Chapter2- GNSS发展历史及基本原理

2012/9/21 1 卫 星 导 航 Satellite Navigation 秦 红 磊 北京航空航天大学-电子信息工程学院-无线电导航教研室 办公室：新主楼F座317房间 Email : qhlmmm@sina.com 卫星导航发展历史1 卫星导航系统结构2 卫星导航定位基本原理3 卫星导航定位误差来源4 第二章：卫星导航基本原理 1957年10月， 前苏联 发 射了第一颗人造卫星。从那 时开始，利用卫星进行导航 和定位的研究引起了各国军 事部门的高度重视。 Sputnik Two scientists of The Johns Hopkins University Applied Physics Laborato ry (APL) - - George Weiffenbach and William Guier -- were able to determine Sputnik's orbit by analyzing the Doppler shift of its radio signals during a single pass. Frank McClure, the chairman of APL's Research Center, went a step further by suggesting that if the satellite’s position were known and predictable, the Doppler shift could be used to locate a receiver on Earth; in other words, one could navigate by satellite. Transit is the first operational satellite-based navigation system. Although Transit was originally intended to support the U.S. Navy’s submarine fleet, the technologies developed for it proved useful to the Global Positioning System (GPS). The first Transit satellite is launched in 1959, finished in 1964. The Transit constellation consisted of two types of spacecraft designated as Oscar (pictured left) and Nova (right). The other name is dopple navigation satellite system, and it is also named NNSS(Navy Navigation Satellite System) Satellite number (卫星数) : 6 Orbit number (轨道数) : 6 Eccentricity (偏心率) : 0(circularity) Orbital inclination (轨道倾角) : 90° Satellite period (卫星周期) : 107min Satellite height (卫星高度) : 1075km Orbits separated by (轨道分离度) : 30° The tone broadcast by Transit at 400MHz by polar orbiting satellites. Actually, two frequencies were transmitted to correct for ionospheric group delay. Transit has four tracking stations, one calculating center, one control center, two ground antenna, one astronomical observatory (navy). 2012/9/21 2 t1 t2 t3 t4 P（X，Y，Z） 基本原理 hyperboloid- hyperbola-point 双曲面 - 双曲线 – 点 • 通过多普勒频移计算出卫星 移动的径向距离. • 接收机每次接收一颗卫星的 15-18分钟的信号. Transit系统的缺点 • Positioning time was too long for one time （每次的定位时间较长）: because the point of intersection was from the same satellite, the two epoch time of satellite couldn’t too short. Otherwise the distance of the two epoch time’s satellite position was too near, the precise of user’s positioning was low. At one positioning process, it was need the observation angle is near 90º, and the time was about 15~18min. Transit was not very useful for high dynamic applications like airplane and missile. • The time was too long for satellite presence（每次卫星出现的时间较长）: the receiver of user couldn’t identified the different satellite signals because the sending signal of the satellite wasn’t adopted TDMA，FDMA and CDMA technique. It was not permitted two satellites appear at same time, so the satellites number couldn’t more than six. The user saw one satellite about 1.5hr at middle latitude region. • The precise was not very high （定位精度较低） because of three reasons: (1) The precise of satellite orbit was low （卫星轨道精度低）; (2) The satellite’s signal frequency was low, it was easy effected by ionosphere （受电离层影响严重）; (3) The satellite clock frequency was not very stable （时钟频率的稳定度低）. Timation satellite system Timation, a Navy satellite system, is developed at the Naval Research Lab (NRL) for advancing the development of high-stability clocks, time- transfer capability. Timation’s work on space- qualified time standards provided an important foundation for GPS. The first Timation satellite is launched in May 1967. Timation project used 12 to 18 satellites to composing the globe positioning system net. The satellite height was 10,000km, orbit were circle, period were 8 hr. Timation was a two dimensional system and could not satisfied high dynamic condition. The Aerospace Corporation launches a study on using a space system as the basis for a navigation system for vehicles moving rapidly in three dimensions. This led directly to establish 621B. In1963, the Air Force begins its support of the Aerospace study, designating it’s system 621B. By 1972, the program has already demonstrated operation of a new type of satellite-ranging signal based on Pseudo-Random Noise (PRN) 621B Air Force project 621B can used at high dynamic condition. 621B system had 3 to 4 constellations, one constellation consisted of 4 to 5 satellites, one satellite at center was geosynchronous satellite, others used oblique orbit which period is 24hr. The disadvantages of 621B was the coverage of polar regions and the station setting. NAVSEG In 1968, DoD establishes a tri-service steering committee called NAVSEG (Navigation Satellite Executive Committee) to coordinate the efforts of the various satellite navigation groups. NAVSEG contracted a number of studies to fine-tune the basic satellite navigation concept. What NAVSEG had to determine? • How many satellites?