Week 02. Assist. Prof. Dr. Himmet KARAMAN

Transcription

1 Week 02 Assist. Prof. Dr. Himmet KARAMAN

2 Contents Satellite Orbits Ephemerides GPS Review Accuracy & Usage Limitation Reference Systems GPS Services GPS Segments Satellite Positioning 2

3 Satellite Orbits The application of operational satellite methods depend substantially on knowing satellite orbits. For single receiver positioning, an orbital error is highly correlated with the position error. In case of baselines, relative orbital errors are approximately equal to relative baseline errors. Orbital information is either transmitted by the satellite as the part of the broadcast message or can be obtained in the form of precise ephemerides from several sources. 3

4 Keplerian Motion Orbits can be described by the 3 laws of Kepler; The orbit of every planet is an ellipse with the sun at a focus. A line joining a planet and the sun sweeps out equal areas during equal intervals of time. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. Illustration of Kepler's three laws with two planetary orbits. (1) The orbits are ellipses, with focal points ƒ 1 and ƒ 2 for the first planetƒ 1 and ƒ 3 for the second planet. The sun is placed in focal point ƒ 1. (2) The two shaded sectors A 1 and A 2 have the same surface area and the time for planet 1 to cover segment A 1 is equal to the time to cover segment A 2. (3) The total orbit times for planet 1 and planet 2 have a ratio a 1 3/2 : a 2 3/2. 4

5 Perturbed Motion Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body. An orbital perturbation is when a force or impulse which is much smaller than the overall force or average impulse of the main gravitating body and which is external to the two orbiting bodies causes an acceleration, which changes the parameters of the orbit over time. 5

6 Orbit Dissemination Orbit determination for global positioning satellites is based on observations at monitor stations of the respective control segment. Global networks lead to higher accuracy and reliability of the orbits compared to those determined from regional networks. The distribution of the sites is essential to achieve the highest accuracy. 6

7 IGS Tracking Network 7

8 Ephemerides (astronomical calendar) An ephemeris (plural: ephemerides) (from the Greek word ephemeros = daily) is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Three sets of data are available to determine position and velocity vectors of the satellites in a terrestrial reference frame at any instant: Almanac data Broadcast ephemerides Precise ephemerides The data differ in accuracy and available either in real time or with some delay. Ephemerides Uncertainty Remark Almanac Some kilometers Depending on the age of data Broadcast Ephem. ~ 1 m Or even better 8 Precise Ephem m Depending on the latency

9 Almanac Data Provide the user with adequate data to facilitate receiver satellite acquisition and for planning tasks such as the computation of visibility charts. Regularly updated and broadcasted as a part of the satellite message. Contains parameters for the orbit and satellite clock correction terms for all satellites. 9

10 Sample Almanac Data Parameter ID WEEK t a a e M 0 ω δi Explanation Satellite Identification number Current GPS week Reference epoch in seconds within the current week Square root of semi-major axis Eccentricity Mean anomaly at reference epoch Argument of perigee Inclination offset from 0.3 semicircles (=54 ) l 0 Longitude of the node at weekly epoch t 0 Ώ a 0 a 1 Drift of node s right ascension per second Satellite clock offset in seconds Satellite clock drift coefficient 10

11 Broadcast Ephemerides Based on observations at the monitor stations of the respective control segment. The most recent of these data are used to compute a reference orbit for the satellites. Additional tracking data are entered into a Kalman filter and improved orbits are used for extrapolation. The master station of the control segment is responsible for the computation of the ephemerides and the subsequent upload to satellites. 11

12 Broadcast Ephemerides (2) Contains records with general information, orbital information, and information on the satellite clock. Orbital information is provided in the form of Keplerian parameters together with their temporal variations or position and velocity vectors at equidistant epochs. The information on the satellite clock is in most cases given in the form of coefficients to model the clock offset from system time by polynomials. 12

13 Sample Broadcast Ephemerides Data Parameter ID WEEK t e a e M 0 ω 0 i 0 Explanation Satellite Identification number Current GPS week Ephemerides reference epoch Square root of semi-major axis Eccentricity Mean anomaly at reference epoch Argument of perigee Inclination l 0 Longitude of the node at weekly epoch t 0 13 n i Ώ C uc, C us C rc, C rs C ic, C is t c a 0 a 1 a 2 Mean motion difference Rate of inclination angle Rate of node s right ascension Correction coefficient (argument of perigee) Correction coefficient (geocentric distance) Correction coefficient (inclination) Satellite clock reference epoch Satellite clock offset Satellite clock drift coefficient Satellite clock frequency drift coefficient

14 Precise Ephemerides Most accurate orbital information is provided by the IGS in the form of various data sets for precise ephemerides. Consist of satellite positions and velocities at equidistant epochs. SP1 and ECF1 data formats contains both position and velocity data SP2 and ECF2 data formats contain just position data. Typical spacing of the data is 15 mins. In 1989 satellite clock offset data were added. (SP3 and ECF3) 14

15 Precise Ephemerides Format GPS satellite precise ephemerides (GPS orbits and clock corrections) are computed from the data collected at the GPS reference stations as well as IGS stations around the world. Replacing GPS s broadcast ephemerides with precise ephemerides improves GPS positioning accuracy. Precise ephemerides are packaged as daily (0:00 to 23:45 GPS Time) Precise Orbit and Clock files are at 15-minute interval. Precise Orbit files are in the NGS-SP3 format) and contain X, Y, Z satellite positions. 15

