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

We analyzed the ionospheric TEC and scintillation distributions in Hong Kong region using three year GPS observations (2001-2003). Figure 4 shows the spatial and temporal distribution of rms value of differential VTEC values for all satellites observed, which can be used to quantify the degree of scintillation in the region. It can be seen that the scintillations were strong around March and September, and most of the strong scintillations occurred around 22 degrees latitude.


Fig. 3 The spatial distribution of TEC in Hong Kong

To achieve high precision of positioning, precise ionospheric models are required. Two different models have been proposed for law latitude applications, one using a regional GPS network and the other based on the PPP method. Unlike the other models that try to model the ionospheric delays in whole space, we only model the ionospheric delays along the satellite tracks. Therefore, our models are capable of providing high resolutions of the ionospheric TEC distribution in both spatial and temporal scales, which is important for low latitude ionosphere modeling as the spatial variation of TEC distribution is very large. Figure 5 shows the estimated ionospheric delay for each satellite observed during a 24 hour period. To test the developed model, we compared the positioning accuracy with dual frequency data and that with single frequency data corrected with the model using the PPP method. The results are in Figure 6. It can be seen that they are compatible, indicating the developed ionspheric model is capable of support centimeter positioning accuracy with single frequency data.

GPS Meteorology
The use of ground-based GPS observations for the estimation of precipitable water vapor (PWV) has several advantages: higher accuracy, high temporal resolution ranging from seconds to hours, all weather capability, and relatively low cost. The GPS-derived PWV have a potential to improve weather forecasting. We, in collaboration with Hong Kong Observatory, have conducted GPS meteorology research since 1998.

The investigations are focused on accurate and real-time/near real-time (RT/NRT) determination of PWV with GPS measurements, construction of three dimensional water vapor fields, and the application of GPS-derived PWV to weather forecasting.

A system has been developed for real-time/near real-time accurate determination of PWV using the data collected in Hong Kong permanent GPS array, and has been used in Hong Kong Observatory for over two years. Figure 7 show data path.


Fig. 4 The spatial and temporal distributions of RMS of differential VTEC values (K= 3 yeaen < K, r mean value, GreK 3K)

In the development of the system studies were conducted to develop the measures for improving the accuracy of GPS-derived PWV as follows.

• Inclusion of a gradient model in the determination of total zenith delay (TZD). This is to reduce the effect of asymmetric distribution of water vapor on the TZD.
• Calibration of dry zenith delay (DZD) calculation model. Extraction of wet zenith delay (WZD) from TZD needs the DZD, which is computed based on the measured surface air pressure and temperature. There are several empirical models available, e.g., the Saastamonien model and the Hopfield model. Their suitability for local conditions was studied. Model calibration was required and conducted using 8 year Hong Kong radiosonde data.
• Estimation of local conversion factor. The conversion from WZD to PWV is done by a factor, which is a function of weighted mean of temperature in the troposphere. The Bevis's formula is commonly used. In similar to the above its modification was required and done using 8 year Hong Kong radiosonde data.

For RT applications the key is to obtain RT accurate satellite orbits. The IGS ultra-rapid orbits are of good quality in general. However, we did observe once in a while a few satellites with large orbital errors (called bad satellites). These bad satellite orbits can degrade the accuracy of GPS-PWV by 2-3 times. Therefore they must be deleted from PWV estimation. We have developed an approach to identify the satellite with bad orbits based on statistical tests.

As an example, Figure 8 shows a correlation plot between GPS derived PWV values and those computed from radiosonde data for a period of 4 month. The slope of 1.0045 was found, which suggests GPS-PWV values coincide very well with the radiosonde data.


Fig. 5 Estimated Ionospheric delay for each satellite 3K)


Fig. 6a Positioning errors with dual frequency ionosphere-free combination 3K)

Construction of a 3D water vapor field needs slant wet delays (SWD) from a receiver to all the satellites in view. There are two approaches to extract SWD from GPS measurements: one based on the PPP and the other on the double difference (DD). The PPP is a direct method, but needs RT precise satellite orbits and clock errors. The DD can cancelled many sources of errors, but needs the GPS measurements at remote station and transformation of final DD residuals to zero differenced residuals. This transformation is not unique, requiring some assumptions imposed. We have developed an approach to obtain SWD with the DD approach in NRT, and validated it against the water vapor radiometer (WVR) and a local weather model (ORSM). Figure 9(a) and (b) are example scatter diagrams for 4 day data.