-多少颗卫星 • What altitude?-卫星轨道高度 • What would be the signal codes?-采用什么样的信号扩频码 • What would be the modulation techniques – 采用什么调制技术 • Cost – 建设经费 . 2012/9/21 3 NAVSEG manages concept debates between the various satellite navigation groups. The Navy APL supported an expanded Transit while the Navy NRL pushed for an expanded Timation and the Air Force pushed for an expanded synchronous constellation―System 621B‖. In 1973, the TIMATION program was merged with the Air Force's 621B program, with the Air Force being named as the Executive Service, to form the NAVSTAR GPS program. NAVSEG NTS-navigation Technology Satellite • 1973 - GPS Joint Program Office established • 1978 - First Global Positioning System satellite launch • 1993 – GPS Standand Positioning Service（SPS）available • 1994 - FAA approves GPS for use in National Airspace System • 1998 - Two new GPS civil signals(L2 and L5) announced • 2000 - Congress funds GPS Modernization in DoD budget • 2000 - Selective Availability set to zero • 2007 - Selective Availability eliminated in GPS III • 2010 – President Obama announces National Space Policy including GPS Summary of GPS Key Events GPS系统建设的重要事件 卫星导航发展历史1 卫星导航系统结构2 卫星导航定位基本原理3 卫星导航定位误差来源4 第二章：卫星导航基本原理 (1) Control Segment – 控制段 (2)Space Segment - 空间段 (3)User Segment – 用户段 Kwajalein Atoll US Space Command Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground AntennaMaster Control Station Monitoring Station 注 入 站主 控 站 监 测 站 2012/9/21 4 SCC:System Control Center TT&C:Telemetry, Tracking and Control ULS:Uplink station MS:Monitoring station CC:Central clock SLR:Laser tracking station The master control station is located on the Schriever （ 施 里 弗 ） Air Force Base (formerly Falcon AFB), about 20 km south of Colorado Springs. The master control station get the data from the monitor stations are processed 24h a day in real time. As results, information about orbits and clocks of the satellites are obtained. Doing this, possible malfunctions can quickly be detected. Master Control Station of GPS - GPS 主 控 站 Additionally, from the raw data new ephemeris data are calculated. Once to twice a day, theses data and other commands are sent back to the satellites via the transmitting antennae on Ascension Islands, Diego Garcia or Kwajalein by means of a S-band signal (S-band: 2000 - 4000 MHz). GPS主控站的轨道确定策略 The ―master control station‖ (Schriever AFB) and four additional monitoring stations (on Hawaii-夏威夷, Ascension Islands-阿森纳群岛, Diego Garcia-迪戈加 西亚环礁 and Kawajalein -夸贾林环礁 ) were set up for monitoring the satellites. Falcon AFB Hawaii Ascension Island Diego Garcia Kwajalein Monitor stations – 监 测 站 The passive monitor stations are GNSS receivers which track all satellites in their range and collect data of the satellite signals. The raw data are then sent to the master control station where the data are processed. Monitor station on Hawaii Monitor stations – 监 测 站 Basic Functions of Monitor Stations 监 测 站 的 基 本 功 能 • These stations are the eyes and ears of GPS, monitoring satellites as they pass overhead by measuring distances to them every 1.5 seconds. • This data is then smoothed using ionospheric and meteorological information and sent to Master Control Station at Colorado Springs. • The ionospheric and meteorological data is needed to get more accurate delay measurements, which in turn improve location estimation. 2012/9/21 5 Monitor stations+ six NGA (国家地理协会) station GPS Ground Antenna – 注入站 Master control station estimates parameters describing satellites' orbit and clock performance. This information is then returned to transportable ground antenna stations which transmit the information to satellites Antenna Specifications Reflector diameter: Nominal 7 meters Feed type: 5-horn Prime Focus Transmit power: 2.1 Kilowatts Uplink frequency: 1783.7 Mhz Downlink frequency: 2227.5 Mhz GPS Constellation • 24 satellite vehicles • Six orbital planes – Inclined 55ºwith respect to equator – Orbits separated by 60º • 20,200 km elevation above Earth • Orbital period of 11 hr 58 min • Five to eight satellites visible from any point on Earth GPS Locations in Sept,2009 Ground-Track (sub satellite path) of the Satellite GPS BeiDou(Compass) Constellation The nominal constellation of BeiDou Navigation Satellite (regional) System is composed of fourteen satellites, including: • Five Geostationary Earth Orbit (GEO) satellites – 5个GEO • Four Medium Earth Orbit (MEO) satellites – 4个MEO • Five Inclined Geosynchronous Satellite Orbit (IGSO) satellites – 5个IGSO. 