Global Positioning system GPS - Dr. S. Balamurugan

Learning is a lifelong process en Sri Raaja Raajan College of Engineering and Technology, Karaikudi
26 de Feb de 2018

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Global Positioning system GPS - Dr. S. Balamurugan

  1. Dr.S.Balakumaran, Geology
  2.  The Global Positioning System (GPS) is a satellite based radio navigation system provided by the United States Department of Defence. It gives unequalled accuracy and flexibility in positioning for navigation,surveying and GIS data collection.  GPS is the shortened form of NAVSTAR GPS.This is an acronym for Navigation system with Time And Rangining Global Positioning System.
  3.  DOD sponsored project puts satellites into orbit  First Sat launched in 1978  24 Sats by mid 1990s  28 Currently in orbit, with more coming  A fundamental change in how positioning is done  What GPS has changed?
  4. What is GPS ? A very precise positioning system • Developed and maintained by the US Department of Defense (DOD) • Satellite Based * 24 satellites * 20,200 km high orbit
  5. The Global Positioning System (GPS) was designed for military applications. Its primary purpose was to allow soldiers to keep track of their position and to assist in guiding weapons to their targets. The satellites were built by Rockwell International and were launched by the U.S. Air Force. The entire system is funded by the U.S. government and controlled by the U.S. Department of Defense. The total cost for implementing the system was over $12 billion. A GPS satellite. The GPS constellation of satellites consists of at least 24 satellites – 21 primary satellites and 3 orbiting spares. They orbit the earth at an altitude of20200 KM (10,900 miles) at a speed of 1.9 miles per second between 60°N and 60°S latitude. Each satellite weighs 1900 lbs and is 17 feet (5.81 meters) wide with solar panels extended. The satellites orbit the earth twice a day. This guarantees that signals from six of the satellites can be received from any point on earth at almost any time.
  6. • The following are the conditions to fulfill the effective operation of GPS. • It should be suitable for all classes of platforms- aircrafts,ships, land vehicles, space satellite and missles. • It should be able to handle a wide variety of dynamics. • It should provide the user with real time positioning, velocity, time determination to an appropriate accuracy. • The positioning results should be available on a single geodetic datum. • Highest accuracy should be restricted to a certain class of users. • It should be resistant to jamming (blocking of radio signal) intentional and unintentional. • It should have redundancy provision to ensure the survivability of the system. • The system should be a passive positioning system ( one way ranging system) that does not require transmission of signal from the user to satellite. • It should be able to provide the service to an unlimited numbers of users. • It should be low cost and low power consumption.
  7.  The above conditions are fulfilled in the design of GPS with the salient features.  A one way ranging system in which the satellites transmit signals but unaware of who is using the signal ( no receiving function) . As a result, the user can not be detected by the enemy ( military context), or the user is not charged for using the system.) Civilian context.  GPS signals are not affected by cloud or rain.  Use of latest atomic clocks for time accuracy and microwave transmission technology including spread spectrum techniques.  A multiple satellite system which ensures visibility of sufficient nos. of satellites anywhere on the world at any time ( four satellites).
  8. • System overview • GPS configuration comprises of three distinct segements. • The space segments-comprising satellites orbiting the earth. • The Control segments-consisting of control stations positioned at various locations to control the satellites. • The user segments- Anybody who receives and uses the GPS signal comes under the segment. • The space segment- consists of a constellation of GPS satellites and the signals broadcast by them. Which allows users to determine position, velocity and time. • Basic functions of satellites-1)To receive and store data transmitted by the control stations. 2) to maintain accurate time by means of several on-board atomic clocks. 3) to transmit information and radio signals to users on two L bands frequencies. L1-1575.42 MHz and L2- 1227.60 MHz.Anoher additional frequency,L5-1176.45 MHz will be use in future. • 4) to provide stable platform and orbit for the L band transmitters. • 24 satellites well placed approximately at an altitude of 20,200 KM orbiting the earth every 12 hours. • The satellites travel at a speed of 11,500 Km per hour which allows them to circle the earth once every 12 hour.
  9.  Satellites are powered by solar energy which lasts for 10years. If solar energy fails due to eclipses, they have back-up batteries on board to keep them running.  Small rocket boosters to keep them orbiting in the corret path.  Fist GPS satellite was launched in 1978- a full constellation 24 satellites was achieved in 1994.  Six orbital planes with inclination of 55 degree with respect to the equator  4 satellites in each orbit.  Space segment is so designed that there will be minimum of four satellite visible above 15 degree cut off angle at any point on the earth surface.  4 satellites are the minimum that must be available for most applications.  Each satellite has several very accurate atomic clocks on board.The clocks operate at a fundamental frequency of 10.23 MHz.This is used to generate the signals that are broadcasted from the satellites.
