch sequence number, – Assigned pseudorandom noise (PRN) code, – Orbital position number, – NASA catalogue number, and – International designation. Cont.Satellite Categories –Block I – Block II, IIA – Block IIR, IIF GPS Satellites Block I Total: 10 99.9Signal27 Feb 9430 Oct 859 Oct 8511 03 133.5Clock18 Nov 953 Oct 848 Sep 8410 12 115.2Power25 Feb 9419 Jul 8413 Jun 849 13 116.8Power4 May 9310 Aug 8314 Jul 838 11 ---Booster----18 Dec 817 --- 126.8Wheels10 Dec 9016 May 8026 Apr 806 09 45.0Wheels28 Nov 8327 Feb 809 Feb 805 05 93.6Clock27 Oct 868 Jan 7911 Dec 784 08 161.3Clock19 Apr 929 Nov 786 Oct 783 06 25.5Clock30 Aug 8014 Jul 7813 May 782 07 Operation (Months) Reason of loss Loss of navigation Available since Launch date Flight PRN No. No. GPS Satellites Block II Cont. Total: 8 D220 act 901 Oct 9020 15 E231 Aug 902 Aug 9019 21 B219 Apr 9025 Mar 9018 20 F314 Feb 9024 Jan 9017 18 D311 Jan 9011 Dec 8916 17 A414 Nov 8921 Oct 8915 19 E313 Sep 8917 Aug 8914 16 B312 Jul 8910 Jun 8913 02 Orbital position Available since Launch date Flight PRN No. No. 3 GPS Satellites: Block IIA Cont.Total: 16 C209 Apr 9628 Mar 9636 03 Cl28 Mar 9410 Mar 9435 06 D429 Nov 9326 Oct 9334 04 B428 Sep 9330 Aug 9333 05 Al21 Ju1 9326 Jun 9332 09 C412 Jun 9313 May 9331 07 C313 Apr 9330 Mar 9330 31 Bl4 Apr 932 Feb 9329 22 F45 Jan 9318 Dec 9228 29 Fl11 Dec 9222 Nov 9227 01 A330 Sep 929 Sep 9226 27 F223 Jul 927 Jul 9225 26 C225 Apr 929 Apr 9224 28 A224 Mar 9223 Feb 9223 25 Dl30 Aug 913 Jul 9122 24 E410 Dec 9026 Nov 9021 23 Orbital position Available since Launch date Flight PRN No. No. GPS Satellites: Block IIF (p. 14) GPS Satellites: Block III (p. 14) 3.3 GPS Satellite Signal The key to the system's accuracy is the fact that all signal components are precisely controlled by atomic clocks. The Block II: 1. Four on-board time standards 2. Two rubidium clocks 3. Two cesium clocks, a stability of 10 -14 to 10 -15 over one day These highly accurate frequency standards being the heart of GPS satellites produce the fundamental L-band frequency of 10.23 MHz. by multiplying the fundamental frequency by 154 and 120 respectively yielding 3000 000 year10 -15 1420405751Hydrogen 300 000 year10 -13 9192 631770Cesium 30 000 year10 -12 683468 2613Rubidium 30 year10 -9 5000000Quartz Time of causing 1 sStability/dayFrequencyClock Type GPS Clock Stability L1 = 1575.42 MHz L2 = 1227.60 MHz These dual frequencies are essential for eliminating the major source of error, i.e., the ionospheric refraction. The GPS signals less subject to intentional (unintentional) jamming 1. C/A-code (Coarse/Acquisition-code ) is – Civilian use, denies full system accuracy to nonmilitary users – Designated as the Standard Positioning Service (SPS), – Effective wavelength of approximately 300 m. – Modulated upon L1 only and is purposely omitted from L2. 2. P-code (Precision-code) is – U .S. military and other authorized users. – Designated as the Precise Positioning Service (PPS), – Effective wavelength of approximately 30 m. – Modulated on both carriers L1 and L2. Technology for Denial of Accuracy and Access Two methods for denying civilian users full use of the system: – Selective Availability (SA) – Anti-spoofing (A-S) Selective Availability-SA – Goal: to deny this navigation accuracy to potential adversaries – Means: by dithering the satellite clock and by manipulating the ephemerides 4 1. GPS fundamental frequency: δ Technology 2. Navigation message: ε Technology 3. P code: encrypting Technology Selective Availability (SA) Anti-spoofing (A-S) – The SA has only been implemented in Block II satellites at various levels of accuracy denial. – Accuracy degraded 100 m for horizontal 156 m for height. 0.3m/s for velocity 340ns for time The predictable accuracy decreases to 300 m for horizontal position and to 500 m for height. All numbers are given at the 95% probability level. At the 99.99% probability level. How to Realize Anti-Spoofing? 1.The design of GPS includes the ability to essentially "turn off" the P-code or invoke an encrypted code as a means of denying access to the P-code to all but authorized users. 2.The rationale for doing this is to keep adversaries from sending out false signals with the GPS signature to create confusion and cause users to misposition themselves. 