WaveMill On-Board Processing
Astrium working with the National Oceanographic Centre
Wavemill is a novel concept proposed by ESA/ESTEC (C. Buck, 2005) to provide high-resolution maps of ocean topography and currents over wide swaths with high precision. The aim of this study is to define the processing algorithms and the implications for the spaceborne hardware that is necessary to achieve the scientific requirements for wide-swath ocean mesoscale and coastal zone mapping. This study provides a roadmap for the development of the on-board signal processor required, thus maintaining the UK industry competitive advantage for the potential future implementation and testing of this concept. This seedcorn project builds a new partnership between UK industry and academia, bringing together two groups with complementary expertise and interests in the application of novel SAR technology to address the need for wide-swath ocean topography and coastal current mapping.
Introduction to the Wavemill Concept
Whereas SAR interferometry over land or ice makes use of SAR images collected over repeat satellite passes, SAR interferometry over the ocean relies on SAR images collected during the same satellite overpass, since ocean features cannot be assumed to remain stable in the time between successive satellite passes. There are two types of SAR interferometry with applications to oceanography: across-track interferometry (XTI), which leads to measurements of ocean topography and along-track interferometry (ATI), which measures the total ocean surface motion.
The Wavemill concept is a hybrid interferometric system which combines both across-track interferometry, for the determination of sea-surface topography, with along-track interferometry, for direct measurement of the ocean surface current. By means of multiple squinted beams fore and aft of the satellite, the surface current can be imaged from different nearly orthogonal directions, from which true two dimensional ocean surface currents may be determined over a broad swath.
Wavemill is a single instrument which combines along track and cross track interferometry to give simultaneously height and surface current along wide swaths either side of nadir. A number of spacecraft and antenna configurations have been studied, most recently the Javelin arrangement which has two antennas, one fore and one aft, oriented with an along track baseline. The underlying principles in terms of beams, swaths and interferograms are common to all configurations, however. Each antenna generates 4 beams, fore and aft, left hand swath and right swath.
Wavemill is a bistatic SAR, the fore antenna transmits and receives, the aft antenna receives only. The squinted beams give rise to an across track baseline even though the antennas are oriented in the along track direction.
The formation of the SAR images in the on-board processor is the key enhancement required for Wavemill compared to a conventional SAR payload such as Sentinel-1.
The project is led by Astrium working with the National Oceanographic Centre in Southampton.