No description available.

No description available.

In this research study, several new algorithms are developed to achieve spacecraft attitude determination from carrier phase information of GPS (Global Positioning System) signals. The first focus is on resolving integer ambiguity in carrier phase difference measurements. A newly developed algorithm based on Gram-Schmidt Orthonormalisation (GSO) is proposed for medium length baseline observations. Using this newly developed attitude algorithm from vector observations, an instantaneous estimated attitude solution is obtained, which we call 'coarse attitude', from only four phase measurements collected from only two baseline observations. Then a 'fine' attitude solution from all phase measurements is estimated, using a sophisticated Kalman filtering estimator, once integer ambiguity has been resolved. The second focus is on estimating the relative phase offset error (line bias) in carrier phase difference measurements. A newly developed block bias search is proposed which finds an initially plausible solution of line bias for each individual baseline. The line bias from all phase measurements collected from each individual baseline is then re-estimated using a developed recursive least squares (RLS) estimator. A newly developed parallel architecture GPS receiver is being flown on the UoSat-12 minisatellite, with the capability for simultaneous measurements from 24 channels for attitude sensing. The final goal of this research study was to apply the developed algorithms to real GPS data, and a number of data files of phase differences of GPS signals logged on UoSat-12 were tested. Independent ADCS (Attitude Determination and Control System) data was used for the reference attitude determination. The results show that an instantaneous attitude error less than 4 degrees is achieved during coarse attitude acquisition, relative to the reference ADCS system. When all measurements are processed during fine attitude tracking, the error in attitude estimation is reduced to one degree error (1 sigma RMS), without any error mitigation for multipath, relative to the reference ADCS system.

No description available.

No description available.

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Using observations from the Global Positioning System (GPS) satellites to determine a three dimensional (3-D) geodetic control network are considered. The repeatability of individual baselines and 3-D vector closures are examined, in order to investigate refraction effects on GPS networks. The effect on GPS baselines of a height bias in the reference point's coordinates is also investigated. A least squares adjustment program is developed and used to obtain a single consistent set of 3-D coordinates for the Tygerberg Test Network (TTN). The results of two GPS processing packages are compared by means of a conformal transformation. It is concluded that single frequency measurements produce better results than the ionospheric free observable on short baselines. Furthermore, a standard atmospheric model shows an improvement over the Marini model to account for tropospheric refraction.

Includes bibliographical references / Global Navigation Satellite Systems (GNSS) are used to provide information on position, time and velocity all over the world at any time of the day. Currently there are four operational GNSS and one of them is GPS (Global Positioning System) that is developed and maintained by U.S Department of Defence (DoD), which is widely used and accessible all over the world. The accuracy of the output or even the availability of the navigation system depends on current space weather conditions, which can cause random fluctuations of the phase and amplitude of the received signal, called scintillation. Interference of GNSS signals that are reflected and refracted from stationary objects on the ground, with signals that travel along a direct path via the ionosphere to the antenna, cause errors in the measured amplitude and phase. These errors are known as multipath errors and can lead to cycle slip and loss of lock on the satellite or degradation in the accuracy of position determination. High elevation cut off angles used for filtering GNSS signals, usually 15-30°, can reduce non-ionospheric interference due to multipath signals coming from the horizon. Since a fixed-elevation threshold does not take into consideration the surrounding physical environment of each GPS station, it can result in a significant loss of valuable data. Alternatively, if the fixed-elevation threshold is not high enough we run the risk of including multipath data in the analysis. In this project we characterized the multipath environment of the GPS Ionospheric Scintillation and TEC (Total Electron Content) Monitor (GISTM) receivers installed by SANSA (South African National Space Agency) at Gough Island (40:34oS and 9:88° W), Marion Island (46:87° S and 37:86° E), Hermanus (34:42° S and19:22° E) and SANAE IV (71:73° S and 2:2° W) by plotting azimuth-elevation maps of scintillation indices averaged over one year. The azimuth-elevation maps were used to identify objects that regularly scatter signals and cause high scintillation resulting from multipath effects. After identifying the multipath area from the azimuth-elevation map, an azimuth-dependent elevation threshold was developed using the MATLAB curve fitting tool. Using this method we are able to reduce the multi-path errors without losing important data. Using the azimuth-dependent elevation threshold typically gives 5 to 28% more useful data than using a 20° fixed-elevation threshold.

No description available.