Techniques for spaceborne remote sensing of earth’s oceans using reflected GNSS signals

Abstract

Global Navigation Satellite System (GNSS) Reflectometry (GNSS-R) is a low mass, power, volume and ultimately cost remote sensing technology that has recently been demonstrated on spaceborne platforms. However, GNSS-R is still relatively immature compared to other remote sensing technologies and there remains significant work to be done before its capabilities are fully realised. In this thesis, techniques to produce and process Delay Doppler Maps (DDM)s using spaceborne receivers with a focus on ocean applications are developed.

A new approach to determine the forward scattered specular point is developed which improves the positioning accuracy by over 20 km compared to the current state-of-the-art. This is followed by the investigation of the pseudo monostatic point and a statistical analysis of the two.

The incoherent range walk compensation technique is developed which focuses the power in the DDM on the iso-delay and iso-Doppler configuration occurring at the midpoint of the integration period. This is shown to mitigate the correlation losses associated with a point scatterer on the surface by over 6 dB that occur due to tracking strategies currently employed by state-of-the-art receivers.

An adaptive window for stare processing which regularises the spatial footprint and reduces the variance of the extracted profiles is developed. Then, the ambiguous stare processing concept is introduced and a simulation study demonstrates sensitivity to wind direction on a spaceborne platform can be achieved.

Multiple techniques applicable to target detection are developed. The first observation of persistent non-specular coherent scattering off an ice sheet is followed by the development of an ice sheet detection method which exploits this. Furthermore, the method is able to integrate an arbitrary number of DDMs without inducing any loss of spatial resolution. Following this, sea target ambiguity resolution is demonstrated with a simulation study. Then, the application of an adaptive filter for blind sea clutter suppression is presented. Finally, a novel matched filter, capable of detecting targets in sequences of DDMs is developed.

The sensitivity of the DDM to satellite attitude is investigated and the pitch of TDS-1 is estimated using DDMs produced on-orbit.