GNSS Notes

Page Contents

GNSS Basic Principles

  • Satellites broadcast their exact ephemerides (data that represent the trajectory of an object over time) and time whilst orbiting about 20,000 Km above the earth's surface.

  • The time for a radio signal to propagate to a point is directly proportional to the distance.

  • Trilateration with multiple distances allows pinpointing the exact location in space.

    • Because the range estimation is based on blocks the satellites have extremely accurate clocks (they use atomic clocks) and the clocks between satellites are synchronised.
      • For example a rubidium clock is accurate to 5 parts per 1E11!
    • Receiver clock is usually much less accurate! To avoid errors introduced by clock bias the receiver must "lock onto" the satellites clock.
      • For example, a quartz crystal might only be accirate to 5 parts per million! If a receiver used its own clock it would get an accuracy works than 1.5 Km, which would be awful!
      • To trilaterate in 3D, 3 satellites are required. To correct the receiver's clock a fourth satellite is required at a minimum, but in practice the more the better.
    • The distribution of satellites in the sky is important - need to be "spread out". Clustered satellites lead to a dilution of precision (DOP).
  • GNSS architecture includes the space segment, control segment and user segment.

    • Space segment is the satellites,
    • Control segment is the base stations that update the satellites orbit parameters and clocks, when necessary. Data upload stations, master control stations and base stations.
    • User segment includes the GNSS receivers
  • There are the following GNSS constellations:

    • GPS - world wide coverage - USA - 24 satellites
    • GLONASS - world wide coverage - Russia - 24 satellites
    • Galileo - world wide coverage - EU - 24 satellites
    • BeiDou - world wide coverage - China - 28 satellites
    • IRNSS - mainly continent coverage - India - 8 satellites
    • QZSS - m=ainly continent coverage - Japan - 4 satellites
  • Satellite Signals

    • Each satellite has its own unique code (PRN -pseudo random noise numbers - which identify the ranging codes). Use in CDMA (code division multiplexing).
    • L1 GPS - Navigation Message
      • A navigation message is transmitted at 50 bps and contains
        1. GPS date and time.
        2. Satellite status and health.
        3. Satellite ephemeris data - allows receiver to calculate satellite position - valid for only 4 hours.
        4. Almanac - consists of information and status of all GPS satellites so that receiver knows which sats are available for tracking - can be valid for up to 2 weeks.
      • Coarse acquisition code (C/A) - freely available to general public.
        • 1023 bits long and repeated every miliisecond.