Stephen Hodgart

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 Kingdoms 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.

Attitude stabilization of an underactuated satellite using two wheels

Failure of mechanical controllers onboard a satellite is a well-known phenomenon that has already occurred during several space missions. Particularly disastrous for attitude control is the loss of thrusters in one or more axes, or in the case studied here, the loss of a reaction wheel. Of course it has been standard practice to employ redundant actuators (e.g. 4 wheels) so that in the event of losing one wheel, full 3-axis control may still be possible. As an interesting alternative to this expensive solution, we present here a theory, which shows how full 3- axis control can still be achieved, despite losing one of the reaction wheels from a standard orthogonal 3-wheel configuration (or even two wheels failures ftom the expensive solution using a redundant 4-wheel configuration). Using a novel nonlinear time invariant and discontinuous approach, we show that the attitude is precisely and rapidly restored, without transient oscillations, to the required earth pointing.

The Effect of Cosine Phased BOC Modulation on the GNSS Receiver Search Process

The modernisation of GPS and the introduction of Galileo will introduce the use of Binary Offset Carrier (BOC) modulation. BOC will be implemented as either a sine or cosine sub-carrier modulation, which affects the spectral properties of the signal and the receiver search, acquisition and tracking processes. This paper addresses the effect of the various forms of BOC modulation on the receiver search process. Theory is developed to draw comparisons between search techniques and verified with computer simulation models.

Modelling of capture effect and analysis in LEO satellite channel

Capture effect can improve the throughput of random multiple access protocols. An evaluation model of the capture effect is proposed for coherent BPSK demodulation. The model introduces the concept of carrier synchronisation probability conditioned upon the carrier-to-interference ratio into the evaluation of the capture probability. Moreover, the effect of bit-sync offset of interfering packets on the capture probability is also taken into account. An attempt is made by means of the proposed model to investigate the capture effect in the LEO satellite channel. The result shows that a relatively large link margin can result in a remarkable capture probability, and consequently benefit slotted ALOHA protocols with a substantial throughput improvement.

A physical explanation of SAR phase function in two dimensional wavenumber domain

The phase function of an SAR signal in the two dimensional wavenumber domain is usually derived by using the principal of stationary phase. From this phase function, the phase error of the conventional range-Doppler algorithm when used for high resolution large squint angle SAR imaging is obvious [Bamler 1992]. The secondary range compression and other approximate wavenumber domain algorithm can be derived based on the phase function. But the physical meaning of the secondary range compression can not be directly perceived. In this paper we derive the phase function from SAR imaging geometry by using the Doppler-azimuth coordinate relationship in different RF frequency. From this point of view, it is easy to understand the phase error originated from the range-Doppler algorithm.