Abstract
Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative technique for remote sensing. It uses reflected signals from the navigation constellations to determine properties of the Earth’s surface. The primary focus of this work is the remote sensing of the ocean by measurement of surface roughness. The most significant unresolved challenge in spaceborne GNSS-R is to verify the accuracy of surface roughness measurements. Existing remote sensing techniques have typically relied on extensive data-sets to validate satellite measurements with the ground truth. This thesis provides a receiver design for collection of the required validation data-sets which can then form part of an operational system for surface roughness measurement. New receiver approaches were investigated through the design of a software receiver to postprocess existing data from the GNSS-R experiment on the UK-DMC satellite. This forms the reflections into Delay-Doppler Maps (DDMs) from which the surface roughness can be determined. The software receiver improves on existing implementations by targeting all available specular reflections using open-loop tracking. A new approach called Stare processing is analysed, which controls the receiver to remain focused at a fixed point on the Earth’s surface as the satellites move. This improves the surface resolution over using the full DDM. Additionally it is shown to be a viable approach for surface roughness measurement through a scattering model and the first demonstration on data collected from space. GNSS-R research has primarily focused on the established GPS navigation system. This research extends the measurement concept to the new Galileo GNSS. A receiver that can target multiple GNSS constellations will allow greater remote sensing coverage. The primary differences between Galileo and GPS are analysed and an approach is developed leading to the first spaceborne demonstration of Galileo-like signals for remote sensing. The system design for the GNSS-R receiver presented in this thesis was carried out in the context of Surrey Satellite Technology Ltd developing a GNSS navigation receiver called the SGR-ReSI, to be launched on the UK Technology Demonstrations Satellite TDS-1. The critical areas identified in the GNSS-R system design were implemented and tested on this receiver. The design overcomes the challenging constraints of GNSS-R in a small satellite platform: principally the mass, power and data downlink capacity. To achieve these, on-board data compression was developed through real-time DDM processing and reflection tracking. An algorithm for real-time DDM processing within the mass and power constraints was designed and demonstrated within the receiver and combined with open-loop reflection tracking. A ground-based test set-up was developed to test the design on existing spaceborne data, from the UK-DMC experiment, before the TDS-1 satellite launch.