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.