The objective of this Polarimetric SAR Field Validation project was to collect ground "truth" data on selected features from the ENVISAT and CV-580 Polarimetric SAR as close to real time after the data acquisition as possible. The project was conducted during the period of February 27th through March 7th, 2003 with this being the first year of a potentially multi-year validation effort. This project was made possible by funding received from the Government Related Initiatives Program (GRIP).
With the launch of RADARSAT-1 in 1995, the Canadian Ice Service became the world's largest operational user of synthetic aperture radar (SAR) in their sea-ice monitoring of native waters. Spaceborne SAR data combines high spatial resolution with independence of cloud cover and light conditions, and it is possible to observe sea ice with better accuracy than visible and passive microwave methods.
RADARSAT-1 is an example of a SAR system which uses one single polarisation antenna for transmitting pulses and receiving their echoes. These were therefore called non-polarimetric systems. Polarimetric systems, on the other hand, are those which have the ability to transmit and receive pulses in both horizontal and vertical propagation. In non-polarimetric radar, the target is characterized by a single scalar; in a full polarimetric system like the future RADARSAT-2 the target is characterized by a complex matrix usually called the scatter (S) matrix or the coherency matrix. Some advantages of polarimetry have already been demonstrated. For example, choosing the correct transmit and receive polarizations can maximize the contrasts between targets and backgrounds. The accuracy of ice type and ice classification results increases.
The Advanced SAR (ASAR) on board of ENVISAT provides dual-channel data. In Alternating Polarisation mode (AP MODE), it provides one of the different channel combinations:
- VV and HH
- HH and HV
- VV and VH
Dual polarisation is important for a wide number of applications including sea ice studies. HH is very suitable for separating marine ice and water, since it is less sensitive to water roughness than VV polarisation producing an improved contrast between the two targets types.
There is considerable interest in the Alternating Polarisation Mode for sea ice applications. It is still unclear whether VV or HH polarisation is generally better for mapping sea ice. For example, one of the current problems using RADARSAT data at low incidence angle is that ice/water discrimination can sometimes be poor. Alternating polarisation HH and VV is expected to give improved edge/water discrimination. Cross polarization data is expected to be particularly useful for mapping ice topography, and is also likely to give improved type discrimination. Discrimination of ice/water is complicated by incidence angle and wind conditions, and is not always distinguishable with either HH or VV polarizations. The ratio of VV to HH seems to be very useful in discriminating ice/water since this ratio is larger to 1 for open water and close to 1 for pack ice independently of incidence angle or wind conditions.
It seems that the cross-pol channel (HV) provides good discrimination between the ice-edge and the sea, and the co-pol channel (HH) provides good ice-type information.
The impetus for this Envisat Validation experiment is to answer some of these questions. Analyzing the information collected should help us better understand the properties of sea-ice as they relate to polarimetric data.
