7.2.1 Basic Modelling Concepts

INTRODUCTION

There are two classes of parameters in GPS observation models: Parameters related to geometric range: where xi, yi, zi are the coordinates of satellite i at time T, and xj, yj, zj are the coordinates of site j in the same reference system.   THIS IS THE QUANTITY OF INTEREST!   All other terms are biases, which must be accounted for during observation modelling. The most common strategy is to difference the data, and construct an observable that is, to a large extent, free of biases. The additional considerations for eliminating biases are: How are differences constructed from one-way observations from R receivers to S satellites? What is the form of the resultant observation weight matrix? What form do the remaining biases (not eliminated by differencing) take?

This chapter is concerned with the following topics:

• The observation models for pseudo-range data and the various differenced phase observables used in GPS surveying, and in particular, the strategies for handling biases in commercial GPS surveying software packages.
  
• The datum definition issues for GPS processing.
  
• The mechanics of a GPS phase adjustment, and in particular the various data differencing options for double- and triple-differenced phase observables.
  
• The nature of correlations in the differenced data.
  
• The ambiguity parameter modelling options, and the associated datum defects.
  
• The Normal Equation structure for a GPS phase adjustment.

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© Chris Rizos, SNAP-UNSW, 1999