Offset Tracking with TerraSAR-X Data

I tried generating a coherence image, but there was no coherence over the glacier at all. This glacier flows at up to 200 metres a year so I guess that the maximum displacement on a set of images with 11 days seperation would be around 6 metres. I get OK results over the glacier tongue (see screenshot) but further up the glacier I have a region of maximum velocity followed by a region of zero velocity.