7.1.5 Introduction

PROPAGATION OF THE GPS SURVEY

In general, a GPS survey involves the use of a small number of receivers (two or more) to coordinate a large number of stations. The survey must therefore proceed in stages. The minimum GPS phase data reduction is therefore for a single baseline. However the data collected in a single session by all receivers has special characteristics. A GPS session solution can be obtained from a combination of separate baseline solutions, or from one simultaneous session solution. A GPS network solution can be obtained from combining individual session solutions, or in one simultaneous network (multi-session) solution.

GPS processing procedures may therefore be partitioned according to:

• Primary adjustment of the (raw) phase data collected during a session by the deployed GPS receivers. The software generally has the following features:
  
  • Provided by the receiver manufacturer, in the case of commercial software.
  • Output is the coordinates, and associated VCV matrix, of the stations processed together.
  • The stations may be processed as a single baseline or, more rarely, in the multi-baseline mode.
  • The coordinate results are expressed in a global GPS satellite datum system.
    
    
• Secondary adjustment that accepts the output of the primary GPS adjustment as input (that is, as "observations"). Such software has the following features:
  
  • Not usually developed by the same institution as responsible for the primary GPS software.
  • Can modify and manipulate separate baseline or network solutions in order to combine together a minimally constrained adjustment from separate primary adjustments (from baselines to multi-baseline, from session to multi-session).
  • Can constrain the network adjustment to fit surrounding geodetic control.
  • Accounts for the datum relationships between global and local geodetic datums.
  • Provides the flexibility to constrain and scale individual GPS adjustments.
  • The results can only be as good as the input primary adjustment data!

The outcome of a GPS adjustment is usually a minimally constrained network solution in which the coordinates of only one station (the "datum" station) have been held fixed.

In subsequent sections the baseline solution will be discussed further. The discussion of session solutions in section 9.2.1 will highlight the essential differences between single-baseline and multi-baseline session solutions. Finally in section 9.3.1, the manner in which a total network solution is obtained from separate session solutions is described.

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© Chris Rizos, SNAP-UNSW, 1999