3.A-S is accomplished by the modulo 2 sum of the P-code and an encrypting W-code. The resulting code Y-code. – Thus, when A-S is active, the P-code on the L1 and the L2 carrier is replaced by the unknown Y-code. – Note that A-S is either on or off. A variable influence of A-S (as is the case with SA) cannot occur. On March 29, 1996, the Presidential Decision Directive (PDD) on GPS was released expressing the "intention to discontinue the use of GPS Selective Availability (SA) within a decade in a manner that allows adequate time and resources for our military forces to prepare fully for operations without SA". Technology for Denial of Accuracy and Access 4. Control Segment Operational Control System (OCS) master control monitor stations ground control Main Tasks: Tracking of the satellites for the orbit and Clock determination and prediction, Time synchronization of the satellites, and Upload of the data message to the satellites. The OCS is also responsible for imposing SA on the broadcast signals. 4.1 Monitor Stations Main Tasks: – Each station is equipped with a precise cesium time standard and receivers which continuously measure pseudoranges to all satellites in view. – Pseudoranges are measured every 1.5 seconds and, using the ionospheric and meteorological data, they are smoothed to produce 15-minute interval data which are transmitted to the master control station. Locations: – Hawaii – Colorado Springs – Ascension Island in the South Atlantic Ocean – Diego Garcia in the Indian Ocean – Kwajalein in the North Pacific Ocean 5 4.2 Master Control Station Tasks of CSOC: – Collects the tracking data from the monitor stations – Calculates the satellite orbit and clock parameters using a Kalman estimator. – These results are then passed to one of the three ground control stations for eventual upload to the satellites. – Be responsible for the master control station. Location: – Formerly, Vandenberg AFB, California for BLOCK I – The Consolidated Space Operations Center (CSOC) at Falcon AFB, Colorado Springs, Colorado for BLOCK II after. 4.3 Ground Control Stations 1. The satellite ephemerides and clock information, calculated at the master control station and received via communication links, are uploaded to each GPS satellite via S-band radio links. 2. Formerly, uploading to each satellite was performed every eight hours; then the rate has been reduced to once (or twice) per day. If a ground station becomes disabled, prestored navigation messages are available in each satellite to support a prediction span so that the positioning accuracy degrades quite gradually. Locations of Ground Station: – Ascension Island in the South Atlantic Ocean – Diego Garcia in the Indian Ocean – Kwajalein in the North Pacific Ocean Receiver Cesium clock Atmospheric data Ephermrides Clock bias Navigation message GPS GPS Monitoring Station Ground Station Master Station Modulation Data processing Commander 5. User Segment 5.1 User Categories Military User Strictly speaking, the term "user segment" is related to the DoD concept of GPS as an adjunct to the national defense program. Even during the early days of the system, it was planned to incorporate a GPS receiver into virtually every major defense system. It was envisioned that every aircraft, ship, land vehicle, and even groups of infantry would have an appropriate GPS receiver to coordinate their military activities. 17,000400080005000 TotalMilitary Receiver Small Receiver Receiver C/A Code Civil Receiver Gulf War GPS Receiver Civilian User There are various other non-military uses. Just one example: A receiver can be connected to four antennas. When the antennas are placed in a fixed array (e.g., corners of a square), the attitude of the array can be determined in addition to its position. For example, placing antennas on the bow, stern, and port and starboard points of a ship would result in the determination of pitch, roll, yaw, and position of the vessel. 