  10.  Control segment  Also referred as ground segment –one master control station,six monitor stations and three uploading stations. The main functions of this segment are  Estimate the on board clock status and define the corresponding parameters to be broadcasted with refer. To constellation master time.  Define the orbit of each satellite in order to predict the ephemeris (precise orbital information) along with the almanac ( coarse orbital information)  Determine the attitude (orientation) and location of the satellites in order to determine the parameters to be sent to the satellites for correcting their orbits and  Uploading the derived clock correction parameters, ephemeris,almanac and orbit correcion commands to the satellites.
  11.  User segment-consists of GPS receiver- composed of an antenna(internal or external) tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock(crystal oscillator) and display system.  Receiver clock is not as precise as the satellite clock.  Receivers are classified by its no.of channels-this signifies signals from how many satellites it can process simultaneously.(4 channels to 48 channels)  Receiver can relay the position data to a personal computer or devices.It can inerface with other devices using methods serial connection ,USB or Bluetooth  It is operated thro many terminals-boaters,ships,pilots, military, land vehicles ect.
  12.  The major tasks of a receiver are  Select the satellites in view  Acquire the corresponding signals and evaluate their health.  Carry out the propagation time measurement  Calculate the location of the terminal and estimate the error.  Calculate the speed of the terminal and  Provide accurate time.
  13. Characteristics of GPS • Free • Precise • Reliable • Anytime & anywhere • All weather • Unlimited user capacity Accurate(precise) Almost!
  14. Segments of GPS 1. Space Segment A constellation of 24 satellites 2. Monitor Station A network of earth-based facilities 3. Users & Equipment Source:Trimble
  15. Segments of GPS Source:Trimble 1. Space Segment A constellation of 24 satellites
  16. GPS Monitoring Station
  17. How GPS Works ……… Uses measurements from 4+ satellites Distance = travel time x speed of light Source:Trimble
  18. The GPS receiver and satellite generate the same pseudo-random code at exactly the same time. When the code arrives from the satellite, the time difference is compared to the same code generated by the receiver. This difference is multiplied by the speed of light (186,000 miles per second) to determine the distance to the satellite.
  19. Determining GPS Position • Suppose the distance from Satellite A to our position is 11,000 miles • At this point we could be located anywhere on the specified sphere Satellite A + • Next, let us take another measurement from a second satellite, Satellite BSatellite B + • Now our position is narrowed down to the intersection of theses two sphere
  20. Satellite C + Determining GPS Position Satellite A Satellite B + + • Taking another measurement from a 3rd satellite narrows our position down even further, to the two points • So by ranging from 3 satellites we can narrow our position to just two points in space • These points are located where the 3rd sphere cuts through the the intersection of first two spheres
  21. Satellite C + How do we decide which one is our true location? Satellite A Satellite B + + • We could make a 4th measurement from another satellite to determine the true point • However, GPS receivers use a 4th satellite to precisely locate our position • We can eliminate one of the two points that gives a ridiculous answer • The ridiculous point may be too far from the earth OR
  22. Determining GPS Position
  23. How accurate is GPS? Depends on some variables • Design of receiver • Relative positions of satellites, technically known as PDOP (Position dilution of precision) • Postprocessing • Time spent on measurement
  24.  D=RxT  Rate is Speed of light  Time is the key! Technology made it possible  One you have distance, its “easy”
  25. Methods of data collection Three methods of positioning • Autonomous 10-20 meters • Differential 2-5 meters • Phase Differential centimeter 2-5m10-20 m cm
  26. PDOP Multipath SNR Sources of error Source:Trimble
  27. Multipath • When GPS signals arrive at the receiver having traveled different paths
  28. What is a PDOP? • Position Dilution of Precision Good PDOP Poor PDOP
  29. SNR (signal-to-noise ratio) • SNR determines the signal strength relative to noise • GPS position is degraded if the SNR of one or more satellites in the constellation falls below certain range Signal Strength Indicators
  30.  GPS has numerous advantages over traditional surveying methods.  1) Inter visibility between points is not required.  2) can be used at any time, day, or night and in all weather conditions.  3) produces result with very high geodetic accurcy.  4) more work can be accomplished in less time with less man power.  5) limited calculation and tabulation works required.  6) large area can be surveyed in short time.  7) network independent site selection hence sites can be placed where needed.  8) economic advantage arise from greater efficiency and speed of survey.  9) three dimensional coordinates are obtained.
  31.  GPS positioning mode  1) Absolute positioning mode-coordinates are in relation to a well defined global reference system.  2) Differential or relative positioning – coordinates are in relation to some other fixed point.This is refered to as baseline determinaion.  3) Static positioning-coordinates of stationary points is either in absolute or relative mode.This is surveying mode of position.
  32.  4) Kinematic posiioning- co-ordinates of moving points is either in absolute or relative mode. This is generally navigation mode based on pseudo range observation.  Absolute positioning – use of single receiver at one station location to collect data from min of 4 satellites.  Not sufficient accuracy for precise surveying. Used for commercial and some military purposes- standard positioning service user can obtain 25 m accuracy- Precision positioning service user with P- code can obtain 10-12 m accuracy.  It can be divided in to two categories-Absolute positioning using carrier phase and Absolute positioning using C/A code. In carrier phase the reciver is tracking both C/A code and carrier phase- tracking real time pseduo-range values with position accuracy of 3 meter.In Psedo-range , C/A code data alone used to calculate approxi. Range.
