Performance of Single-frequency GPS Precise Point Positioning
3.2 Establishment of Reference Coordinates
In order to obtain accurate reference coordinates of all stations, the 1-day batches were submitted to the automated GPS data analysis service, the so-called AUSPOS service, provided by Geoscience Australia. With the use of 24-hr data set, the coordinates obtained from the AUSPOS service should be accurate to within a centimetre (Dawson et al., 2001). Therefore, the averaged coordinates of an individual station are used as reference coordinates for subsequent analysis.
3.3 Data Processing Strategy
The additional information required for processing are the IGS final orbits and satellite clock corrections. The IGS final orbits and satellite clock corrections can be downloaded from http://igscb.jpl.nasa.gov/components/prods_cb.html. The GPS data were cut into 5-min, 10-min, 15-min and 30-min data segments, and each data segment was individually processed with the developed PPP software to produce final coordinates. It should be noted that only single-frequency data were selected and used in the data processing step.
4. Analysis of Results
After all data segments were processed, the solutions were compared with the reference coordinates. The performance of the developed PPP software can be characterized by the Root Mean Square Error (RMSE). Since horizontal positioning results are the quantities of interest in this analysis, the 2-d RMSE values are calculated to estimate the horizontal accuracy obtained from all session lengths for each period of data. The 2-d RMSE values of all stations have been presented in Figures 2 to 5, which show the 2-d RMSE values obtained from the 5-min, 10-min, 15-min and 30-min session lengths, respectively. Table 2 presents the averaged RMSE values from each session length for each individual station. Subsequently, the final averaged RMSE values of all stations for each session length were calculated and shown in Table 3.
Figure 2. Horizontal RMSE values obtained from the 5-min session length at the 5 IGS stations
Figure 3. Horizontal RMSE values obtained from the 10-min session length at the 5 IGS stations
Figure 4. Horizontal RMSE values obtained from the 15-min session length at the 5 IGS stations
Figure 5. Horizontal RMSE values obtained from the 30-min session length at the 5 IGS stations
Table 2. RMSE values for different session lengths
|Session|| RMSE (m)|
|length|| ASC1|| CAS1|| KERG||MAS1||METS|
|5-min|| 2.88|| 1.38|| 2.90||3.71||1.32|
|10-min|| 2.75|| 1.25||1.61||3.13||1.48|
|15-min|| 2.60|| 1.20||1.43||3.26||1.21|
|30-min|| 2.03|| 0.99||1.21||2.36||0.97|
Table 3. Averaged RMSE values of all stations at different session lengths
|Session length|| RMSE (m)|
With reference to Figures 2 to 5, it is evident that only the ASC1 and MAS1 stations have shown significant seasonal variations in the RMSE values. Interestingly, both stations are located in low latitude region. It can be seen from Table 2 that the results obtained from the stations located in high latitude region (CAS1, KERG and METS) tend to be more accurate than the other two stations. Results presented in Table 3 have shown that the better positional accuracy can be expected if the longer session length is used.
5. Concluding remarks
A simple PPP software based on the use of single-frequency carrier phase and pseudorange measurements has been developed and tested in this paper. Test results indicate that the effect of season on positional accuracy becomes more pronounced when stations are located in low latitude region. In addition, the results obtained from the high latitude stations tend to be more accurate than the low latitude stations. As expected, results show that the better positional accuracy can be achieved when a longer data span is used. Overall results also reveal that the use of only 5-min data span can produce a horizontal positioning accuracy at the same level as a traditional pseudorange-based differential GPS technique.
This research is supported by a Ratchadaphisek Somphot Endowment Grants for Invention from Chulalongkorn University. The first author would like to thank to the Geo-Image Technology Research Unit for supporting the first author to attend the MapAsia2006 conference.
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