Sect_7.3.1

7.3.1 Getting Started: Pre- & Initial Processing Steps

INTRODUCTION

The general initial / pre-processing steps for a GPS phase adjustment are:

Transfer, compress and reformat the data (including the recorded Navigation Message) from the GPS receivers onto computer storage media. This is may be done after each observation session, for each receiver separately, using the manufacturer's software.
Prepare data, for example form a common ephemeris file (from the multiple copies of the recorded Navigation Messages), or obtain "precise ephemerides" from an external source. If processing is to be done using third-party software (as in the case of scientific GPS processing), data files may need to be reformatted.
Screen data: edit data according to quality flags, or if to satellites below elevation cutoff angle, unhealthy satellites, unequal observation sessions, etc. This step usually takes place when merging data from several receivers.
Obtain preliminary point position solution, usually through the processing of pseudo-range data. This has the advantage of, in addition, giving apriori station coordinates as well as observation time-tag corrections.
Obtain approximate baseline solution(s) using triple-differenced phase data (no ambiguities). Such a solution helps subsequent cycle slip detection and repair.
Detect and repair cycle slips by any of a number of methods.

The most important steps are time-tag synchronisation and cycle slip editing. The triple-difference solution, although usually carried out before the cycle slip screening step, will be discussed in section 8.1.3.

There are a number of comments that can be made in relation to data handling:

Data files should be downloaded, backed up on suitable media, and archived as soon as possible -- for example at the field office.
Data processing can commence on a session basis when data files from several receivers are brought together at the processing centre.
Different receivers output one or more files per observation session in a proprietary format.
Data from a particular brand of receiver is usually processed with that brand's software -- exceptions possible using RINEXed data files.

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