Sect_7.1.4

7.1.4 Introduction

THE NATURE OF GPS SOLUTIONS

The most distinctive feature of GPS surveying is the determination of 3-D coordinates. The following additional remarks can be made:

The most convenient coordinate system is that provided by the rectangular Cartesian system. This is the system, in the first instance, in which the satellite coordinates are defined in the Navigation Message (Table in section 3.3.3). Furthermore, the coefficients of the design matrix (the partial derivatives of phase with respect to the site coordinates) are most easily computed in the Cartesian system (eqn (7.1-21)). If the results are required in any other coordinate reference system, the Cartesian components can be easily transformed at the result presentation stage (Chapter 11).
GPS phase processing result in relative 3-D coordinates. In the case of a two receiver scenario, it is the 3-D baseline components that are computed. In the case of more than two receivers (say R), the coordinates of the R-1 stations are determined relative to the datum station in the adjustment. (There is always a minimum of one station held fixed in a GPS adjustment, whether a hundred stations are involved, or just a single baseline.)
GPS pseudo-range solutions are single point, or absolute, solutions in which the 3-D coordinates are independently determined for each receiver. These coordinates are not as accurately determined as those from carrier phase (relative) solutions, but are often used to provide apriori values for the subsequent phase solutions.
All results relate to the GPS antenna phase centres and must be corrected to the groundmark by applying the antenna height and eccentric station offsets.
All results are, nominally, referred to the WGS84 reference system.

The phase data may be processed in the:

baseline mode, taking the data of any two receivers at a time, which is the minimum configuration possible for data reduction, or
session mode, where all data collected together during an observation session is processed together, or
campaign mode, in which all data, collected over several sessions, is processed simultaneously.

The mode of processing is generally a function of:

The software that is available -- most commercial software is only able to process a single baseline at a time.
The degree of rigor required for the phase data reduction, and hence the accuracy sought -- for example, scientific software can accommodate all the phase data collected during a campaign in a rigorous manner.

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