| Abstract: |
A spaceborne imaging microwave radiometer system, having hitherto unseen performance, has been designed and proposed to ESA as the next generation ocean mission. The instrument focuses on C-band, which is important for sea surface temperature (SST), wind vector measurements, and sea ice parameters. The instrument is designed to fly behind MetOpSG, and the Microwave Imager will thus provide necessary data at higher frequencies. This means that we can utilize a large mesh antenna reflector that can be folded for launch (much like the SMAP). The baseline design uses a 5 m aperture providing a 20 km -3 dB footprint on the ocean surface.
The instrument is augmented by an L-band channel thus providing sea surface salinity (SSS) and thin sea ice measurements at 100 km -3 dB footprint.
Land/sea contamination (and sea-ice/sea contamination) has been a challenge ever since the start of microwave radiometry from space! The problem is the large brightness temperature contrast when passing over the coast (or ice) line in combination with realistic antenna patterns. No matter how well you design the feed horn it is just not good enough when considering this issue. The problem can be solved by using a focal plane array (FPA) as feed: many small feed elements (around 30) illuminate the reflector, and by properly adding the output of each element in amplitude and phase an almost perfect antenna beam can be generated. The challenge is that where a traditional system would have 1 radiometer receiver, the FPA based system will have 30 receivers, 30 fast A/D converters (the full RF bandwidth must be digitized), as well as significant and fast digital processing hardware – all on-board and real-time. Technology has developed such that this is now totally realistic.
Radiometric sensitivity also becomes an issue in high-spatial resolution, wide swath imagers. By using 2 simultaneous beams at each frequency, and the latest technology, a ΔT = 0.2 K can be achieved at C-band, and 0.10 K at L-band.
The new instrument, having a 5 m antenna rotating with 12 RPM, will from an 817 km orbit measure a 1524 km swath at 53° incidence angle, thus providing frequent coverage of the Earth (especially in ice infested Arctic regions). Two C-band channels at 6.9 GHz and 7.3 GHz will measure all 4 Stokes parameters with ΔT = 0.2 K (0.3 K for Stokes 3 and 4) and will provide meaningful measurements of the ocean as close to coast (and sea ice) as 15 – 20 km. This is just so much better than anything seen hitherto! Also cross-polarization issues are handled by the FPA concept, and the instrument will provide unsurpassed V & H-polarization purity.
RFI detection and mitigation will be handled on-board using the latest algorithms and developments. Members of the team are presently developing an RFI processor for MetOpSG. This processor is based on anomalous amplitude, kurtosis, and cross frequency algorithms. The processor for the new instrument will be augmented by polarimetric algorithms.
The instrument will have 33 dual polarized C-band elements hence 66 receivers, and 27 dual polarized L-band elements hence 54 receivers, as well as substantial FPGA hardware carrying out the beam formation and RFI processing. Using off-the-shelf commercial low-power components, the power consumption for the basic receiver and processing units is calculated to 318 W.
The instrument will provide unique all-weather sea surface temperature globally, i.e. also in cloudy areas like near equator and far north/south, as well as wind vectors under severe weather conditions. The measurements have excellent quality even very close to coasts and sea ice edges.
|
