Iceberg detection using the three modes of SIRAL on Cryosat
Event: 2015 Ocean Surface Topography Science Team Meeting
Session: Science III: Large scale and global change ocean processes: the ocean's role in climate
Presentation type: Type Poster
Contribution: PDF file
Its primary goal is to monitor the thickness of land ice and sea ice and help explain the connection between the melting of the polar ice and the rise in sea levels and how this is contributing to climate change. The main instrument is the SAR/Interferometric Radar Altimeter SIRAL-2
It operates in threee modes:
-Low-resolution mode (LRM) like a conventional altimeter, on land ice or sea which are composed of few rough surfaces,
-SAR mode (Delay Doppler Altimetry) operating a high resolution measurement on sea ice,
-SAR interferometer (SARIn) mode operating on rough surfaces like on the sea ice/land limit.
This three modes capabilities allows to investigate in details the advantages of each mode for the detection of icebergs and ships. The method of detection of icebergs using conventional altimeters data was developed byDelay Doppler Altimetry (DDA), proposed by R.K. Raney (1998), offers improved altimetric precision and better along-track resolution than conventional pulse limited altimeters. DDA altimeters have a high pulse repetition frequency to ensure pulse-to-pulse coherence, leading to an along-track resolution about 300 meters, improved signal-to-noise ratio and enhanced altimeter ranging performance. The delay doppler maps, the beam stacking capabilities and the improved along-track resolution offers new possibilities for the detection and the determination of the characteristics of both icebergs and ships. Using Cryosat SAR L1b and L2 date, a method of detection of target emerging the sea has been developed. This method is based firstly on the analysis of the pseudo LRM waveforms obtained without stacking. Within these waveforms, the signature of a target emerging from the sea is a parabola that can be easily detected using the method developed by Tournadre et al (2008) for classical pulse limited altimeters. It should be noted that within the stacked waveforms the signature is bright spot that can also be easily detected by classical image processing methods. Using the estimate of the iceberg's position the Delay Doppler Maps (DDM) at 85 Hz are then used to estimate the area of the icebergs by averaging the DDM taking into account the iceberg displacement in both range and doppler. This allows to compute an image of the iceberg backscatter at high resolution. The results are then compared to the SAR (refocused) waveforms to estimate the precision of the area estimate using the waveforms alone.
The new ESA SARVATORE project nows allows to obtain full resolution stack data. These data include more than 300 range bins before the mean sea level that can be exploited to improve the iceberg and ship detection by increase more than twofold the detection swath of the altimeter. Several examples of such detection of icebergs and ships are analyzed and presented.
DDA altimetry offer improved capabilities of both iceberg and ship compared to classical altimetry as it is demonstrated using a limited Cryosat SAR dataset. In the perspective of Sentinel 3 it can be of importance for the scientific community whose interest for icebergs and its impact on both the southern ocean circulation and ecosystems has increased during the recent year to develop an operational processing chain design to detect icebergs.
The SAR interferometer mode uses the phase difference between returning radar waves: in order to measure the arrival angle, a second receive antenna is activated so that the radar echo is received by two antennas simultaneously. When the echo comes from a point not directly beneath the satellite there is a difference in the path length of the radar wave, which is measured by the phase difference. Simple geometry provides the angle between the baseline joining the antennas and the echo direction. An iceberg being a target emerging from the sea surface its echo lies within the noise part of the waveforms where the power received by the two antennas is incoherent. The phase difference has thus a zero mean as well as the coherence. An iceberg can be simply detected using either the parabola detection in pseudo LRM or the bright spot detection within the stacked waveforms. The signature are also characterized by a high (>0.6) coherence between the two antennas and a non zero phase difference. The phase difference is then converted to incidence angle using the interferometric equation. Knowing the satellite altitude the incidence is converted in distance from nadir. As the range of the iceberg's signature (i.e. the range bin where it is detected) depends on the distance from nadir and the freeboard elevation, it is possible to compute the icebergs freeboard. As the DDM maps are also available he surface of the icebergs can also be estimated using the DDM. SARin allows to estimate all the iceberg's characteristics: surface, freeboard, surface backscatter with a better precision than any other sensor.