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

Gravity gradient and magnetorquing attitude control for low-cost low earth orbit satellites - The UOSAT experience.

An important concern in spacecraft engineering is attitude determination control and stabilisation (ADCS) - the combination of applied mathematics, classical physics and modern technology which maintains the pointing direction of one or more axes of an Earth-orbiting satellite. This thesis is a detailed study of a particular type of ADCS which exploits the gravity-gradient effect, which is just the weak tendency for an appropriately shaped body to point naturally in preferred directions; reinforced by magnetorquing, which is the active interaction of the geomagnetic field with a switched current passing through coils in the spacecraft body. The advantages of this technology is that it is low-cost, non-consumable and has no moving parts - so constituting no limitation to the satellite's life. The thesis is a detailed study of this form of ADCS with specific application to low Earth polar-orbiting (LEO) satellites, for which it is particularly suitable. The work is also a study in attitude determination based solely on a 3-axis magneometer measurement of the geomagnetic field, which is in principle a simple way, in terms of technology, of determining the attitude of the spacecraft, and from this controlling the attitude, if mediated by an on-board computer implementing appropriate algorithms. The results are for the most part practically based on the author's involvement with two satellites over a six year period with the satellites UOSAT-1 and UOSAT-2, which were designed, built, and continue to be controlled from the University of Surrey. A practical innovation in 2-axis attitude control is described: the active 'delibration' by active damping of a gravity-gradient controlled LEO satellite in an attitude-stabilised state, using a threshold comparison algorithm. A new theory and algorithms are then developed for 3-axis attitude control, based on a complementary use of magnetorquing and gravity gradient. Subject to further development these could alter the general perception of the most effective way of controlling low Earth orbiting satellites

Gravity gradient and magnetorquing attitude control for low-cost low earth orbit satellites - The UOSAT experience.

An important concern in spacecraft engineering is attitude determination control and stabilisation (ADCS) - the combination of applied mathematics, classical physics and modern technology which maintains the pointing direction of one or more axes of an Earth-orbiting satellite. This thesis is a detailed study of a particular type of ADCS which exploits the gravity-gradient effect, which is just the weak tendency for an appropriately shaped body to point naturally in preferred directions; reinforced by magnetorquing, which is the active interaction of the geomagnetic field with a switched current passing through coils in the spacecraft body. The advantages of this technology is that it is low-cost, non-consumable and has no moving parts - so constituting no limitation to the satellite's life. The thesis is a detailed study of this form of ADCS with specific application to low Earth polar-orbiting (LEO) satellites, for which it is particularly suitable. The work is also a study in attitude determination based solely on a 3-axis magneometer measurement of the geomagnetic field, which is in principle a simple way, in terms of technology, of determining the attitude of the spacecraft, and from this controlling the attitude, if mediated by an on-board computer implementing appropriate algorithms. The results are for the most part practically based on the author's involvement with two satellites over a six year period with the satellites UOSAT-1 and UOSAT-2, which were designed, built, and continue to be controlled from the University of Surrey. A practical innovation in 2-axis attitude control is described: the active 'delibration' by active damping of a gravity-gradient controlled LEO satellite in an attitude-stabilised state, using a threshold comparison algorithm. A new theory and algorithms are then developed for 3-axis attitude control, based on a complementary use of magnetorquing and gravity gradient. Subject to further development these could alter the general perception of the most effective way of controlling low Earth orbiting satellites

P.D.: Dual estimate receiver of binary offset carrier modulated signals for global navigation satellite systems

Abstract A dual estimate receiver of binary offset carrier (BOC) signals for GNSS applications is described. A unique estimate is derived solely from the code modulation. A more accurate but ambiguous estimate is derived solely from the sub-carrier modulation. Any time difference between the two is rounded to the nearest sub-chip in order to correct the latter estimate. Implementation in the receiver is by a triple digital-loop. The full potential accuracy of BOC is then reliably and unambiguously realised

Detection and processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing

We will show that ocean-reflected signals from the global positioning system (GPS) navigation satellite constellation can be detected from a low-earth orbiting satellite and that these signals show rough correlation with independent measurements of the sea winds. We will present waveforms of ocean-reflected GPS signals that have been detected using the experiment onboard the United Kingdom's Disaster Monitoring Constellation satellite and describe the processing methods used to obtain their delay and Doppler power distributions. The GPS bistatic radar experiment has made several raw data collections, and reflected GPS signals have been found on all attempts. The down linked data from an experiment has undergone extensive processing, and ocean-scattered signals have been mapped across a wide range of delay and Doppler space revealing characteristics which are known to be related to geophysical parameters such as surface roughness and wind speed. Here we will discuss the effects of integration time, reflection incidence angle and examine several delay-Doppler signal maps. The signals detected have been found to be in general agreement with an existing model (based on geometric optics) and with limited independent measurements of sea winds; a brief comparison is presented here. These results demonstrate that the concept of using bistatically reflected global navigation satellite systems signals from low earth orbit is a viable means of ocean remote sensing