Instruments

Hydros instrument makes a conical scan at constant incidence angle over a wide swath.

The radar and radiometer systems will share a lightweight, deployable mesh reflector. A feedhorn will be used with the shared aperture to form an offset beam with a surface incidence angle of 39.3 deg.

The antenna system will operate at 1.26 GHz for the radar and 1.41 GHz for the radiometer and will be capable of both horizontal and vertical polarizations. The reflector will rotate about the nadir axis at 14.6 rpm to provide contiguous coverage over the 1000-km swath.

A 6-m reflector will be required to produce the radiometer footprint of approximately 40 km (root ellipsoidal area), where the resolution is defined by the antenna 1-way 3-dB beamwidth. Similarly, the radar 2-way 3-dB real aperture footprint will be 30 km.

The antenna reflector will most likely employ either a folding radial rib or perimeter truss wire-mesh design. Both are high heritage space-proven designs which will result in a reflector surface that meets or exceeds the performance requirements at the desired L-band frequency. A selection decision of the final reflector design will be made early in the Hydros mission formulation phase.

To obtain the required 3-km x 3-km and 10-km x 10-km resolution geophysical products, the radar will employ range and Doppler discrimination. While similar to standard synthetic aperture radar (SAR), the aperture length of Hydros will be quite short (32 ms), simplifying the processing. Due to squint angle effects, the high-resolution products will not be achieved within a 300-km swath region centered on the nadir track. Due to the slowly precessing orbit, those regions will be imaged at the full resolution on later passes.