7) Global Positioning System

7) Global Positioning System

1. Satellite based navigation system

   • Developed for the use of the US Department of Defense
   • Navigation and positioning tool

2. Satellite positioning

   • Point positioning
     • Position of a point wrt a well defined coordinate system
   • Relative positioning
     • Position of two points relative to another

3. Basic concept

   • Rangers are measured simultaneously from at least 3 satellites to a single ground station
     • Satellite positions are known
       • Through a 3D trilateration the position of the ground station can be determined
         • Rangers are deduced from the time taken for a code to reach the ground station

4. Limitations on accuracy achievable

   • Multipath
     • Major error of GPS
   • Unmodeled biases in satellite orbits

5. Carrier phase measurements

   • Pseudo-range measurements are not sufficient
   • Carrier signal has higher frequency (wavelength of 19m)
     • Used for GPS surveying

6. Practical aspects

   • Outcome
     • Vector consisting of cartesian coordinate differences in WGS84 system (h)
       • Can be converted to differences of lattitude, longitude and height
   • Heights used in SA are orthometric heights (H)
     • Geoidal Height (N)
       • N=h-H
       • N ranges from -100m to 100m
         • Within SA N ranges from 10m to 36m