GNSS applications in Deformation monitoring, Intelligent transport systems, Precise Point Positioning and...

GNSS applications in Deformation monitoring, Intelligent transport systems, Precise Point Positioning and Atmosphere study

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Chen W
Chen W
Department of Land Surveying and Geo-Informatics The Hong Kong Polytechnic University
[email protected]

Chen YQ
Chen YQ
Department of Land Surveying and Geo-Informatics The Hong Kong Polytechnic University
[email protected]

Ding XL
Ding XL
Department of Land Surveying and Geo-Informatics The Hong Kong Polytechnic University
[email protected]

The Department of Land Surveying and Geo-Informatics at the Hong Kong Polytechnic University has been active in GNSS research and development, and made several significant developments.

This article introduces some of these developments, including a multi-antenna system for deformation monitoring, integrated navigation system for intelligent transport systems, precise point positioning technique (PPP), GPS/Met, and study of ionosphere.

Precise Point Positioning (PPP)
The PPP is to use single receiver to determine the absolute position of a point. With the improvement of the IGS (International GPS Services) orbit and satellite clock error products, the PPP has been attracted more and more attention in recent years. It has been demonstrated that the positioning accuracy at centimeter level can be achieved in its static mode, comparable with conventional relative positioning. In the relative positioning mode, most of GPS errors can be cancelled out or significantly reduced. Thus, some of the GPS error sources, such as the earth tide, phase wind-up effects, and satellite antenna offset, are not crucial for short baseline relative positioning. However, with the PPP all the error sources have to be considered carefully. We analyzed the contribution of all major errors in the PPP observation models to the positioning accuracy and have developed programs for static and kinematic PPP data processing. A new PPP algorithm based on parameter elimination method which separates time-variable parameters (i.e. position coordinates and receiver clock) and ambiguity and tropospheric delays has been proposed for data processing. The positioning accuracy of 1 cm has been achieved with static PPP method after the solution converges (an example shown in Figure 1).


Fig. 1 Static PPP positioning error

To test kinematic PPP positioning accuracy, we installed a GPS receiver on a buoy in the sea and a reference station about 200 m away from the buoy. Then the buoy positions are estimated using both relative positioning and PPP methods, and the accuracy of the PPP positioning method is compared with the relative positioning results. Figure 2a and 2b shows the positioning errors of the kinematic PPP method in horizontal and vertical components respectively, with the RMS of the differences of 3 cm for horizontal components and 8 cm for vertical components.

Ionosphere monitoring and modeling at low latitude
Ionospheric delay is one of the major error sources in GPS positioning. Modeling of the ionosphere becomes more difficult in both lower and higher latitude region. In Low latitude, ionospheric plasma from equatorial region moves upward and then diffuses downward along sloping magnetic field lines to low latitudes on both sides of geomagnetic equator. The electron concentration is thus depleted on the geomagnetic equator and enhanced in two regions, which locate approximately north and south 15° of the geomagnetic equator. Figure 3 shows the TEC distribution above Hong Kong, where different colors represent different satellites. Also, electron density is extremely variable in the equatorial zone between sunset and midnight due to the presence of electron density irregularities. The electron density irregularities cause inhomogeneities in the refractivity of RF signals, which pass through the ionosphere. Rapid random fluctuations of the phase and field strength (amplitude) will appear on the RF signals and the phenomena are known as ionospheric scintillations.


Fig. 2a GPS buoy horizontal position difference estimated by the RTK and the Kinematic PPP Fig. 2b GPS buoy height difference estimated by the RTK and the Kinematic PPP