Data transfer

146 7. Surveying with GPS Table 7.5. Control abstract

Receiver Time (UT)

Session A B C Start Stop

Site Height Site Height Site Height

a HCA 1.234 HCB 1.574 HCC 1.342 01:00 02:00 b HCA 0.987 001 1.782 002 1.543 02:29 03:30 c 003 1.344 010 1.328 002 1.452 04:01 05:00 d 011 1.324 010 1.563 012 1.437 05:31 06:30 e 001 1.564 013 1.453 012 1.455 06:59 08:00

and rename files that have been misnamed.

Most of the batch file processing software automatically extracts the antenna height from the site file data stored in the receiver. After correcting the names of the various files, one should then check and correct all antenna heights. Again a form such as Table 7.5 will be helpful in keeping track of the various antenna heights. Also, a table listing the various occupations and antenna heights is useful for inclusion in the project report.

As soon as all files have been corrected, the observed data should be backed up on at least two sets of storage medium (Le., floppies). A good practice is to keep one set of original data in a different secure location.

Today, many organizations keep a complete set of storage medium in a vault.

7.4.2 Data processing

In the case of remote and extended GPS surveys, a quality control check of the data at least once per day should be made. This control may also include cycle slip detection and repair which, when not performed in the receiver during operation, is normally part of the preprocessing procedure, see Sect. 9.1. The quality control check could also include preliminary com-pu tations of baseline vectors in the field before leaving the survey area. The on-site vector processing on a daily basis helps that adequate measurements are being made.

Today, most of the routine processing is performed by batch file process-ing. All batch files are generated using the three or four digit site identifica-tion so that the first task in processing GPS data is to ensure that all sites are properly named. A good practice is to first number the control points and then consecutively assign numbers to adjacent points. This number-ing should have been accomplished durnumber-ing the plannnumber-ing and reconnaissance phase to facilitate record keeping.

7.4 In situ data processing 147 Processing of static surveys. Modern processing software uses batch pro-cessing to compute baseline vectors. Normally, the data for a given day are loaded into a subdirectory on a hard disk. Processing software is normally in another directory and the "path" of the computer has to be set to access the programs. Once the software has been initiated (generally by menu com-mands), the lines are computed in order, automatically. There are two types of processing software: (1) vector by vector, and (2) multipoint solutions.

The vector by vector or single baseline solution type is presently the most common, and in any case should be used prior to processing with the multipoint software. In some instances one of the points in an observation session will be corrupted and if all points are processed together, the errors from the bad point are distributed among the vectors and the error is masked.

The single vector software provides a better check on bad lines or points.

The bad point can be more easily isolated by noting that the statistics (Le., root mean square error, standard error) to lines leading to this one point are worse than the statistics for the other lines. Additionally, the vectors can be summed for the lines in the session, and if the sum around the perimeter is not a small value (e.g., 1 ppm), this indicates that one of the points in the session is bad.

The individual vector processing software performs the following steps:

1. Computes best fit value for point positions from code pseudoranges.

2. Creates undifferenced phase data from receiver carrier phase readings and satellite orbit data. Time tags may also be corrected.

3. Creates differenced phase data and computes their correlations, cf.

Sects. 8.2.1 and 8.2.2.

4. Computes estimate of vector using triple-difference processing. This method is insensitive to cycle slips but provides least accurate results.

5. Computes double-difference solution solving for vector and (real) val-ues of phase ambiguities.

6. Estimates integer values of phase ambiguities computed in step 5, and decides whether to continue with fixed ambiguities.

7. Computes fixed bias solution based upon best ambiguity estimates computed in step 6.

8. Computes several other fixed bias solutions using integer values differ-ing slightly (e.g., by 1) from selected values.

148 7. Surveying with GPS 9. Computes ratio of statistical fit between chosen fixed solution and the next best solution. This ratio should be at least two to three indicating that the chosen solution is at least two to three times better than next most likely solution.

Processing of kinematic surveys. The basic steps are similar for static and kinematic processing. The data files are downloaded from the receiver to the computer and the file names and antenna heights are checked. The ac-tual processing differs depending on the software used; however, much of the newer software is automated so that hands-on interaction is not needed.

The main check for kinematic vectors is to compute positions of the roving receiver and check that similar values are obtained on separate visits to the same point. Also, it is good survey practice to visit points whose coordinates are known during the survey as a further check of the method.

Differential code range processing. Processing code pseudorange data is ac-complished using software provided by manufacturers and third party ven-dors. Most programs are automated so that only the precise position of the fixed station is entered and the coordinates ofthe second point are computed.

The data seems to have a strong central tendency so that averaging several observations rapidly improves the determined position. Positions can be de-termined for either fixed or moving receivers. Most uses of this technique are radial surveys so that no check of the results is possible.