Module 2 : Global Positioning System
  Lecture 13 : GPS Measurements Techniques II
Carrier based positioning techniques
  • This method provides a more accurate range resolution due to the short wavelength (approximately 19 cm for L1 and 24 cm for L2) and the ability of a receiver to resolve the carrier phase down to about 2 mm.
  • Primary application in engineering, topographic, and geodetic surveying, and may be employed with either static or kinematic methods.
  • Several carrier-based methods are in use such as
    • static
    • rapid static
    • kinematic
    • stop and go kinematic
    • pseudo kinematic
    • real time kinematic (RTK) or on-the-fly (OTF) kinematic, etc.
 
Carrier phase based real-time DGPS
In carrier phase based real time DGPS, the salient features at reference station and rover station are as follows:
At reference station (on a known point)
  • Monitor receiver tracks all satellites in view
  • The monitor receiver transmits phase and code measurements to a roving receiver via a reliable data link along with the reference station coordinates.
At the rover station
  • The rover receives the GPS measurements and reference station coordinates via the communication link and evaluates the differential phase and code measurements in real time.
  • Rover carries out computations to resolve ambiguities and reliable results are available as soon as the phase-processing software can resolve ambiguities and the quality of coordinates.
  • Generally, positions in real-time are available for every data interval as long as the lock is maintained to a minimum of 4 satellites on L1 carrier.

    For RTK surveys, initialization (the process of ambiguity resolution, explained later in Lecture 14) could be static, moving or on a known point with 5 satellites or more on L1 and L2, with GDOP< 8
  • As soon as the initialization is successful real time surveying can begin.
  • The rover can move at almost any speed to get cm level accuracy for
    • determining trajectories
    • surveying detail point
    • both together (trajectories with detail points)
Advantages
  • Coordinates are available in real time in the field in global WGS-84 or local system
  • Quality control - one knows in the field that the ambiguities are resolved and that the results are correct
  • No post-processing is required
  • It is a truly one person system
  • Several rovers can use one reference station
  • The approach has given rise to many applications
 
Disadvantages
  • Needs a radio modem (data link)
  • Radio contact can be interrupted by obstructions (hills, valleys buildings, etc.)
 
 
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