Abstract's details
Separation of Coherent and Incoherent Scattering Components from Delay/Doppler Altimeter Waveforms
CoAuthors
Event: 2015 Ocean Surface Topography Science Team Meeting
Session: Instrument Processing: Measurement and retracking (SAR and LRM)
Presentation type: Type Oral
Contribution: PDF file
Abstract:
One of the main benefits of the delay/Doppler altimeter (DDA) is the improved resolution of the system along the satellite track. By means of an unfocussed Synthetic Aperture Radar (SAR) processing technique, the altimeter footprint along the flight direction can be reduced by an order of magnitude with respect to conventional altimeters. This has allowed to resolve small-scale features on the ocean, and to provide altimetry data up to several hundreds of meters off the coast.
However, with the delay-Doppler processing the resolution improvement occurs only on the along-track direction, while the across-track direction remains pulse-limited. The result is an elongated footprint perpendicular to the satellite flight path. This is particularly significant in the case of coastal altimetry, as the DDA’s response in coastal regions depends on the relative orientation between the coastline and the spacecraft orbital plane.
The combination of the effects of several scatterers with different backscattering intensities on the surface can lead to random variations of the DDA waveforms, preventing conventional retracking techniques from retrieving geophysical parameters from altimeter data. In the case of coastal areas, the scattering is essentially composed of a coherent component, from static targets on the land, which are able to maintain their phase history, and an incoherent component from reflections of the radar echoes off the ocean surface, which acts as wide distributed target. If the land surface scattering is much higher than the one of the ocean, a strong coherent component is expected to arise from these areas, which will highly distort the ocean-like waveform shape.
We have developed a processing technique that allows the separation of the coherent and incoherent scattering components from SAR altimetry waveforms. The technique is similar to the one used in imaging SAR systems, and is based in the exploitation of the phase history of coherent targets during their illumination period with the antenna beam.
For the development of the technique we have used the CryoSat-2 SAR Mode data. The starting point of our processing is the full bit rate (FBR) I/Q complex echo samples. By accounting for the phase evolution of the static targets in the scene, it is possible to counter-rotate the phase of the FBR complex echoes along the aperture, which allows to perform an inter-burst coherent averaging, potentially, as long as the target illumination time. This reduces the incoherent components of the radar signal, which results in a radar waveform that contains only the coherent scattering component. The coherent component can later be removed from the original delay/Doppler waveform.
For the coastal areas, this can be used to remove the contribution of static coherent targets on the land from the SAR mode L1b waveform. This is expected to remove most of the land contamination from the DDA waveforms, thus enabling the application of ocean-like retrackers at much closer distances to the coast, regardless of the coastline/satellite track relative orientation. This technique has also promising applications for the cryosphere, as it can be used for the separation of the contribution of sea-ice freeboard and leads, as well as for in-land waters, where small water bodies act as perfect coherent scatterers.
However, with the delay-Doppler processing the resolution improvement occurs only on the along-track direction, while the across-track direction remains pulse-limited. The result is an elongated footprint perpendicular to the satellite flight path. This is particularly significant in the case of coastal altimetry, as the DDA’s response in coastal regions depends on the relative orientation between the coastline and the spacecraft orbital plane.
The combination of the effects of several scatterers with different backscattering intensities on the surface can lead to random variations of the DDA waveforms, preventing conventional retracking techniques from retrieving geophysical parameters from altimeter data. In the case of coastal areas, the scattering is essentially composed of a coherent component, from static targets on the land, which are able to maintain their phase history, and an incoherent component from reflections of the radar echoes off the ocean surface, which acts as wide distributed target. If the land surface scattering is much higher than the one of the ocean, a strong coherent component is expected to arise from these areas, which will highly distort the ocean-like waveform shape.
We have developed a processing technique that allows the separation of the coherent and incoherent scattering components from SAR altimetry waveforms. The technique is similar to the one used in imaging SAR systems, and is based in the exploitation of the phase history of coherent targets during their illumination period with the antenna beam.
For the development of the technique we have used the CryoSat-2 SAR Mode data. The starting point of our processing is the full bit rate (FBR) I/Q complex echo samples. By accounting for the phase evolution of the static targets in the scene, it is possible to counter-rotate the phase of the FBR complex echoes along the aperture, which allows to perform an inter-burst coherent averaging, potentially, as long as the target illumination time. This reduces the incoherent components of the radar signal, which results in a radar waveform that contains only the coherent scattering component. The coherent component can later be removed from the original delay/Doppler waveform.
For the coastal areas, this can be used to remove the contribution of static coherent targets on the land from the SAR mode L1b waveform. This is expected to remove most of the land contamination from the DDA waveforms, thus enabling the application of ocean-like retrackers at much closer distances to the coast, regardless of the coastline/satellite track relative orientation. This technique has also promising applications for the cryosphere, as it can be used for the separation of the contribution of sea-ice freeboard and leads, as well as for in-land waters, where small water bodies act as perfect coherent scatterers.