16 16 ISTANBUL TECHNICAL UNIVERSITY - DEPARTMENT OF GEOMATICS ENGINEERING

17 Introduction to GPS 17

18 Historical Review of GPS Global Positioning System, as known GPS or NAVSTAR- GPS (NAVigation System with Timing And Ranging- Global Positioning System), is a radio navigation positioning system developed by the Department of Defence (DoD) to meet the military needs in In 1978, the launch of development satellites began. Civilians were allowed to use GPS in In 1989, first operational satellite was launched. Initial Operational Capability (IOC) was attained in Full Operational Capability (FOC) was achieved in 1994 but declared after

19 Historical Review of GPS (2) In 1996, first UN national policy on GPS was released. In 1998, second civilian signal was provided by GPS. In 1999, GPS modernization program was announced. Two new civil signals were also announced to be provided. In 2000, the degrading in GPS accuracy was stopped by the U.S. government. (SA: selective availability) 19

20 Accuracy & Usage Limitation SA: Selective Availability (removed on May 2, 2000) Error on satellite clocks Error on satellite coordinates AS: Anti Spoofing No availability of real P code 20

21 Reference Systems Coordinate System Time System 21

22 Coordinate System Referring to coordinates, the GPS terrestrial reference system is the World Geodetic System 1984 (WGS-84). This geocentric system was originally realized by the coordinates of about 1500 terrestrial sites which have been derived from Transit observations. The ellipsoid is defined by: Semi-major axis a Normalized second-degree zonal gravitational coefficient Ć 2,0 f =(a b)/a Truncated angular velocity of the earth ω e Earth s gravitational constant µ This frame has been used for GPS since

23 Coordinate System (2) WGS-84 frame has been refined in 1994 (WGS-84 (G730), 1996 (WGS-84 (G873), 2002 (WGS-84 (G1150) With respect to ITRF2005, current WGS-84 frame shows insignificant systematic differences in order of 1 cm. Hence, both frames are virtually identical. 23

24 Time System The system time of GPS is related to the atomic time system and referenced to coordinated universal time (UTC). Nominally the GPS time has a constant offset of 19 seconds with TAI (international atomic time). TAI = GPS time s GPS time is exactly 14 seconds ahead of UTC. Starting at the GPS standard epoch (0hr 6 January, 1980), the system time of GPS is counted in terms of GPS weeks and seconds within the current week. 24

25 Time System (2) For the calculation of the GPS week; WEEK = INT[JD )/7] JD (Julian Date): defines the number mean solar days (each of which is SI second in length) elapsed since the epoch 1.5 (midday) January, 4713 B.C (on the proleptic Julian calendar). The Modified Julian Date (MJD) is obtained by subtracting from the JD. The JD can be computed from the year number Y (a full four digit integer), integer month number M, integer day number D, and the real-valued time in hours H: JD = Int[ y ] + Int[ (m+1) ] + D + H / y = Y - 1 and m = M + 12 if M 2 y = Y and m = M if M > 2 25

26 GPS Services For positioning and timing, GPS provides two levels of services. Standard Positioning Service (SPS) access for civilian users. Precise Positioning Service (PPS) access for authorized users only. GPS Information services provide GPS status information, orbital and other data to civilian users. 26

27 Standard Positioning Service (SPS) Positioning and timing service. Uses the C/A-code. Provided on the L1 signal only. SPS performance refers to the signal in space (SIS). Contributions of ionosphere, troposphere, receiver, multipath, topography or interference are not included. 27

28 Precise Positioning Service Uses the P-code (Y-code respectively) on the L1 and L2 signal. Use of PPS is restricted to US armed forces, federal agencies, and some selected allied armed forces and governments. 28

29 GPS Segments Space Segment Control Segment User Segment 29

30 GPS Space Segment 24 satellites 6 orbiting planes 55 inclination km above Earth 12 hours of orbit 5 hours view in horizon GPS Satellite Types: Block I, Block II, Block IIA, Block IIR, Block IIR-M, Block IIF, Block III 30

31 GPS Control Segment Consist of; Master control station Monitor stations Ground antennas Tracking of satellites for the orbit and clock determination and prediction Time synchronization of the satellites Upload of navigation data message to the satellites Imposing SA on the broadcast signals 31

32 GPS Control Segment (2) Colorado Springs (Main control & monitoring) Hawaii (Monitoring) Ascension Island in South Atlantic Ocean (Monitoring and ground control station) Diego Garcia in Indian Ocean (Monitoring and ground control station) Kwajalein in North Pacific Ocean (Monitoring and ground control station) 32

33 GPS Control Segment (3) 33

34 GPS Control Segment (4) 34

35 User Segment Users; Civilians (universities, private and state sectors, etc.) Military Receivers; Trimble Ashtech Rogue Leica Javad etc... 35

36 Satellite Positioning 1 satellite 2 satellites 3 satellites Latitude Longitude Latitude Longitude Height 36

37 Satellite Positioning 4 satellites Latitude Longitude Height Time or X, Y, Z, t 37

38 Satellite Positioning 38

39 Most Important Features of GPS Cloudy Rainy Suny sea, land space Worldwide Day & night 24 hours 39

40 40 ISTANBUL TECHNICAL UNIVERSITY - DEPARTMENT OF GEOMATICS ENGINEERING