During a research visit to the University of Alaska Fairbanks (UAF), Anthony Doulgeris (member of the University of Tromsø EO Lab) acquired some ALOS PALSAR L-band polarimetric radar images of the sea ice near Point Barrow on the northern coast of Alaska, where he had taken part in concurrent field work. To his disappointment, he discovered that the images were subject to severe noise, which rendered them unusable for the purpose of sea ice mapping. There were large intensity variations across the image and the polarimetric information was clearly disturbed, resulting in RGB images with a psychedelic hew.
After some investigation, it became evident that the noise came from military over the horizon radars belonging to the North Warning System, that happen to transmit at the same frequency. Further enquiries to JAXA (the Japanese Space Agency, who owns the ALOS satellite) identified the intensity variations as an artefact of a noise filter implemented in the standard ALOS processor, which apparently did not handle this particular kind of interference very well. It was further recommended that the problem should be resolved on raw data level.
Franz Meyer, a research professor at UAF, took on this job. He implemented a filter which matched the characteristics of the observed interference: high power, wide bandwidth, short time extent pulses whose center frequency changed randomly with time. In a common conference paper, Doulgeris and Meyer then demonstrated the successful recovery of the polarimetric L-band images, that were now fit for the purpose of sea ice analysis. Meyer further went on to produce software which logs the location, strength and time-frequency characteristics of the inference detected and processed by his filter. This software is now being used to reprocess the Alaska Satellite Facility’s entire archive of ALOS PALSAR data. When completed, the result will be a map of radio frequency intererence at L-band for the American continent.
The whole development is described in a recent paper presented at the 9th European Conference on Synthetic Aperture (EUSAR 2012), authored by Meyer together with Jeremy Nicoll (UAF) and Anthony Doulgeris. For this contribution, they received the Best Paper Award. Meyer takes a modest stand, claiming that technical part of the work was not very advanced. However, the award can also be seen as a recognition for making efforts that go beyond the immediate interest of their own work, and for providing information that is of high value when planning the design and use of future L-band missions, as well as highlighting the important battle over the electromagnetic spectrum and its allocation to remote sensing.