INS/GPS Vector Gravimetry Along Roads in Western Montana
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Publisher:Ohio State University. Division of Geodetic Science
Series/Report no.:Ohio State University. Geodetic Science and Surveying. Report. no. 477
This document reports on a comprehensive look at the data collected by the GPSVan (OSU, Center for Mapping) in western Montana in April and June of 2005. The data consist of inertial measurement unit (IMU) data, extracted from high-accuracy inertial navigation systems, and differential GPS data that are combined to estimate the (3-D) gravity vector along the roadways traveled by the vehicle. The key to the evaluation of these tests and to their deemed success was the repeated runs of the traverses, rather than the existing control data in the region. The fairly dense network of gravity data provided only some overall corroboration of the accuracy in the vertical components of the estimates. The deflection of the vertical (DOV) data and other independent sources of computed DOV’s provided barely some long-wavelength confirmation of our estimates, while the repeatability of the traverses verified fine detail in the recovered horizontal components. However, this precision was not consistent and large errors remain in the horizontal components. The single largest detriment to our estimates was the inaccuracy in the kinematic GPS positioning solution. Due to road overpasses and other obstructions, the GPS solution was often degraded significantly due to the inability to solve for the cycle ambiguity. This had a direct and demonstrable effect on the gravity estimation. When all systems were working at peak performance, we showed better than mgal repeatability in the down component of the gravity disturbance and standard deviation of 2-3 mgal with respect to the interpolated control data. No attempt was made in this first analysis to solve for biases and linear trends, nor to take advantage of the multiple traverses to arrive at final along-track gravity disturbance estimates. Three essential conclusions were obtained from our analysis: 1) GPS solutions must be improved, e.g., using INS to help recover the cycle ambiguity after a GPS outage; 2) more direct, along-track control data are necessary, particularly in the horizontal components, to obtain a meaningful assessment of the vector gravimetry capability of the system; and 3) an operational system would clearly benefit from redundancy in instrumentation in order to imitate and take advantage of multiple traverses along each surveyed road. The first chapter summarizes the instrument setup, the survey routes, the data collected, and the control data available. The second chapter briefly reviews the techniques used to obtain the gravity vector estimates, relying heavily on previous publications and reports. Results of applying these techniques to the data are shown in Chapter 3; followed by the concluding chapter with comments and analyses, and an outlook toward further data processing.
This project is supported under a contract with the National Geospatial-Intelligence Agency, contract no. NMA- NMA401-02-1-2005; and this report serves as an Interim Technical Report for the project.