cET 318 1. Basic Knowledge of GPS In 1973, DOd organized a Joint P Office (JPO) located at the U.S. Air Force Systems Command's Space The s Second Lecture Division, Los Angeles Air Force Base(AFB)to what? to establish, develop, test, acquire, and deploy a spaceborne positioning system, called A GPS Overview and Ranging (NAVSTAR) (GPS)is the result of this in Purpose: Military Functions: All-weather, 24hrs :p.11-24 Principle: ranging system Other Application: Technical Problem: design, Potential Users Dr Guoqing Zhou Cost: 30 billions 1. GPs was conceived as a ranging system from 4. Goals: known positions of satellites in space to unknown I-weather, all-day, and space-based navigation system positions on land, sea, in air and space To satisfy the require To accurately dete heir position, velocity, and 2. The satellite signal continually ---be ed with time In a com ence system, anywhere and 3. The original objectives of GPs were the instantaneous determination of Position Three Segments 2. Basic Principle Control segment steering the whole system, ≡
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
s=vt(pseudorange) 3. Space Segment S: range v:light velocity seudorange receiver 3.1 Constellation t: time o GPS Nearly circular orbits s=t Ea(i)b(i)cos( o)(carrier phase) Altitude of about 20200 km s: range 24-hour worldwide coverage phase of electronic magnetic wave Time -21+3 satellites in six orbital planes (A to Et Carrie ase receiver Four satellites per plane. Furthermore, four active spare Frequencies: L1,L2 Coordinate This constellation provides global coverage with four to eight simultaneously observable satellites above 15 Single frequencies receiver System elevation at any time of day ency receive Receiver -I0, occasionally up to 10 satellites visible; 3.2 GPS Satellites -5. occasionally 12 satellites visible. 1. General Remarks he GPs satellites. essentially provide a platform for radio transceivers. atomic clocks. commuters, and various ancillary The auxiliary equipment of each satellite I Solar panels for power supply 2. A propulsion system for orbit adjustments and stability control The satellites have various systems of aunch ☆BcCK Assigned pseudorandom noise(PRN)code Orbital position number NASA catalogue number, and International designation. Satellite Categories GPS Satellites Block ll Cont. Block IIR, IIF Total: 8 GPS Satellites Block I Orbital 10Jun8912J89 1617Aug8 Sep 89 151921ot8914Nov89 26 Apr 0 16 May 80 10 Dec 90 Wheel 126.8 31Aug90 BBAD ep 84 3 Oct 84 18 Nov 95 Clock Oct90 20 act90 9 Oct 85 30 Oct 85 27 Fcb 94 Signal 99.9
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(ϕ) (carrier phase) s: range ϕ : 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. Satellite Categories Cont. – Block I – Block II, IIA – Block IIR, IIF GPS Satellites Block I Total: 10 11 03 9 Oct 85 30 Oct 85 27 Feb 94 Signal 99.9 10 12 8 Sep 84 3 Oct 84 18 Nov 95 Clock 133.5 9 13 13 Jun 84 19 Jul 84 25 Feb 94 Power 115.2 8 11 14 Jul 83 10 Aug 83 4 May 93 Power 116.8 7 --- 18 Dec 81 ---- Booster --- 6 09 26 Apr 80 16 May 80 10 Dec 90 Wheels 126.8 5 05 9 Feb 80 27 Feb 80 28 Nov 83 Wheels 45.0 4 08 11 Dec 78 8 Jan 79 27 Oct 86 Clock 93.6 3 06 6 Oct 78 9 Nov 78 19 Apr 92 Clock 161.3 2 07 13 May 78 14 Jul 78 30 Aug 80 Clock 25.5 Operation (Months) Reason of loss Loss of navigation Available since Launch date Flight PRN No. No. GPS Satellites Block II Cont. Total: 8 20 15 1 Oct 90 20 act 90 D2 19 21 2 Aug 90 31 Aug 90 E2 18 20 25 Mar 90 19 Apr 90 B2 17 18 24 Jan 90 14 Feb 90 F3 16 17 11 Dec 89 11 Jan 90 D3 15 19 21 Oct 89 14 Nov 89 A4 14 16 17 Aug 89 13 Sep 89 E3 13 02 10 Jun 89 12 Jul 89 B3 Orbital position Available since Launch date Flight PRN No. No
