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Performance of Single-frequency GPS Precise Point Positioning
Chalermchon Satirapod
Geo-Image Technology Research Unit
Department of Survey Engineering
Chulalongkorn University, Bangkok, THAILAND
Tel. 66-2-2186662 Fax. 66-2-2186653
chalermchon.s@chula.ac.th
Somchai Kriengkraiwasin
Geo-Image Technology Research Unit
Department of Survey Engineering
Chulalongkorn University, Bangkok, THAILAND
Tel. 66-2-2186662 Fax. 66-2-2186653
somchaimo_mo@yahoo.com
Abstract:
This research aims to assess the performance of GPS Precise Point Positioning (PPP) with code and carrier phase observations from L1 signal collected from geodetic GPS receiver around the world. A PPP software developed for processing the single frequency GPS data is used as a main tool to assess a positioning accuracy. The precise orbit and precise satellite clock corrections were introduced into the software to reduce the orbit and satellite clock errors, while ionosphere-free code and phase observations were constructed to mitigate the ionospheric delay. The remaining errors (i.e. receiver clock error, ambiguity term) are estimated using Extended Kalman Filter technique. The data retrieved from 5 IGS stations located in different countries were used in this study. In addition, three different periods of data were downloaded for each station. The obtained data were then 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. Results indicate that the use of 5-min data span can provide a horizontal positioning accuracy at the same level as a pseudorange-based differential GPS technique. Furthermore, results confirm effects of station location and seasonal variation on obtainable accuracies.
1. Introduction
Based on the measurements made on the GPS signals, the determination of the receiver’s position can be classified into two techniques, Absolute Positioning and Relative Positioning. The absolute positioning technique, also known as the single point positioning (SPP) technique, permits one receiver to determine the ‘absolute’ coordinates of a point with respect to a coordinate system such as WGS84. The relative positioning technique, sometimes called the differential positioning technique, requires the use of two receivers, one as a reference station and the other one as a user station, in order to determine the coordinates of the user with respect to the reference station. Each technique can be further divided into two classes depending on the measurements used, namely pseudorange and carrier phase. Since the differential positioning technique uses the measurements simultaneously made at both receivers, many biases (e.g. satellite orbit bias, satellite clock bias, ionospheric and tropospheric delays) can be largely reduced by forming the difference between the measurements made at both stations. Therefore, the differential positioning technique is extensively used for applications that require high accuracy. However, the effectiveness of the differential positioning technique is largely dependent on the distance between the two receivers. If the distance between the receivers increases, the residual errors will become larger, and hence the quality of the positioning results degrades. This is a major limitation of the differential positioning technique. Furthermore, the requirement of operating at least two GPS receivers simultaneously during data acquisition further complicates the field procedure making the differential positioning technique less attractive for most applications.
With the availability of precise GPS orbits and satellite clock corrections from the International GPS service (IGS) and several organizations, it is possible to improve the positional accuracy of the single point positioning technique. Furthermore, the use of carrier phase measurements can lead to centimetre or decimetre positional accuracy. This positioning method that uses both un-differenced pseudorange and carrier phase measurements with the precise orbits and satellite clock information as well as many corrections (e.g. satellite antenna offset, earth tide, ocean tide loading, atmosphere loading) in the estimation procedure is known as the precise point positioning (PPP) technique. The PPP technique using measurements from dual-frequency receiver was first proposed by Zumberge et al. (1997). Using the PPP technique, the repeatability of positioning results ranges from 10 to 20 cm for one hour of static observation down to a couple of centimetres for 12 hours (Witchayangkoon and Segantine, 1999). Table 1 summarises positional accuracies obtained from each GPS positioning technique.
Table 1. Obtainable accuracy from each GPS positioning technique
| Positioning technique | Measurement used | Accuracy |
| Absolute | Pseudorange | 10 metres |
| Absolute | Carrier phase and pseudorange | cm-decimetres |
| Relative | Pseudorange | 1-5 metres |
| Relative | Carrier phase | cm-decimetres |
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