DATA PROCESSING SPECIFICATIONS |
Specifying the number of triangle and loop closures, the model for phase
data reduction, observation weights, error analysis tests, data editing
techniques, etc.
Specifications for processing data might include (as a function of accuracy desired):
As with GPS instrumentation, the only way of ensuring realistic results
with confidence is to use data reduction software which has been tested
on a three-dimensional calibration network, or with a standard test dataset.
A minimally constrained three-dimensional adjustment would reveal whether realistic observation weights were used and what the standard errors of the three-dimensional vector components are. A constrained adjustment using existing control coordinates accounting for scale, orientation, and geoid undulation relationships between the GPS satellite and the local geodetic control network datums would provided proper integration of the GPS relative positioning results (but obviously at a level corresponding to the existing control network accuracy).
The Australian S& S give no guidelines on office checking procedures (loop tests, etc.). Rather they give minimum data reduction procedures for all the common GPS surveying techniques (see Table below). (The meaning of terms such as "double- and triple-differencing", "ambiguity fixed and ambiguity float", etc., as they apply to GPS data processing were given in Chapter 6 and Chapter 7.)
Australian recommended GPS reduction procedures (ICSM, 1994).
| CLASS | 3A | 2A | A | B | C | D | E |
|---|---|---|---|---|---|---|---|
| c-values (one sigma) |
1 |
3 |
7.5 |
15 |
30 |
50 |
100 |
| < 8 km | D*, DD, FX | D*, DD, FX | S, DD, FX | S, DD, FX | S, DD, FX | S, DD, FT | S, DD, FT |
| 8-25 kms | D, DD, FX | D, DD, FX | D, DD, FX | D, DD, FX | S, DD, FX | S, DD, FT | S, DD, FT |
| 25-50 kms | D, DD, FX(25)-FT(50) | D, DD, FX(25)-FT(50) | D, DD, FX-FT | D, DD, FX-FT | D, DD, FX-FT | D, T | D, T |
| 50-90 kms | D, DD, FT | DD or T**, D, FT | DD or T**, D, FT | DD or T**, D, FT | DD or T**, D, FT | D, T, NCP | D, T, NCP |
| > 90 kms | D, T | D, T | D, T | D, T | D, T | D, T, NCP | D, T, NCP |
Notes:
S = single frequency D = dual frequency DD = double differences FX = ambiguity fixed solution FT = ambiguity float silution, with repaired cycle slips T = triple difference solution with sufficient observation length, allowing change of geometry. NCP = Narrow correlation, C/A code or pseudo-range methods, e.g. DGPS * L1 solution, from a dual frequency receiver, in order to enable ambiguity resolution by widelaning. ** Double difference preferred, triple difference solution increasingly acceptable the longer the distance, if the observation length allow sufficient geometry change.
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© Chris Rizos, SNAP-UNSW, 1999