The radar that is used in DRYMON is the so-called Wind Scatterometer on the ERS-satellites for the period 1991-present, and the ASCAT sensor on the MetOp satellites (2006-2020). The 'land' archive was by chance built as the ERS satellites were designed for typical 'sea' observations, and it was too costly to turn off the instrument when it flew over land. The ERS-2 and METOP satellites orbit around the Earth at 800 km altitude. The satellites fly from pole to pole while the Earth turns underneath, so that every spot on Earth can be observed.
A radar (RAdio Detection And Ranging) emits and receives a signal on the antennae, so it can also work at night. Furthermore, clouds are no obstruction for the radar signal, as clouds act as a transperant medium for the frequency used (i.e., C-band radar). The SCAT and ASCAT have large antennae consisting of the three panels illustrated in the DRYMON-logo. While flying over an area, this area is illuminated and than observed by the aft- central- and forward looking panel of the instrument: Each antennae looks at the area from a different angle. This permits to separate influences from vegetation and soil moisture in the signal. Take the example of the spike bed. If one looks at it vertically, it is hard to be aware of the danger of sitting on it.
When the radar signal has been received still a large number of complex steps have to be taken to arrive at the information that is useful to use as input for yield forecasts and drought assessments. In summary these steps are:
- Retrieval of information from the soil surface.
- Moisture in the whole soil profile.
- Moisture and crop performance modelling.
As any validated method DRYMON has its limitations.
Of course there are alternatives to obtain information not dissimilar to the DRYMON info. However if these alternatives are analysed, these methods have large disadvantages in comparison with DRYMON. .