Multi-Beam Passive Microwave Radiometer Design using Multi-Path Cross Correlation Radiometry

Orbital Micro Systems, Inc. (OMS) proposes “Multipath Cross-Correlation Radiometry” (MXCR) as a new means of continuous multi-beam radiometer “self-calibration” that will obviate the need for conventional switched calibration looks, whether from internal noise sources or external blackbody targets. It will simultaneously implement multiple independently radiometric antenna beams that provide a simultaneously sampled push broom imaging capability, thus reducing or eliminating mechanical scan requirements. Importantly, MXCR will facilitate high spectral resolution (0.1-1 MHz bandwidth channel) radiometry over wide (multi-GHz) bands with rejection of out-of-band radio frequency interference due to it’s a unique path gain cancelling inversion algorithm that is unavailable using conventional single-path or two-path correlation radiometry. As a result of this algorithm, 1/f gain fluctuations are entirely cancelled out of the inversion process. This specific study address the implementation of several simultaneous radiometer antenna beams using FPGA-based MXCR detection architecture.

Radiometer receiver gain and offset fluctuations that produce striping errors at the level of 0.1-1K or more preclude the use of satellite radiometers for many climate monitoring purposes. Such fluctuations result from calibration looks that are too infrequent to accommodate the gain and offset fluctuations for many MMW and SMMW receiver technologies, specifically InP and GaAs LNA-based receivers. In many such spaceborne radiometers (specifically, ATMS and GMI) calibration looks are accomplished only approximately once per second due to scanning mirrors that are limited in the angular velocity at which they can be spun. Moreover, the calibration looks typically last only tens of milliseconds, thus preventing long enough calibration views to lower levels of ΔTrms for accurate estimation of gains and offsets. Such views of thermal targets also do not address gain compression that can occur when observing scene temperatures with large spectral excursions, particularly those caused by radio frequency interference (RFI). Finally, such views of thermal blackbody targets and/or cold-space reflecting splash plates necessarily occur with changes in near-field antenna geometry that can result in 50-200 mK errors due to feedhorn standing wave effects. These problems have limited the absolute traceable precision of spaceborne earth remote sensing radiometers – which are inherently mechanical scanning instruments and thus do not permit long integration times – to no better than typically 100-300 mK at best. Errors of this order require on-orbit bias correction to be used for numerical weather forecasting purposes and are entirely too large for traceable measurements of climatologically critical variables such as mid-tropospheric temperature. MXCR radiometry has a wide range of applications including satellite and ground based weather instruments, electronic test equipment, medical devices, jam resistant RF receivers for a variety of applications, and potential planetary science missions.

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