GPS Satellites: Block IIA Total: 16 Co GPS Satellites: Block IIF(p. 14) GPS Satellites: Block Ill(p. 14) 3.3 GPS Satellite Signal L=1575.42MHL2=1227.60MHz These dual frequencies are essential for eliminating the The key to the systems accuracy is the fact that all signal of error, i.e the ionospheric refraction. olled b The gPs signals less subject to intentional(unintentional) The block il. I Four on-board time standards 2. Two rubidium clocks GPS Clock Stability 3. Two cesium clocks, a stability of 10-14 to 10-l5 over Clock'TypeFrequency Stability/day Time of causing I being the Rubidium6834682613101230000ear tal L-band 9192631770 300000year frequency Hydrogen1420405751105300 1. C/A-code( Coarse/Acquisition-code )is Technology for Denial of Accuracy and Access Civilian use, denies full system accuracy to wo methods for denying civilian users full use of the system Designated as the Standard Positioning Service Selective Availability(SA Effective wavelength of approximately 300 m Anti-spoofing(A-S) Modulated upon LI only and is purposely omitted from L Selective Availability-SA Goal: to deny this navigation accuracy to potential adversaries IS Me Designated as the Precise Positioning Service(PPS) Effective wavelength of approximately 30 m by dithering the satellite clock and Modulated on both carriers ll and L2 by manipulating the ephemerides
3 GPS Satellites: Block IIA Total: 16 Cont. 36 03 28 Mar 96 09 Apr 96 C2 35 06 10 Mar 94 28 Mar 94 Cl 34 04 26 Oct 93 29 Nov 93 D4 33 05 30 Aug 93 28 Sep 93 B4 32 09 26 Jun 93 21 Ju1 93 Al 31 07 13 May 93 12 Jun 93 C4 30 31 30 Mar 93 13 Apr 93 C3 29 22 2 Feb 93 4 Apr 93 Bl 28 29 18 Dec 92 5 Jan 93 F4 27 01 22 Nov 92 11 Dec 92 Fl 26 27 9 Sep 92 30 Sep 92 A3 25 26 7 Jul 92 23 Jul 92 F2 24 28 9 Apr 92 25 Apr 92 C2 23 25 23 Feb 92 24 Mar 92 A2 22 24 3 Jul 91 30 Aug 91 Dl 21 23 26 Nov 90 10 Dec 90 E4 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 10 3000 000 year -15 Hydrogen 1420405751 10 300 000 year -13 Cesium 9192 631770 10 30 000 year -12 Rubidium 683468 2613 10 30 year -9 Quartz 5000000 Clock Type Frequency Stability/day Time of causing 1 s 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
Selective Availability(SA) satellites at various levels of accuracy denial. Anti-spoofing(A-s) Accuracy degraded 100 m for horizontal 1. GPS fundamental frequency: 8 Technology 0.3m/s for velocity Ons for time 3. P code encrypting Technology The predictable accuracy decreases to 300 m for horizontal position and to 500 m for height. How to Realize Anti-Spoofing? Technology for Denial of Accuracy and Access On March 29. 1996 residential Decision Directive(PDD)on vas released expressing the 2. The rationale for doing this intention to discontinue the use of GPS Selective nding out false signals with the gPs signature to create Availability(A)within a decade in a manner that confusion and caus s to misposition themselves. allows adequate time and resources for our military 3. A-s is accomplished by the modulo 2 sum of the p-code and forces to prepare fully for operations without SA Thus. when A-S is active. the P-code on the li 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. 4. Control Segment 4.1 Monitor Stations Main tasks Operational Control System(Ocs) Each station is equipped with a precise cesium time andard and receivers which continuously measure monitor stations - master control ound control Pseudoranges are measured every 1.5 seconds and Main tasks thed to produce 15-minute interval data which are Tracking of the satellites ansmitted to the master control station. Clock determination and Time synchronization of Upload of the data message to the satellites Colorado Springs Ascension Island in the South Atlantic Ocean The OCS is also responsible for imposing SA on the Diego garcia in the Indian Ocean Kwajalein in the North Pacific Ocean broadcast signals
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) monitor stations master control 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
4.2 Master Control station 4.3 Ground control stations 1. The satellite ephemerides and clock Tasks of csoc alculated at the master control and Collects the tracking data from the monitor stations Calculates the satellite orbit and clock parameters using a received via communication links. are each GPS satellite via S-band radio links. These results are then passed to one of the three ground 2. Formerly, uploading to each satellite was ontrol stations for eventual upload to the satellites performed every eight hours; then the rate has Be responsible for the master control station been reduced to once(or twice)per day Location rg AFB, Califomia for BLOCK I navigation satellite to The Consolidated Space Operations Center(CSOC)at support a prediction span so that the positioning Falcon AFB, Colorado Springs, Colorado