Journal of Surveying Engineering

Control of reference mark stability is essential for monitoring of deformations of engineering structures or displacements of points in geodetic networks. Some robust estimates, which are based on median application, can be used for such purposes. For example, some R-estimates can be applied to determine displacements of geodetic points thus to identify unstable reference marks. However, such simplest approach can sometimes fail. Thus, a more complex strategy for monitoring of geodetic point stability is elaborated and presented in this paper. The concept presented here concerns leveling networks and applies some R-estimate and also other median-based estimates, i.e., the median distance to the median and the average distance to the median. The method proposed in this paper is illustrated with some practical examples and is compared with the traditional method for geodetic point stability monitoring.

In deformation analysis, it is important to know whether the points detected by conventional deformation analysis (CDA) are really displaced or not, and also if there are any more displaced points in the network. It is impossible to answer these questions unless the actual positions of the displaced points before the analysis are known. Moreover, the efficacy of the analysis method used must be known. The efficacy of CDA methods can be measured using the mean success rate (MSR). When a displacement occurs at a point, both the observations that belong to the displaced point and to those undisplaced points closest to it are affected. The least-squares estimation (LSE) spreads these effects to various degrees over all the coordinates. As a result, the actual displacements are not exactly reflected in the coordinates of that point. Consequently, CDA methods may wrongly identify a point as being displaced. Also the F-test is known not to be successful in some cases. Hence, the MSRs of the CDA methods are smaller than what can be expected. To eliminate the smearing effect of the LSE and the indifference of the F-test, in order to obtain more specific results a new strategy that works on absolute deformation monitoring network has been developed based on division into subnetworks.

Carrier phase ambiguity resolution is the key to precise positioning with a global navigation satellite system (GNSS); therefore, quite a few ambiguity resolution methods have been developed in the past two decades. In this paper, a new ambiguity searching algorithm by treating part of normal equations as constraints is developed. The process starts with the truncation of the terms with respect to the small eigenvalues from the normal equations of a least-squares estimation problem. The remaining normal equations are employed as the constraint equations for the efficient searching of integer ambiguities. In the case of short single baseline rapid GNSS positioning with double differenced phase measurements, there are only three real parameters of the position to be estimated. Therefore three terms of the normal equations should be truncated off due to the fact that there is a large difference between the last three eigenvalues of the normal matrix of the float solution and the others, and then the remaining ambiguities can be trivially solved with three independent ambiguities by means of the remaining normal equations. As a result, only three independent ambiguities are necessarily searched and the searching efficiency is dramatically enhanced. Moreover, a new indicator of minimizing the conditional number of the subsquare matrix of the remaining normal equations is introduced to select three independent ambiguities. Once the correct integer values of the selected three independent ambiguities are applied to solve the remaining ambiguities, the estimated real-valued solutions are very close to their integers, which can be applied as additional strong constraints to further improve the searching efficiency. Finally, two case studies, from real dual-frequency global positioning system (GPS) data of about 10-km baseline and random simulations, respectively, are carried out to demonstrate the efficiency of the new algorithm. The results show that the new algorithm is efficient, especially for the scenarios of high-dimensional ambiguity parameters.

Multipath is a major sidereally repeating error affecting precise GPS positioning and deformation monitoring. Because satellite-receiver geometry repeats almost exactly every sidereal day, filtering can reduce multipath in near-static situations. Here, we investigate how sidereal filtering can be used in a switched multiantenna array system providing semicontinuous GPS data, which may be adopted in order to reduce hardware costs. Depending on the receiver’s setup, the optimum session switching interval is 119 s or a multiple thereof. This provides sufficient reliable epochs using a short switching interval, and is synchronized with the appropriate geometry repeat interval for sidereal filter application. The semicontinuous sidereally filtered GPS technique is efficient in detecting horizontal and vertical displacements, surpassing 5 and 8 mm precision, respectively, per epoch in near-static environments with moderately high multipath. This provides the capability of monitoring deformations occurring at periods greater than double the switching interval, although it can also be used for monitoring rapid structural deformations despite data gaps which may affect temporal resolution. Consequently, this combined method presents an efficient, cost-effective, and precise GPS technique.

NOAA’s National Geodetic Survey has developed the horizontal time-dependent positioning (HTDP) software to provide a way for its users to estimate the coordinate changes associated with horizontal crustal motion in the United States. HTDP contains a model for estimating horizontal crustal velocities and separate models for estimating the displacements associated with 29 earthquakes (two in Alaska and 27 in California). This software is updated periodically to provide more accurate estimates for crustal velocities and earthquake displacements, as well as to include models for additional earthquakes. In June 2008, NGS released version 3.0 of HTDP (HTDP 3.0) that introduces an improved capability for predicting crustal velocities, based on a tectonic block model of the western contiguous United States (CONUS), that is, from the Rockies to the Pacific coast. Values for the model parameters that predict the velocity at any point within the domain were estimated from 4,890 horizontal velocity vectors (derived from repeated geodetic observations), 170 fault slip rates, and 258 fault slip vector azimuths. Extensive testing indicates that this model can predict velocities within CONUS with a standard error of less than 2 mm/year in both the north and east components. HTDP 3.0 also introduces a model for the combined coseismic and postseismic displacements associated with the magnitude 7.9 Denali earthquake that occurred in central Alaska on November 3, 2002.

Referencing small engineering surveys to the national spatial reference system (NSRS) presents advantages in terms of spatially coordinating infrastructure and facilitates the inclusion of the infrastructure in a geographic information system. In cases where the total station is the appropriate tool to conduct the survey, online positioning user service rapid static (OPUS-RS) can be employed to economically establish azimuth pair points to orient the survey to the NSRS. Using OPUS-RS coordinates as observations along with global positioning system (GPS) vectors and total station observations in a least-squares adjustment to establish the azimuth pair points adds robustness to the least-squares adjustment to isolate systematic errors. All the observations contribute to the least-squares adjustment but some observations contribute more than others do. Redundancy numbers can be used to identify those observations that contribute little to the adjustment results. By analyzing the results of three least-squares adjustments, it will be shown that under normal conditions total station observations contribute little to the overall adjustment. Under obstructed conditions total station observations contribute to the vertical precision of the azimuth pair points. OPUS-RS appears to be less susceptible to vertical biases than GPS vectors at obstructed sites.