Journal of Surveying Engineering

This paper discusses the benefits of using dual-frequency receivers for attitude determination applications. The ambiguity resolution performance, baseline, and attitude solution accuracy is compared between single and dual-frequency modes. Evaluation is performed both in static and kinematic environments. While the static data were collected in an open-sky environment, the two kinematic data sets were collected in “degraded” environments including partial blockages due to trees or overpasses. The level of dynamics observed during these tests is somewhat limited, as the vehicle often travels at constant speed, but is, however, representative of car navigation. Results indicate that for baselines as long as $10\mathrm{m}$ , the proposed WL/L1 cascaded ambiguity resolution scheme significantly outperforms its single frequency counterpart by providing faster and more reliable fixed ambiguity resolution. Assuming a correct ambiguity fix, dual frequency observations do not provide significant improvements in terms of baseline or attitude accuracy. However, the improved speed and reliability of the dual frequency ambiguity resolution algorithm does translate, especially in kinematic mode, into significant improvements in terms of attitude solution availability and reliability.

NOAA’s National Geodetic Survey (NGS) has developed the Rapid Static GPS software for use as the major processing engine in the OPUS-RS utility (online positioning user service—rapid static) (http://www.ngs.noaa.gov/OPUS/OPUS-RS.html). The software was written specifically to support the computation of static positions from GPS tracking sessions as short as $15\min$ , while using reference station data from the NGS archive of continuously operating reference stations (CORS). When the reference stations are close $\left(50\mathrm{km}\right)$ to the user’s station, it is relatively easy to obtain an accurate solution. However, the CORS stations in the NGS archive are separated by $200\mathrm{km}$ or more in many areas of the country. In this situation, much care must be taken in conditioning the data sets and in selecting appropriate weights for the observations and constraints. This paper describes methods and weights that have been found to work well for most (but not quite all) data sets, and, therefore, can be used in an automated procedure such as OPUS-RS.

The Thai geodetic network has been regularly observed with GPS since 1994. The continuous tectonic motions in Thailand located on the Sundaland block are well determined with uncertainties $\sim 1\mathrm{mm}∕\mathrm{yr}$ . The mega thrust earthquake occurred near Banda-Aceh on Dec. 26, 2004 with Mw 9.3 magnitude and caused significant surface displacements in many surrounding countries. The geodetic network within Thailand was also significantly deformed during the earthquake at the cm to dm level and the geodetic network will continue to deform for many months and possibly even years to come. The magnitude of displacements certainly has a direct impact on the Thai geodetic control network. The Royal Thai Survey Department (RTSD) has been carrying out GPS field campaigns in an attempt to monitor the postseismic displacement. However, the situation became more complicated when the second major earthquake occurred near Nias Island, Sumatra on Mar. 28, 2005 with Mw 8.7 magnitude. This paper will analyze the GPS observations obtained from the RTSD GPS campaigns and existing permanent GPS stations in Thailand up to the end of 2006 using the precise point positioning strategy of the GIPSY-OASIS II software. Results obtained from the latest campaign (Nov. 2006) indicated that the earthquakes have resulted in the horizontal displacements, ranging from $55\mathrm{cm}$ in the south to about $6\mathrm{cm}$ in the north of Thailand. The behavior of postseismic motions is investigated in this study and it was found that the postseismic motions can be fitted well by a logarithmic decay function. Finally, a strategy for updating the Thai geodetic network is proposed.

The sampling rate in automatically recording instruments is usually assumed stable. Small fluctuations in this rate have practically little influence on low recording rates, but they might be important in high rates for three reasons. First, they lead to wrong estimation of certain parameters, equations of motions, etc. Second, errors accumulate and become very important in certain cases, for instance, displacements deduced from double numerical integration in instruments such as the accelerographs. Finally, they lead to noisy or wrong spectral characteristics in periodic functions. Fluctuations in the sampling rate were studied on the basis of experiments with a robotic total station, the built-in software of which was configured to display time of recordings with centisec resolution. The conclusion of these experiments is that at high sampling rates, the signal-to-noise ratio decreases, introducing additional noise in final results. Such instabilities in the sampling rate are not easy to identify in most automatically recording instruments, but if modeled, they have the great advantage that they permit high-rate aliasing-free estimates.