for BLOCK II 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 Ephermrides Receiver H Modulation Cesium clock Master Station .Data processing tmospheric data Commander Monitoring station Ground Station 5. User Segment Civilian user 5.1 User Categories There are various other non-military uses trictly speaking, the term user Just one example Even during the early days of th A receiver can be connected to four antennas. When the antennas are placed in a fixed array(e.g, comers of a system. It was envisioned that every aircraft, ship, land vehicle, square), the attitude of the array can be determined in groups of infantry would receiver to coordinate their military activities to its position. For example, placing antennas on Gulf war GPs Receiver stern,and port and starboard points of a ship result in the determination of pitch, roll, yaw, and C/A Code Civil Small ReceiverMilitaryTotal 5008004001700
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. 5000 8000 4000 17,000 Military Total 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
5.2 Receiver Types By Positioning Style By Load Body Based on the type of observables(i.e, code pseudoranges or arrier phases) and on the availability of codes(ie, C/A-code, Static Receiver Potable receiver P-code, or Y-code), one can classify GPS receivers into. Kinematic Receiver Back Bag Receiver By Code Vehicle Load receiver (1)C/A-code Middle kinmatic 2)C/A-code Airplane Load Receiver (4)Y-code carrier phase receiv Missile load receiver By F Navigation Receiver tellite load receiver elver (2)Dual frequency receiver In general 2)Carrier phase receiver 1. C/A-code Pseudorange Receivers 2. C/A-Code Carrier Receivers Characteristics Characteristics: 1. Only code pseudoranges using the C/A-code are carrier p he LI e obtained because the C/A-code is ually a hand-held device powered by not modulated on L2 2. Most instruments have a minimum of 3. Typical from one to six independent receiver channels independent receiver channels and some more recent designs have L2 channels more channels are erred for 3. To store the time-tagged code range and carrier ranges phase in laptop computers and magnetic tap can be measured to produce more accurate early, later in memory chips 3. P-code receivers e the phases of the l2 that the L This signal-to-noise ratio(SNR)is cor to the ype of receiver uses the P-code and is able to lock on carrier he C/A-code measurements on L 2. In the absence of A-s, the observables are derived by first orrelating the signals with a replica of the p-code. After nt to red emoving the P-code from the received satellite signal, phase us, prov vector measurements can be performed. determination(especially for long lines) 3. One of the first the P-code TI-4100 completed This type of receiver can be used in 1984 and tested bi precise surveys incIuding static, koir match 4. This rece more from a military pseudokinematic methods perspective than a civilian or 5. Manufacturers of civilian receivers ple to justify P. code work around 1989-1990
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 Pcode work around 1989-1990
6. With A-s activated. in the emitted signal the P-code replaced by the unknown Y-code. Thus, traditional P-code Two main advantages of the p-code receiver: correlation technique can no longer be applied To measure long (100 km) lines with a fet 7. This type of receiver can operate in noderate length lines(20 km) providing carrier phase data and code pse sudoral nges for the L2 frequency without knowledge of the minutes of measured phases of Ll andL, ear tion of the 8. The L2 tracking is accomplished using four techniques 1)Signal squaring 3)Code correlation followed by squaring 4)Z-tracking technique 4. Y-code Receivers 1. This type of receiver provides access to the P-code Summary 2 code ranges and phases derived from LI What have we learnt? L2 signals by the de correlation Which parts are important? 3. The access to the posed by SA. 4. Only users authorized by the doD have access to 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 riefly describe the 4 types of GPs receivers (characteristics and drawback)
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 ycode 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)