2012/9/21 6 BLOCK IIR-M Rocket (2). GPS Functions and structure • Three or four atomic clocks • Three nickel-cadmium batteries • Two solar panels – Battery charging – Power generation • S band antenna—satellite control • 12 element L band antenna—user communication GPS 卫星 The space segment consists of at least 24 satellites. The first of the satellites was brought to its orbit as early as 1978. During the years the satellites became more and more sophisticated and meanwhile seven different types of these satellites exist (Block I, Block II, Block IIA, , Block IIR, Block IIR-M, Block IIF and Block III). GNSS 接收机 GNSS receivers are highly useful tools. They can provide information on your current location,velocity,directions to your destination, and feedback on your progress and performance to your destination. In different fields, the different performance receiver are needed, like anti- jamming receiver, high dynamic receiver,high sensitivity. GNSS接收机的基本结构 2012/9/21 7 卫星导航发展历史1 卫星导航系统结构2 卫星导航定位基本原理3 卫星导航定位误差来源4 第二章：卫星导航基本原理 • Satellite ranging – Satellite locations – Satellite to user distance – Need four satellites to determine position • Distance measurement – Radio signal traveling at speed of light – Measure time from satellite to user Your location is: 37o 23.323’ N 122o 02.162’ W • Easily blocked and reflected by solid surfaces and water bodies • Can penetrate standard atmosphere, water vapor, clouds for travel over long distances • Primary purpose is to measure signal transit time 2012/9/21 8 The important of time – 时间的重要性 • Each GNSS satellite is equipped with two atomic clock at least – accurate to 3 nanoseconds (0.000000003 s) – +/- 1 second every 360,000 years • Ground control stations correct satellites time • Satellites correct GNSS receivers time • A discrepancy between satellite and receiver of 1/100 sec could result in an error of 1860 miles! • From the base frequency of the atomic clocks (10.23 MHz) all other frequencies that are required for the satellite are derived. T2 T1 • Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. • To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time. • By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us. • Multiply that travel time by the speed of light and you've got distance. Composition of the GPS signals • Carrier • L1 Band: 1575.42 MHz • L2 Band: 1227.60 MHz • Pseudorandom code • C/A Code: 1.023MHz, 1023bit • P Code: 10.023MHz, 266.4days • Navigation data • 50Hz,1500bit/sub-frame • information： ephemeris, clock correction coefficients system health status ionospheric model 2012/9/21 9 Receiver PRN Satellite PRN t First Method: Using Pseudo Random Noise(PRN) Code Second Method: Using Carrier Phase 卫星导航发展历史1 卫星导航系统结构2 卫星导航定位基本原理3 卫星导航定位误差来源4 第二章：卫星导航基本原理 2012/9/21 10 Receiver Errors are Cumulative! User error >100m System and other flaws = < 9 meters Stationary Clock Motive Clock Fast Slow 2012/9/21 11 High Gravity Planet Slow Low Gravity Space Fast ③ Ionospheric Delay Correction ④ Tropospheric Delay Correction Ionospheric Delay Correction Global occurrence characteristics of scintillation equator Varying effects of scintillation on GNSS 2012/9/21 12 Tropospheric Delay Correction Signal BlockedSignal Blocked Signal Obstruction (Buildings, Mountains, Foliage, etc ) Selective Availability (S/A) – 选择性可用 • The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users. • S/A was designed to prevent America’s enemies from using GPS against US. • In May 2000 the DoD reduced S/A to zero meters error. • S/A could be reactivated at any time by the DoD GPS Satellite Geometry  Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.  Dilution of Precision (DOP：精度因子) reflects each satellite’s position relative to the other satellites being accessed by a receiver.  There are five distinct kinds of DOP.  Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position.  It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.  Some GPS receivers allow DOP to be manipulated by the user. 2012/9/21 13 Good DOP Low error Poor DOP High error 2drms=2×GDOP×UERE 2012/9/21 14 Differential GPS (差分GPS) Differential GPS (DGPS) • Use two receivers – Base station with known location and elevation – Mobile receiver • Velocity of signal from satellite to receiver is dependent on atmospheric conditions • Known error at base station can be applied to measurements by mobile receiver • Works best for measurements within 300 miles of base station – Same atmospheric conditions over both receivers Local Area Augmentation System- LAAS 2012/9/21 15 National Differential Global Positioning System - NDGPS