GPS concept

• GPS Space Segment


  The GPS satellites
• GPS satellite constellation


  • 24 core satellites
  • Orbit the earth twice a day at 10,900 NM (20,200 km)
  • Arranged in 6 orbital planes with 4 satellites in each

  
• The GPS satellite orbits are elliptical and cross the equator at a 55° angle, so it is not possible to see a GPS satellite directly overhead north of 55° N or south of 55° S latitude

  
• Space Vehicle Number (SVN)


  Indicates where in the launch sequence this satellite was

  
• Pseudo Random Noise number (PRN)


  The number that identifies an SVN to the GPS receiver
• PRN codes 1-37 have been reserved for GPS use
• Additional satellites


  • In the past few years there have been spare satellites operating in the spare orbital slots resulting in 26 to 31/32 since 2007 operational satellites in orbit
  • The latest of GPS III satellites are planned with one in full operational as of 2019
• Satellite Service Outages


  Scheduled satellite maintenance and orbital manoeuvres or component failures may cause satellites to be out of service
• Temporary periods of satellite unavailability are generally not of significant consequence, since the GPS constellation is arranged so that there are usually between seven and nine satellites visible at any time, ensuring a reasonable level of redundancy

  
• GPS Frequencies and Signals


  Each GPS satellite transmits unique ranging code signals on two frequencies: 1575.42 MHz (L1) and 1227.60 MHz (L2)

  
• Coarse Acquisition (C/A) code


  Transmitted on L1 and can be received by any type of GPS receiver

  
• Precision (P-code)


  Transmitted on L1 and L2, encrypted and available only to users with appropriate decryption equipment provided by the USA Department of Defence
• L1C, L2C and L5 are signals that will be transmitting from the GPS III satellites, which are additional civilian signals that are resilient to signal jamming
• A GPS Navigation Data Message is combined with each ranging code and transmitted on both L1 and L2 frequencies
• Power Levels


  The strength of the transmitted GPS signals is very low and cannot be easily viewed on a spectrum analyzer, making them susceptible to both intentional and unintentional interference
• The minimum received power levels at the surface of the earth are: L1 C/A code -160 dBW or -130 dBmW, L1 P code -163 dBW or -133 dBmW, L2 P code -166 dBW or -136 dBmW
• The received signals are at least 16 dB below the noise level of the receiver and require code matching (correlation) technique to recover the PRN code
• GPS Navigation Data Message

  Consists of 25 frames, each 1500 bits long, transmitted at a 50 Hz rate, and contains the transmitting satellite's predicted path (Ephemeris), and information on all satellites in the constellation (Almanac)
• Ephemeris Data


  Contains precise orbital parameters that permit the receiver to predict the exact position of the satellite at any time

  
• Almanac Data


  Contains an indicator of the health of all the satellites, satellite clock corrections and coarse orbital data, atmospheric delay parameters, and the current GPS time and offset from UTC time

  
• Each satellite transmits almanac data for the entire constellation, and the entire almanac is broadcast over a period of 12.5 minutes
• GPS Control Segment


  Consists of: One Master Control Station (MCS), Five Monitor Stations (MS) + 10 National Geographic Monitoring Stations, and Three Ground Antenna (GA)
• Master Control Station (MCS)


  Located at Shriever AFB, Colorado and operated by the United States Air Force, manages and controls the GPS constellation
• Monitor Stations


  Positioned around the world, passively track and monitor GPS satellites as they come into view, and transmit the received data to the MCS for processing

  
• Ground Antennas


  Collocated with three of the Monitoring Stations, provide the links used by the MCS to transmit messages up to the satellites

  
• If a problem is detected with a satellite by the monitoring station(s), the satellite is commanded by MCS to send an "unhealthy" status indication, causing receivers to exclude it from the position solution
• Due to monitoring stations visibility restrictions, and delays at the MCS it can take upwards of 45 minutes before the satellite is declared unhealthy, which is unacceptable for aviation services, and so Satellite Based Augmentation System (SBAS) or Ground Based Augmentation System (SBAS) provide much more timely integrity information to onward GPS tracking systems for support on signal and positioning precision

  
• GPS Receiver Operation


  GPS provides a standard positioning service (SPS) and a precise positioning service (PPS)

  
• Range Determination


  The receiver uses code matching techniques to determine the time it took the signal to travel from the satellite to the receiver, and computes the distance from the receiver to the satellite (pseudorange) by multiplying the signal travel time and the average speed of the signal

  
• When computing position, the receiver also requires the position of the tracked satellite which is provided in the Ephemeris data
• Computation of Position


  A receiver needs four pseudoranges to calculate a three-dimensional position and to resolve the time difference (errors) between receiver and satellite clocks, and uses triangulation methods to determine the best estimate of position

  
• Factors Affecting GPS Position Accuracy

  Position accuracy is a function of satellite geometry and range measurement errors, including satellite clock errors, satellite orbit errors, atmospheric delays, multipath errors and receiver noise

  
• Satellite Geometry


  Measured by a term known as DOP (Dilution of Precision), where a DOP of less than 4 is considered acceptable for most position solutions
• Selective Availability


  A feature that was incorporated into the GPS design to degrade the accuracy of the pseudorange measurements for civilian users, but was discontinued on May 1, 2000
• Multipath Errors


  Caused by GPS signals reflected from objects surrounding the GPS receiver antenna, combining with the direct signal and causing errors in range measurements
• RF Interference


  Can result in downgraded operation, hazardously misleading information or complete loss of receiver tracking, from both unintentional and intentional sources
• Above errors, except for RF interference, are independent and can be root-sum-squared, and today's receivers employ sophisticated algorithms resulting in better position accuracies, with GPS horizontal and vertical positions currently accurate to 6 m and 8 m, respectively, 95% of the time
• Wide Area Augmentation System (WAAS)

  Designed to provide GPS augmentation throughout North America, by broadcasting integrity data on all GPS satellites, differential corrections and Ionospheric data to improve position solution accuracy, and residual errors which permit computation of WAAS position error bounds

  
• WAAS Initial Operating Capability (IOC) was commissioned for aviation IFR use in July 2003
• WAAS Accuracy


  The basic GPS service is inadequate to support certain aviation operations that require a greater level of accuracy, integrity, and availability, which WAAS is designed to provide