New Approach to Achieving Stand Alone GPS Attitude Determination using Dual Short Baselines for Small-Satellite

This paper proposes a new approach to GPS (Global Positioning System) attitude determination for small satellite application in LEO (low Earth orbit). Prior knowledge of attitude and integer resolution is not required. The methodology of the new approach includes integer ambiguity search, initial estimation of attitude and line bias, attitude initialisation, path difference estimation and fine attitude determination. The observable is the carrier phase difference measurement between two GPS antennas. A dual short baseline (typical baseline length up to 30 cm) is assumed in this research. The key point to initialising attitude is to estimated the attitude of individual baseline vectors with respect to the reference frame. Elimination of integer ambiguity is a simple task. Two set of vectors are required to determine an initial attitude. Once attitude is initialised, an estimation algorithm based on the extended Kalman filter starts to determine the attitude. The integer ambiguities and cycle slips can be resolved properly. The filter now is converged and, fine attitude is estimated. The robustness of the filtering estimator is tested with simulated anomalous conditions

Study of spacecraft attitude determination from phase information of GPS signals.

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

Study of spacecraft attitude determination from phase information of GPS signals

Available from British Library Document Supply Centre-DSC:DXN040819 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Study of spacecraft attitude determination from phase information of GPS signals.

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