The Advanced Scatterometer (ASCAT) will make use of external calibration targets to allow the generation of radiometrically calibrated scattering coefficient data. These external targets will take the form of well-calibrated and stable transponders located at various points on the ground.
Ground reflections (clutter) can interfere with the return signal from the transponder. To overcome this, the transponder has a time delay. This means that the transponder echo is only present after the ground clutter has decayed. A special calibration mode is used in the instrument in which echo data is collected in the second pulse repetition interval after pulse transmission, instead of the usual first. Like the real target, the transponders will need to appear as large radar targets, this means that their internal electronics need to have a high gain, which may be temperature sensitive. An internal calibration loop is used to stabilise this gain. The transponders will track the satellite during its overflight and will allow the fore, mid and aft beams to be calibrated in sequence.
MetOp's orbit results in a large number of closely spaced ground tracks. Calibration of the instrument can therefore be made at closely spaced intervals across the instrument swaths. This means that the complete external calibration process can be accomplished in about one month.