Marshall Airborne Polarimetric Imaging Radiometer (MAPIR)


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The Marshall Airborne Polarimetric Imaging Radiometer (MAPIR) is a dual beam, dual angle polarimetric, scanning L band passive microwave radiometer system developed by the Observing Microwave Emissions for Geophysical Applications (OMEGA) team at MSFC. MAPIR observes naturally-emitted radiation from the ground primarily for remote sensing of land surface brightness temperature from which we can retrieve soil moisture and possibly surface or water temperature and ocean salinity.

MAPIR consists of an electronically steered phased array antenna comprised of 81 receiving patch elements and associated electronics to provide the required beam steering capability. The antenna produces two independent beams that can be individually scanned to any user-defined scan angle. The antenna is connected to four microwave radiometers and a microwave spectrum analyzer. Two radiometers operate over a narrow band (science band) between 1400-1427 MHz. Two other radiometers operate over a wider bandwidth (1350-1450 MHz) and are used for Radio Frequency interference (RFI) surveillance. The outputs of the four radiometers are routed to the digital back end module that digitizes and filters the signal into 16 well isolated spectral sub-bands and computes the first four statistical moments in each sub-band from which the radio brightness temperature and kurtosis (a statistical measure, indicative of RFI) can be computed in post-processing.

MAPIR can operate in two user-selectable modes: Single-Beam Dual (simultaneous) Polarization and Dual (simultaneous) Beam Single Polarization. In the first mode, both beams of the antenna are directed to scan to the same angle, but the radiometers are observing orthogonal polarizations (horizontal and vertical) at the same time. In the second mode, the two antenna beams can be directed to different azimuth and/or angles and the radiometers observe the same polarization at the same time. The instrument is capable of electronic beam steering to one-degree of resolution from 0-40 degrees in elevation and 0-360 degrees azimuth in both beams. MAPIR precision is 0.01K and brightness temperature accuracy is 5 degrees K accuracy over a 10 ms integration interval, but is capable of achieving 0.5K sensitivity over a 1 second integration interval. Near-term improvements to MAPIR will bring that accuracy to 3 K over a 10 ms integration period.

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