Measuring Earth from afar: Microwave radiometers

04 Feb 2022

Satellites in low orbit, as opposed to those in geostationary orbit, inherently move with respect to Earth and therefore constitute a viable way for monitoring and measuring the state of our planet. Instruments onboard these so-called LEO satellites may include microwave radiometers that act as remote thermometers of brightness temperatures, giving insights into oceans, atmosphere, sea ice, forestation, among others.

A key element of a radiometer is its antenna. While the electrical size of the antenna determines the spatial resolution on ground, the accuracy of the measured brightness temperature is controlled by the sidelobes and cross-polarisation of each antenna beam.

In 2012, TICRA won a contract with the European Space Agency (ESA) to make, for future ocean observation missions with stringent requirements, an initial assessment of two types of radiometers: a conical scan and a pushbroom radiometer. Together with DTU Space, Chalmers University of Technology and HPS, it was found that a 5 m-diameter deployable reflector antenna was needed to obtain the required spatial resolution, while the required beam purity (low sidelobes, low cross-polarisation), which is especially important in vicinity of the coastline, could only be achieved with a multi-feed-per-beam configuration. Here, each beam is generated by a number of electrically small feed array elements, properly excited in amplitude and phase.

In a subsequent ESA project, started in 2016 also with TICRA as prime contractor and with DTU Space and Chalmers University, the multi-feed-per-beam focal plane array concept was further matured, and a proof-of-concept prototype was designed, manufactured, and measured.

The prototype consisted of 67 densely packed Vivaldi antenna elements on a ground plane, which was simulated by the full-wave solver of TICRA’s ESTEAM software to account for mutual coupling and the finite size of the ground plane. A number of array element patterns were finally measured at the DTU-ESA spherical near-field antenna test facility in Denmark. The correlation between the predicted and measured antenna patterns was excellent, and subsequent computations showed that ESA’s stringent ocean observation requirements were met thanks to the designed Vivaldi focal plane array.


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