Thursday, August 20, 2009

Staying coherent - come rain come sunshine

SAR is widely touted as an all-weather system, but weather can limit the applicability and vastly affect the interpretation of SAR data. Not only is tropospheric weather we are familiar with the cause of concern, at longer wavelengths ionospheric effects and Faraday rotation can become an issue, impacting polarimetry.
One of the impacts moisture in the atmosphere is attenuation and path length extension as the electromagnetic properties of the transmission medium change. This can impact interferometric applications.

In addition to instantenous effects on the wave propagation, weather be it wind, rain or may be a long spell of dry weather can have complex effects on the target backscatter. Grass may dry up and disappear from view. Canola crops and trees may wave and become motion blurred ( saw some marvellous examples of that at IGARSS). Rain and hail may saturate the ground making things suddenly darker/brighter. In such cases direct comparison even accurate coregistration becomes difficult. Theoretical modelling and experiments put the wind based decorrelation of trees to a few tens of milliseconds, putting limits on integration time achievable.

All this makes repeat pass interferometry very fickle just like the weather. Coherence can only be achieved in very limited areas.

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