Performance Evaluation Of Rtk Gps Without Sa Effect
3. Methodology of RTK GPS
The key of RTK GPS positioning is the ambiguity solution in the movement, when the dual frequency phase measurements of master and rover stations are put together through the radio communication. If the position of master station is known, then the position of rover station is solved by double differencing algorithm, so the merits of RTK GPS are rapid positioning and cm level of accuracy [Langley, 1998; Leick, 1996].
The phase ambiguity could be resolved in a short period and only few epoch of observations, which is contributed by the full constellation of GPS satellites, the complete full wavelength of dual frequency receiver, and the advanced decoding ability. Therefore, the ambiguity function method, the fast ambiguity resolution approach, the least squares ambiguity search technique, the fast ambiguity search filter, and the on-the-fly approach are commonly adopted to gain the integer ambiguity [Abidin, 1994].
The general procedures for estimating the ambiguity are listed as the followings:
-
initial location estimation;
- search space generation;
- phase ambiguity search;
- phase ambiguity verification.
Certainly, the effectiveness of phase ambiguity is related to the amount of satellite, the geometry of satellite, the location of master station, the length of baseline, the significance of multipath, the appearance of cycle slips, the carrier-to-noise ratio of observations, and so on.
4. Description of Experiments
The experiments performed on campus of Chung Cheng Institute of Technology, National Defense University. The GPS control network consisted of 31 points and the situation of control points is displayed in Figure 1. Since point A001 is considered as the fixed point for static GPS surveying, the tasks of baseline vector processing and network adjustment were done by software package. The three-dimension outcomes taken from the network adjustment are used for reference standard. Moreover, the RTK GPS accomplished over 14 points with SA and 30 points without SA. The instruments employed include Ashtech GPS receivers and Leica RTK GPS positioning system. In addition, the software packages involved GeoGenius from Spectra Precision, SKI from Leica, Excel from Microsoft, and Survey generated from this research group.
Figure 1 Experimental Network
5. Performance Evaluation of RTK GPS
The evaluation tasks will be separated into RTK GPS with SA and without SA cases.
5.1 RTK GPS with SA
The static GPS surveying transacted and the outcomes are deliberated as the known facts for evaluating the capability of RTK GPS technique. Eventually, 14 points in the experimental network occupied with RTK GPS when B001 was fixed, but points B031, B033, B036, B037, and C030 were phased out during the statistical analyzing task because of no outcome appeared that may caused by the thoughtfully obstruction and the bad signal. The statistical analysis of geographical coordinates is listed in Table 3. The difference of latitude, longitude, and height does range about 1arc-second, 0.02arc-second, and 0.9m, respectively. In addition, the average deviation is around 0.1arc-second in latitude, 0.001arc-second in longitude, and 0.09m in height. In fact, the significant latitude deviation does happen in B010 and B035; the notable longitude deviation does occur in B035; the noted height deviation does appear in B035. If B010 and B035 points are not included, then the consequence may be more reasonable that is almost 1cm in horizontal vector and 2cm in vertical vector. Table 4 is the description of baseline length deviation between RTK and static GPS. The deviation range and average deviation are about 0.9m and 0.01m, respectively. The evidence of portion baseline length is not enough stability that is similar to the previous remark. Besides, the lager deviation is arisen in the longer baseline.
Table 3 Statistical Analysis of Geographical Coordinates
| Point |
Latitude (arc-second) |
Longitude (arc-second) |
Height (m) |
| B010 |
0.99989 |
0.00011 |
-0.011 |
| B013 |
0.00042 |
-0.00041 |
0.003 |
| B032 |
-0.00053 |
0.00193 |
0.005 |
| B034 |
-0.00985 |
-0.00038 |
0.006 |
| B035 |
-0.05912 |
0.01671 |
0.890 |
| BL01 |
0.00006 |
-0.00676 |
-0.017 |
| C026 |
0.00038 |
-0.00010 |
-0.036 |
| C027 |
-0.00023 |
-0.00016 |
-0.037 |
| C031 |
0.00036 |
0.00029 |
-0.015 |
| Max |
0.99989 |
0.01671 |
0.890 |
| Min |
-0.05912 |
-0.00676 |
-0.037 |
| Mean |
0.10349 |
0.00125 |
0.088 |
| RMS |
0.31745 |
0.00592 |
0.284 |
