Presentation on theme: "Hakan Velioğlu 27.03.2012 Remote Sensing and Satellite Comminication Instructor: Dr. Sedef Kent."— Presentation transcript:
Hakan Velioğlu Remote Sensing and Satellite Comminication Instructor: Dr. Sedef Kent
CONTENTS: Short History Transit Satellite System Kosmos(Cosmos), Parus, Tsikada Global Positioning System (GPS) GLONASS Satellite Navigation Techniques Doppler Effect Based Navigation System Trilateration Based Navigation System GPS (NAVSTAR) Space Segment Control Segment User Segment GPS Signal Structure GPS Pseudorange Measurements From PRN code From Carrier Phase measurement GPS Position Calculation GPS Positioning Services GPS Positioning Point Positioning Relative Positioning (Differential Positioning -DGPS) GPS Error Sources Applications of Navigation References
Short History Definition of Navigation: "the guidance of ships or airplanes from place to place" (http://www.audioenglish.net/dictionary/navigation.htm) What we actually need from Navigation is to measure our points/objects POSITION and SPEED. Early navigation for sailing and air travel -- LORAN (Long Range Navigation). Military needs.
Short History Transit Satellite System USA Navigation System The first navigation satellite system in the world Transit I launched at 13 April 1960 (~3 years after the first artificial satellite Sputnik). Transit is operational for military at 1964 for civilian 1967 Transit had used 6 satellites (3 active and 3 spare) at LEO orbit of altitude ~1000km. Transit System navigation working principle is Doppler effect. Transit System propogates signals in two frequencies (150Mhz, 400Mhz) Last Transit satellite launched 1988 Transit System gives positions for two dimension (longtitude, latitude) not height. Low altitude = Small coverage & Short service time
Short History Kosmos(Cosmos), Parus, Tsikada Soviet Union (Russian) First Kosmos navigation satellite is launched at 1967 Similar principles and frequency used with Transit system. Operational until 1978 Kosmos system is superceded by Parus and Tsikada system. Parus is for military 6 orbital plane, 30 ◦ longitude intervals. 98 satellites (last one at 21 July 2009) were launched. Tsikada is for civilian use 4 orbital planes, 45 ◦ longitude intervals. 20 satellites (last one at 21 January 1995) were launched.
Short History Global Positioning System (GPS) USA Navigation System GPS formally known as NAVSTAR (Navigation Satellite Timing and Ranging) First effort began in late 1972 First GPS satellite launched in 1978 System is fully functional on 17 July 1995 with 24 operational satellites. Most common system for navigation. Uses trilateration techniques. There are 32 operational satellites in constellation We will dig into GPS in later sections.
Short History GLONASS Soviet Union (Russian) Firs satellite launched on 12 October 1982 MEO circular orbit of altitude ~19100km Uses trilateration techniques System fully operational on 23 September Due to short life time of satellites GLONASS had 10 satellites in August Now there are 24 satellites constellation in operation.
Satellite Navigation Techniques Doppler Effect Based Navigation System Trilateration Based Navigation System
Satellite Navigation Techniques Doppler Effect Based Navigation System Christian Doppler found that if reciver and transmitter is moving accourding to each other then, the transmitted signal frequency is changed according to this movement (1842). It can be observed from a moving car. While a car is getting closer or away, it's sound changes.
Satellite Navigation Techniques Doppler Effect Based Navigation System
Satellite Navigation Techniques Doppler Effect Based Navigation System Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 511
Satellite Navigation Techniques Doppler Effect Based Navigation System Transit and Kosmos navigation satellites used this system. Satellites transmit signal in two frequencies (150Mhz, 400Mhz) Signal contains the satellite path and timing information Receiver calculates doppler effects and satellite path to find its position 1 satellite is enough to calculate position Accuracy is 500m for single frequency, 25m for dual frequency Position can be calculated in two dimension (latitude, longitude)
Satellite Navigation Techniques Trilateration Based Navigation System GPS and GLONASS use this system. Glileo will use this system. Trilateration system has better accuracy and wide coverage Receiver calculates its position by measuring its distence to three or four satellites.
Satellite Navigation Techniques Trilateration Based Navigation System
Satellite Navigation Techniques Trilateration Based Navigation System
GPS (NAVSTAR) GPS comprises of three segments. 1.Space Segment Satellites 2.Control Segment Control Stations Monitor Stations Antenna Stations 3.User Segment Military Users Civil Users
GPS (NAVSTAR) Space Segment 24 ? satellites in operation and 32 satellites in orbit. Medium Earth Orbit (MEO), altitude of ~20200km 6 orbital planes Satellites inclined at 55◦ to the equator Orbital period 11 hours 58 minutes 5 hours view in horizon All satellites have a Rubidium atomic clock (accuracy of 1 second in years, 3ns in a second) Satellites use solar energy and rechargable battery for power Small rocket boosters for station keeping. Satellites also have nuclear blast detectors Satellites transmit signals in two Microwave bands L1 ( Mhz) and L2 ( Mhz)
GPS (NAVSTAR) Space Segment
GPS (NAVSTAR) Control Segment The purpose of control segmen is to track all satllites position and health and manage/control them. Ground Antenna Stations: Send commands to satellites using S band. Receive temeletry data Monitor Stations: Track satellite positions (they have very accurate GPS receiver and Cesium oscilators) Sent track data to Master Control Station Master Control Station Get data from Monitor Stations Calculate this data in every 15 minutes. Upload path correction data to satellites with Ground Antenna Stations ones or twice in a day.
