August 2, 2016

Instrument

A Microwave Radiometer using Aperture Synthesis:
Instrument Principle

The spatial resolution requirements for passive imaging from space of soil moisture and ocean salinity at L band ( = 21 cm) leads to large antenna apertures for which thinned arrays using synthetic aperture principles are the only valuable candidates by comparison with mechanically and electronically steered antennas and push brooms.

A two-dimensional interferometer approach for Earth observation has been studied in several cases including the ESA SMOS contract, showing a drastic stowed volume improvement by comparison with a classical radiometer. In the early phase of study, the T geometry, suitable for space platform accommodation, was merged with the Very Large Array approach (Y shape exponential distribution of feeds), leading to a Y shape with equally spaced antenna elements. That structure has many advantages in terms of ground resolution, and is suitable for space-borne instrumentation (deployment).
The instrument is tilted by 20-30° with respect to the nadir to guarantee an incidence angle range within [0°, 55°]. The principle of this instrument is to sample the Fourier transform of the scene. To do so, a set of elementary antennas are regularly distributed on a Y structure. The correlation between the output signals of all the couples of antennas gives an estimator of the visibility function. These visibility functions are transmitted to the ground where—long story short—a pseudo inverse Fourier transform is performed to retrieve the brightness temperature image.