  33.  Differential Positioning( carrier phase tracking)-carrier phase signals are tracked in order to obtain more accurate range resolution due to shorter wavelength of L1 and L2 frequencies-  Ability of receiver to get 2 mm accuracy. Hence, primary applications in engineering, topographic an geodetic surveying.  There are several techniques using DGP  1) Static surveying  2) Rapid static surveying  3) Stop- and Go Kinematic surveying  4) Real time Kinematic surveying  5) Real time DGPS surveying.  Some important points for a GPS survey solution  The fundamental unit of GPS solution is a three dimentional baseline vector joining the antennas of two GPS receivers that are tracking same satellites simultaneously- GPS software in the
  34. Differential GPS To obtain more accurate measurements than is possible from a single GPS unit, a GPS receiver broadcasts the signal it receives from a known position. The GPS unit in the field simultaneously receives data from the GPS satellites and the other GPS receiver on the ground through a radio signal. The GPS error from the known position is compared to that of the GPS receiver in the unknown location.
  35.  To carry out the solution task  One en d of baseline is fixed with known coorinates and coordinates of other stations are determined relative to it.  All results are obtained in WGS reference system  Length of observation time , No of satellites tracked by receiver,signal delay correction factors applied,software used for processing the data.  Static surveying-mostly used technique for control and geodetic surveying-long observation time-1-2 hours to resolve ambiguities bewteen satellite and receiver.  Relative static positioning involves several GPS receivers collect ing data simultaneously from at least 4 stallites for 30 min to 2 stationary carrier phase mode-  Requirements for static mode- 1) More than one receiver 2)chance
  36.  Of getting direct signal from four or more staellites  3) unobstructed sky above the stn  4) enough power supplyto complete the observation  5) enough memory space to store all data for post processing.
  37. One particular negative characteristics of this technique is the requirement that phase lock must be maintained by the roving receiver as it moves from site to site. This requires special hardware mounts on vehicles if the survey is carried out over a large area. An additional requirement is that the stationary reference receiver must continue to track all the satellites being tracked by the roving receiver. The accuracy attainable is about the same as for the "rapid static" technique. As with the "reoccupation" technique, the receiver must have the ability to handle data files from several different sites. The software then has to sort out the recorded data for the different sites, and to differentiate the "kinematic" or "go" data (not of interest) from the "static" or "stop" data (of interest). It can be implemented in real-time if a communications link is provided to transmit the "carrier-range" data from the reference receiver to the roving receiver(s).  
  38. GPS Receivers
  39. GeoXT Versus Garmin Mostly used for GIS data collection Mostly used for recreational purposes
  40. • Wide Area Augmentation System. • It provides FREE GPS differential correction data for visible satellites • Developed & operated by the FAA (Federal Aviation Administration) for flight navigation but it’s available free to GPS users • WAAS-enabled receivers can provide sub-meter level accuracy anywhere in most locations of the US and southern Canada.
  41. • Differential corrections are computed from ground stations and then uploaded to geostationary satellites for broadcasting • WAAS-enabled GPS receiver automatically uses such correction data to enhance the positional accuracy
  42. Using Garmin12 Quit ENTER Goto PAGE MARK Scroll keypad Cycle through pages Save a waypoint On/Off switch
  43. Common use of GPS A. GIS data collection & mapping B. Navigation C. Recreation
  44. GPS for Navigation
  45. GPS in recreation
  46. GPS in Farmland
  47. Future GPS A. GPS in USA B. GLONASS program from Russia C. GALILEO from European countries
  48. GPS in recreation
  49.  :  Advantages  Higher accuracy than pseudo-range solutions  Appropriate for many survey applications  High productivity  Similar procedures to modern terrestrial surveying  Disadvantages  Special hardware and software  Susceptible to orbit, atmospheric multi-path disturbances  Higher capital costs  Ambiguity-fixed or continuous lock required  Two negative characteristics of these modern GPS techniques are:  They are susceptible to multipath disturbance to an axtent (affecting the receiver signals during both the kinematic and static stages of the tracking) than the conventional static technique. Multipath during the ambiguity resolution period is especially dangerous, as wrong ambiguities may result.  The results from short observation sessions are more sensitive to bad satellite geometry (large GDOP) than the conventional static techniq

Notas del editor

  1. At the cost of 15 billion dollar of your tax money
  2. Speed of the light: 186,000 mile/sec 300,000 km/sec 3x10^8 meter/sec
  3. SNR is also known as signal strength
  4. When SA was on, the position could be anywhere on 1st, 2nd, Alpine, or Spruce street. With SA removed, the position is located with in the road width of +/- 15 feet. (road = 35’, the circle is of 20’ radius). Before the location was within 100 meters, that is the point is about a football field away. Now it’s within the 10 meters. Removal of SA made GPS more useful and handy for our day to day use in cell phone, car, watch etc.
  5. Farm equipment capable of producing yield map during harvesting shows how crop yield varies across the field. Allowing farmers to plan fertilizer application for future crops.