Lect 3

• 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)

  
• C/A-code


  Also called the "Civilian Code"
• The L1 frequency carries C/A code, Navigation Message and P-Code
• The L2 frequency carries P-Code and Navigation message
• Ionospheric delay error


  Major error in GPS, can be corrected by measuring the difference in transmission delay between L1 and L2 frequencies
• 1. Code is much more accurate than C/A-Code for single GPS receiver navigation applications
• Basic Positioning Principles of GPS

  1. Trilateration from satellites

  2. Measure distance by measuring transmission time of radio signals

  3. Determine position of satellites with distance

  4. Receive data including PRN code, ephemeris and almanac

  5. Calculate distance to each satellite and triangulate location

  
• Trilateration


  • Measures distances only from the satellites to one single receiver on or above the surface of the earth
  • A measurement of high orbit and precision positioning
• Pseudo Random Noise (PRN) code

  Allocated for every single 32 satellite, used as ID code

  
• Ephemeris data


  Contain the status, current date and time of the satellite

  
• Almanac


  Transfers orbit data to inform the receiver the location of each satellite

  
• What the receiver does

  1. Saves almanac and ephemeris data as reference

  2. Calculates distance to each satellite

  3. Triangulates location using signals from 3+ satellites
• Satellite geometry


  • Influences the accuracy of the GPS receiver
  • Refers to the relative position of individual satellites from the receiver
  • The angle of signal receiving will influence the accuracy of positioning
• When the angle among satellites is small

  A bigger error will occur
• When the adjacent angle of 2 satellites is big


  The correction segment will be small, and the positioning will be more accurate
• When 4 satellites are located in different directions

  The positioning accuracy will be enhanced
• The Five logical steps (The Concept)

  1. Trilateration from satellites

  2. Measure distance using the difference of travel time of radio signals

  3. Accurate timing is needed

  4. Know the exact location of satellites in space

  5. Correct for atmospheric delays
• Trilateration


  A method of determining the relative positions of objects using the geometry of triangles
• Trilateration is only possible due to GPS coordinates at any point in time over the period of orbit that the coordinates are known with reference to the X, Y, Z the focal point or the Zero X, Zero Y, and Zero Z of the centre of the universe
• The USA department of defence transmit corrections including its locations with respect to the orbit with reference to the origin of the X, Y, Z coordinate system every minimum of 4 hours 24/7 days a week, non-stop for 366 days
• 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