Sect_4.3.3

4.3.3 GPS Surveying Software:

C0MMENTS ON PRE-PROCESSING SOFTWARE

The pre-processing component of the software package generally:

(1) Downloads the recorded data from internal memory or removable memory cards, to a computer.

(2) Prepares the files containing the raw observations and Navigation Message data. If a Data Management System is integrated within the software, the appropriate file names, directories and "projects" are assigned and catalogued for later use. In addition, the station record file is interrogated and an appropriate entry made in a log file. This file contains such items as user-entered station name, receiver serial number, antenna height, etc.

(3) Reformats the data files if necessary. For example, if the data is to be archived, or transmitted to a processing centre where different phase reduction software is to be used, a "standard" (receiver-independent) format may be used. The RINEX (Receiver INdependent EXchange) format is the preferred format at present (GURTNER, 1994), see section 7.3.2.

(4) Computes preliminary station positions using pseudo-range data. As a by-product, the receiver clock offsets from GPS time can be calculated.

(5) Repairs cycle slips in the phase data (section 7.3.5). This is often undertaken after a triple-difference phase solution (see below).

The outcome of the pre-processing step is a set of "clean" data files, and ancillary information, to support subsequent phase data processing. The following additional comments need to be made:

The pre-processing software is invariably written by the GPS instrument manufacturers, and is specific to the operation of their receivers. Some pre-processing modules (particularly the ones that carry out operations (3), (4) and (5) above) are an integral part of third-party GPS software as well.
Step (4) may not be necessary if the code-correlating GPS receiver automatically resets its own clock to GPS Time during data tracking (so-called clock "steering" -- section 6.3.8).
Cycle slip repair is generally a time consuming and laborious task if carried out manually. This is no longer necessary for standard short baseline GPS surveying, even in the "kinematic" mode. These days "standard" pre-processing for cycle slip detection and repair is carried out automatically (receivers may correct them in the field as data is being recorded). Only in the case of precise, scientific, long baseline applications may some level of manual screening of the data still be required.
Some pre-processing software creates differenced data files (for example, between-receiver differences, or double-differences), perhaps requiring the analyst to make a decision on the processing strategy at the pre-processing step.
Some pre-processing steps (data downloading, reformatting, receiver point positioning) can occur on a single site basis, but other steps require that the data from all receivers that were operated simultaneously be first brought together.
The RINEX format has received strong endorsement as the "official" GPS data format. Many GPS receivers output data either directly in this format, or via a utility program to translate the (downloaded instrument-specific) internal format into the RINEX data format (GURTNER, 1994).
There is a trend to include pre-processing software within the receiver itself, and hence making it largely "transparent" to the user. This is particularly true for real-time GPS surveying!

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