components of GPS

• L1 Carrier


  1575.42 Megahertz (or 1.57542 Gigahertz, part of the so called "L-Band")
• L2 Carrier


  1227.60 Megahertz (or 1.2276 Gigahertz, also of the L-Band)
• Pseudo-Random Noise (PRN) codes


  Unique to each individual satellite, used to modulate the carrier signals
• Codes used in GPS

  • Coarse Acquisition Code (C/A-code)
  • Precise, or Protected Code (P-Code)
• The L1 frequency carries C/A code, Navigation Message and P-Code

  
• The L2 frequency carries P-Code and Navigation message

  
• Difference between C/A-code and P-Code

  1. Code is more accurate than C/A-Code because ionospheric corrections can be made by measuring the difference in transmission delay between the two frequencies (L1 & L2)
• Basic Positioning Principles of GPS

  1. Trilateration from satellites

  2. Receiver measures distance by measuring the transmission time of radio signals

  3. Receiver determines the position of satellites with distance

  4. Receiver receives data including PRN code, ephemeris and Almanac

  5. Receiver calculates distance to each satellite and triangulates location

  
• Satellite geometry


  • Influences the accuracy of the GPS receiver
  • Angle of signal receiving influences the accuracy of positioning

  
• If the angle among satellites is small, a bigger error will occur
• If 4 satellites are located in different directions, the positioning accuracy will be enhanced

  
• The Five logical steps (The Concept)

  1. Trilateration from satellites

  2. Receiver measures distance using the difference of travel time of radio signals

  3. GPS needs very accurate timing

  4. Receiver needs to know exactly where the satellites are in space

  5. Corrections for time delays of signal as it travel through the atmosphere

  
• Trilateration


  A method of determining the relative positions of objects using the geometry of triangles, unlike triangulation which uses angle measurements
• Trilateration on its own without coordinate origin is not valid when receiver are able to record coordinates, indicating that there is an original source of coordinates between the GPS receivers and the source (satellites)

  
• Other factors affecting positioning accuracy include the ionosphere and troposphere in the atmosphere, internal clock error and all other sources
• Carrier Signals


  
  
• High-frequency signals that carry information.
• Transmitter Signals