Speaker
Description
Spaceborne imaging microwave radiometers for Earth observations can determine a multitude of properties of land and oceans, by measuring the brightness temperature emitted by the Earth. For ocean observations, ocean surface wind vector, sea surface temperature and sea ice characteristics are important parameters, normally measured in C band, at 6.9 GHz.
The above instruments are defined by specific radiometric requirements on spatial resolution (also known as footprint), radiometric resolution and accuracy. An important characteristic for radiometers for ocean observation is additionally the so-called distance to coast, which quantifies how close to land and sea ice the radiometer can accurately measure. Radiometric requirements can be converted into antenna requirements: the spatial resolution determines the reflector antenna projected aperture, while the radiometric resolution determines the number of beams generated by the antenna in the along track and across track direction. Accuracy and distance to coast set a limit on the maximum cross-polar power that can be received by the antenna, and on the sidelobes of the antenna beams illuminating the land, respectively. An accuracy of 0.25 K implies a maximum value of cross-polar power on the Earth smaller than 0.29% of the total power. An accuracy of 0.25 K and a distance to coast of maximum 20 km at 6.9 GHz, as required by future missions, cannot immediately be satisfied by traditional conical-scan or wide-scan push-broom reflector antennas in single-feed-per-beam configuration, due to the non-negligible cross-polar power of the antenna beams and unless the footprint is sacrificed and accepted being larger than 20 km.
The challenging requirements can however be met in a multi-feed-per-beam configuration, where more feed array elements take part in the formation of each beam, and the same element takes part in the formation of multiple beams. This is realized in practice by a feed array with elements placed less than one wavelength from each other, properly excited in amplitude and phase, and connected to a digital beamformer.
In this work, we have focused on a 5 m conical scan radiometer working at 6.9 GHz, and designed its feed array in a multi-feed-per-beam configuration, in order to achieve 20 km of spatial resolution, 0.3 K of radiometric resolution, 0.25 K of accuracy and less than 20 km of distance to coast. The feed array is analyzed in detail in two commercial software, i.e. the MoM add-on to GRASP and CST, including mutual coupling between the array elements and the effect of a finite ground plane, and is later manufactured and tested. The work constitutes the proof of concept and generalization of a previous TRP activity “Study on advanced multiple beam radiometers” ESA contract 4000107369/12INL/MH, where the electrical model of the feed array did not include mutual coupling and edge truncation effects, and the optimization algorithm used to find the proper array excitation thus considered identical element beams as input.
| ESA Technical Officer | Benedetta Fiorelli |
|---|