6 5.2 Receiver Types Based on the type of observables (i.e., code pseudoranges or carrier phases) and on the availability of codes (i.e., C/A-code, P-code, or Y-code), one can classify GPS receivers into: By Code: (1) C/A-code pseudorange receiver, (2) C/A-code carrier phase receiver, (3) P-code carrier phase receiver, and (4) Y-code carrier phase receiver. By Frequency: (1) Signal frequency receiver (2) Dual frequency receiver In General: (1) Pseudorange receiver (2) Carrier phase receiver By Positioning Style: Static Receiver Kinematic Receiver Low kinmatic Middle kinmatic High kinmatic By Application: Navigation Receiver Positioning Receiver Timing Receiver By Load Body: Potable Receiver Back Bag Receiver Vehicle Load Receiver Ship Load Receiver Airplane Load Receiver Missile Load Receiver Satellite Load Receiver 1. C/A-code Pseudorange Receivers Characteristics: 1. Only code pseudoranges using the C/A-code are measured. 2. The receiver is usually a hand-held device powered by flashlight batteries. 3. Typical from one to six independent receiver channels and output the three-dimensional position in longitude, latitude, and height. 4. Receivers with four or more channels are preferred for applications in motion since simultaneous satellite ranges can be measured to produce more accurate positions. 2. C/A-Code Carrier Receivers Characteristics: 1. Code ranges and carrier phases from the L1 carrier only are obtained because the C/A-code is not modulated on L2. 2. Most instruments have a minimum of four independent receiver channels and some of the more recent designs have L2 channels. 3. To store the time-tagged code range and carrier phase in laptop computers and magnetic tapes early, later in memory chips. 5. Now measure the phases of the L2 carrier by the use of some codeless technique. The drawback is that the signal-to-noise ratio (SNR) is considerably lower than the C/A-code measurements on Ll. 6. Normally, the L2 phase is used in combination with the Ll measurement to reduce the ionospheric effect on the signal and, thus, provide a more accurate vector determination (especially for long lines). This type of receiver can be used for all types of precise surveys including static, kinematic, and pseudokinematic methods. 3. P-code Receivers 1. This type of receiver uses the P-code and is able to lock on to the Ll and L2 carrier. 2. In the absence of A-S , the observables are derived by first correlating the signals with a replica of the P-code. After removing the P-code from the received satellite signal, phase measurements can be performed. 3. One of the first receivers was the P-code TI-4100 completed in 1984 and tested by the FGCS. 4. This receiver was developed more from a military perspective than a civilian one. 5. Manufacturers of civilian receivers were able to justify P- code work around 1989-1990. 7 6. With A-S activated, in the emitted signal the P-code is replaced by the unknown Y-code. Thus, traditional P-code correlation technique can no longer be applied. 7. This type of receiver can operate in codeless mode, providing carrier phase data and code pseudoranges for the L2 frequency without knowledge of the Y-code. 8. The L2 tracking is accomplished using four techniques: 1) Signal squaring 2) Cross correlation 3) Code correlation followed by squaring 4) Z-tracking technique Two main advantages of the P-code receiver: – To measure long (100 km) lines with a few centimeters. P-code instruments can measure moderate length lines (20 km). – Centimeter level with as little as some minutes of data collection based on a linear combination of the measured phases of Ll and L2. 4. Y-code Receivers 1. This type of receiver provides access to the P-code with A-S invoked. 2. The code ranges and phases can be derived from L1 and L2 signals by the P-code correlation technique. 3. The access to the P-code is achieved by installing Auxiliary Output Chips (AOC) in each receiver channel. These chips allow the decryption of the y- code into the P-code and also correct the degradation imposed by SA. 4. Only users authorized by the DoD have access to the AOC. Summary What have we learnt? Which parts are important? Assignment 2 1. Detailed description to basic GPS positioning principle. 2. What are the SA and A-S technology? 3. Describe the GPS satellite constellation. 4. Describe the GPS control segments. 5. List the classification of GPS receivers, and briefly describe the 4 types of GPS receivers (characteristics and drawback)