REOCCUPATION GPS SURVEYING TECHNIQUES
|Centimetre positioning accuracy with two occupations per site, each for a short static observation period (few minutes) ...|
Also known variously as pseudo-kinematic and pseudo-static. This technique exploits changes in satellite geometry across conventional observation sessions. Phase data from two short sessions of just a few minutes in length (perhaps up to 10 minutes), collected about one hour apart is sufficient to ensure a good quality ambiguity-free solution.
The field procedure is otherwise similar to the "rapid static" or conventional static techniques. One receiver is located at a known point (the "reference" receiver) while the second "roving" receiver moves from point to point. For example, the roving receiver stops at a site, where it is static for a short period, and then moves on to the next point. The roving receiver must revisit the same point one or more hours later (see Figure below). The second occupation is the same as the first: the receiver stops on the point, is static for 10 or so minutes, and then is off again. (To increase redundancy, the points may be revisited more than twice.) The receiver need not be tracking satellites between the sessions (it can in fact be switched off), however continuous tracking (no cycle slips) should be maintained during the on-site observation period. Furthermore, the satellite geometry should be favourable. Two separate sets of ambiguities must be estimated, one for the first session, the other for the second session. It is not necessary for the same constellation of satellites to be observed for both sessions.
Field procedure for the "reoccupation" surveying technique.
The following are some of the characteristics of the technique:
No special tracking hardware is required other than that used for conventional GPS surveys. However, the receiver must be able to record the data files in a manner that permits the processing software to sort out the pairs of observation sessions for a single site. Appropriate software is necessary for the subsequent data processing.
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© Chris Rizos, SNAP-UNSW, 1999