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Over the course of 30 years at the European Space Research and Technology Centre (ESTEC), Kees van ‘t Klooster was involved in antenna related subjects for Earth Observation and Science in space applications. His personal desire to document his work led him to pursuing a PhD at ºÚÁϸ£ÀûÍø.

BIOMASS

Earth observations can provide crucial and unique information about the planet from orbit. Global or regional applications can be addressed in the domains of meteorology, climatology, geology, seismology as well as effects caused by human activity on the planet.

The BIOMASS instrument, which is part of the ESA Explorer program, seeks to monitor the status of forests on the planet, whose status is influenced by both changes in climate and human activities. BIOMASS as an Earth explorer mission was launched in 2025.

Large reflector antenna

BIOMASS carries a unique synthetic aperture radar, which operates at a low frequency of 435 MHz A synthetic aperture radar, or SAR for short, is a radar system on a moving platform that use the motion of antenna to generate high-resolution images. In the case of BIOMASS, a single active transponder assists the mission for instrument characterization and calibration, from an ESA-location near New Norcia in Australia.

Large reflector antennas can be used as a radar target for low-frequency space-based SAR instruments, such as BIOMASS. It is important to know key details about the reflector antenna to provide predictions with electromagnetic tools. This can be used to derive an indication of the absolute level and a phase response (a stable response is most desired) of the mono-static backscattering. A stable phase response is most desired.

SAR interferometry

SAR interferometry techniques can benefit from phase-stable radar targets, a function which such large reflectors can possibly serve, provided they can be pointed towards the SAR satellite. This has motivated the research of Kees van ‘t Klooster into the behavior of the mono-static backscattering of large reflector antennas, when pointed towards a satellite with a SAR.

Reflector materials of such large antennas are assumed to be perfectly electric conducting (PEC) at the low SAR frequency. Measurements of reflector materials by Van ‘t Klooster confirm this assumption.  He also showed that one needs to know the precise configuration of the antenna to make predictions for the mono-static backscattering of pointed antennas. A loading of the antenna often has an impact. Van ‘t Klooster has suggested ways to reduce effects due to the antenna load in his work.

Spherical main reflector

Van ‘t Klooster also investigated an example of a less frequently encountered reflector antenna with a spherical main reflector. The mono-static backscattering from such an antenna led to unexpected results, predominantly determined by the compete spherical reflector, while an antenna function only uses a part of the spherical reflector.

SAR observations of Westerbork 25 m antennas and the Dwingeloo 25 m antenna have been initiated using the Sentinel-1 SAR satellite. The Sentinel-1 SAR operates at 5.5 cm wavelength; therefore, the mono-static backscattering response is too strong for accurate absolute results. However, the use of such SAR observations is initially informative and serves the observation process.

A start

A start has been made to use SAR interferometry, even for antenna reflection data which are too strong in response for the SAR. Van ‘t Klooster shows that there is strong correlation for antennas that were pointed similarly and almost no correlation for when antennas were pointed differently in the two datasets, as required for SAR interferometry.

The Sentinel Application Platform (SNAP) software is available for such processing approaches upon registration and can handle Sentinel-1 SAR data available from the Copernicus program. It illustrates also the rich possibilities available in general for other remote sensing applications.