GPS receivers and signal corrections

GPS Receivers

  • portable (roving) receivers (handheld, cars, airplanes)
  • permanent “base stations” (surveyed location, tracks error in satellites/transmission)
  • temporary survey stations (days/weeks)

How does it work? Triangulation (from up to 12 satellite signals) based on time

  • uncorrected, single receiver GPS
    • 3 satellites make a positional triangulation based on distance between satellite and receiver
      time-delay = distance (approx. 0.05 seconds to receiver from a satellite directly overhead)
    • time is established by “psuedo-random” code transmitted from each satellite and compared to receiver-generated code
    • This is called “coarse acquisition” GPS. The C/A code repeats every 1023 bits
      . source
    • A 4th satellite is required to correct the non-atomic clock time at the receiver.
    • PDOP (Position Dilution of Precision) from
      • location of satellites (ephemeris error) and satellite clocks (SV is “space vehicle” I think)
      • obstruction of satellites, poor geometry (clustered, not spread out).
      • time errors due to
        • atmospheric changes (“billows” in the ionosphere—what’s the ionosphere? 50-200 km)
          status map for ionospheric delay over North America
        • multipath (bounce) signals
        • receiver/atmospheric noise

    • differential GPS

      image sources: and
      corrects by comparison to base station

      • satellite time and position uncertainty (drift in clocks, ephemeris errors, etc)
      • atmospheric/ionospheric conditions
      • real-time differential GPS (DGPS) using radio signals from Air Traffic Control system, coast guard, local base station, etc
    • Different kinds of DGPS
      • real-time vs post-processing
      • code vs carrier phase
      • satellite, radio, local, or RTK source for correction
    • real-time DGPS
    • post-processing DGPS
      • base stations—data is downloaded “reference station” and compared with rover positions
        • national base station system of public receivers
          (Trimble, Topcon  and other companies have private networks)
        • local temporary base stations— placed locally to establish “relative” precision”
          Dave and Jeff’s work in the Eastern Cordillera of Argentina

          place a receiver that doesn’t move all day (unless someone steals it!), it has to be higher than the rover, and see the same set of satellites. Result is <2-3 cm horizontal precision, day to day, but unknown accuracy.
      • For differential correction, the degree of precision in the correction depends on the distance from the base station (same ionospheric distortion and lower atmospheric noise)
    • Code phase vs Carrier phase corrections
      • code – phase
        • The C/A code repeats every 1023 bits (see above)
        • approx 1 MHz (one millisecond)
      • carrier – phase
        • need 15 to 30 minutes of continuous signal from satellites and receiver
        • 1575.42 MHz (1.5 GHz) or potentially 1000 time more precision defining the start of the “code”
        • how does “carrier” frequency transmit information?
          • changes in nature of signal make ones and zeros for pseudorandom code (or in fact, a tenor singing an “A”)
          • AM/FM/phase modulation

            modified from

        • “sub-meter” data: 1 m down to 1 cm !
      • real-time kinetic GPS (RTK)
        is carrier phase differential GPS in real time with radio links to base stations ( < cm precision as rover is moving)