GPS (NAVSTAR) Control Segment
GPS (NAVSTAR) Control Segment Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 519
GPS (NAVSTAR) User Segment User segment includes all receiver devices. A receiver must do three main task 1.Get the radio signal correctly One or more antenna Filter Amplify Remove the carrier signal (down converter) 2.Process the signal (DSP). Calculate the Pseudorange code (PRN) Correlate signal with appropriate satellite 3.Position computation Compute position (according to WGS84), velocity, time, etc... Display this data (on a screen, on a map)
GPS (NAVSTAR) Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 519
GPS Signal Structure GPS signal contains 3 types of information 1.Pseudorandom code (PRN): ID of satellites that transmits signal PRN Code is two type: C/A Code Civilian Access Code 1023 bits with bitrate of 1.023Mbps Code repeats itself in every milisecond Carried with L1 signal ( Mhz) P Code Military use and Encrypted 2.35 x 10^14 bits with bitrate of Mbps Code repeat itself in every 266 days. 38 codes each of them is 7 days long. Carried with L1 and L2 ( Mhz) signal Could be further encrypted to Y code 2.Ephemeris data: Information about health of the satellite and current date and time 3.Almanac data: Information of each satellites position during the day.
GPS Signal Structure Almanac and Ephemeris data is named as Navigation data Navigation data is transmitted at a bitrate of 50Kbps All information transfer is made with BPSK with 180 degree phase shifting for 1 and 0 All GPS satellites uses same frequency bands GPS use CDMA ( Code Division Multiple Access) technique to discriminate each satellites information from PRN code.
GPS Signal Structure Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 523
GPS Pseudorange Measurements From PRN code Receiver gets satellite signal which has PRN code in it. At the same time receiver crates same PRN code While receiver gets satellite signal, it contains a delay This delay is calculated by compairing receivers code with detected signals code. This delay is multiplied with the speed of electromagnetic wave to found the distance to the satellite. PROBLEM: Receiver clocsks are not as accurate as satellites clock. This problem can be solved by getting signal from at least 4 satellite.
GPS Pseudorange Measurements From PRN code Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 524
GPS Pseudorange Measurements From PRN code Accuracy of C/A code: C/A code is 1.023Mbps. Duraiton of 1 bit 1/(1.023 * 10^6) ~= 10^-6 sec. Distance = Velocity * Time Inaccuracy of C/A code 3*(10^8) * 10^-6 = 300m Accuracy of P code: C/A code is 10.23Mbps. Duraiton of 1 bit 1/(10.23 * 10^6) ~= 10^-7 sec. Inaccuracy of C/A code 3*(10^8) * 10^-7 = 30m
GPS Pseudorange Measurements From Carrier Phase measurement Range is the sum of carrier phase and some fraction. This sum is multiplied by wavelength 19cm (L1) or 24cm (L2) As the vawelengths are in cm, so inaccruacy is as short as milimeter. However, calculation of this technique requires two receivers and complex signal processing. It is used in Diferential GPS.
GPS Position Calculation (x1 − Ux) 2 + (y1 − Uy) 2 + (z1 − Uz) 2 = (PR1 ± EC) 2 (x2 − Ux) 2 + (y2 − Uy) 2 + (z2 − Uz) 2 = (PR2 ± EC) 2 (x3 − Ux) 2 + (y3 − Uy) 2 + (z3 − Uz) 2 = (PR3 ± EC) 2 (x4 − Ux) 2 + (y4 − Uy) 2 + (z4 − Uz) 2 = (PR4 ± EC) 2 xn, yn, zn =x, y and z coordinates of the nth satellite Ux,Uy,Uz =x, y and z coordinates of the user receiver PRn = pseudorange of the user receiver from the nth satellite EC = error correction
GPS Positioning Services Standart Positioning System (SPS) Available to all GPS receivers worldwide Calculations made with C/A code in L1 frequency ( Mhz) Horizontal accuracy m Vertical accuracy 140m Timing accuracy < 340ns. Selective Availability (befora 1 May 2000) Precise Positioning System(PPS) Availably to only authorized users only Calculations made with P code in L1 and L2 frequencies Horizontal accuracy 16m Vertical accuracy 23m
GPS Positioning Point Positioning Positioning with one GPS receiver Receiver calculates its position from three or four satellites by using PRN codes. Low accuracy
GPS Positioning Relative Positioning (Differential Positioning -DGPS) This is used for high accuracy applications Two GPS receivers used whileall of them sees same satellites Measurement is done with PRN code or from phase measurement Main principle is to calculate GPS error from a base locaiton and propogate this correction data to other receivers
GPS Error Sources Signal Propogation Error Delays in signal caused by the medium (ionosphere, troposphere) Change in signal propogation speed Lower the frequency greater the delay, so L2 is much more affected than L1 This error can be eliminated by using DGPS or calculating P code from two frequencies. Multipath Reflection This error occurs when same signal comed from different paths. This multipath occurs when GPS signal is reflceted from objects like tall buildings or large rock surfaces. This error can be eliminated by using ring antenna or selection of good position for surveying. Clock Errors Receiver clocks are not accurate as satellite clocks. This error can be reduced by calculation from different satellite.
GPS Error Sources Ephemeris Errors Inaccurcies in the satellites reported popsition This error is about 2 to 5 m. Can be eliminated with DGPS Number of visible satellites More visible satellites means uch more accurate positioning Buildings, mountains, rocks may affect GPS signal. GPS signal can’t propogate from objects like water, buildings Satellite Distribution Geometry If satellites are distributed to wide angels than positioning is good If satellites are grouoped to one direction than positioning is not good GPS Signal Interference GPs signal power is weak and for this reason it can be interfere from near frequencies
GPS Error Source Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal, Page 532
Applications of Navigation Military Applications Navigation for soldiers Navigation for military vehicles Navigation for missiles Civilian Applicaitons Vehicle tracking and navigation Rescue operations Mapping and construcitons Geodesic sciences Precise timing