Research at MWGP
The Microwave Geophysics Group (MWGP) at the University of Michigan studies Earth surface hydrology, both through Land Surface Process (LSP) models and radio-brightness field experiments. This is an important research area because, for example, latent energy transport associated with soil moisture is a dominant summer time land-atmosphere process governing weather and climate. A better understanding of the LSP interactions such as net energy flux at the air/soil interface can significantly increase the precision and relevance of weather and climate prediction which is of significant economic and societal importance.
To explore and refine the land surface process models, the MWGP group is involved in numerous field experiments and measurements. It is relatively easy to take a sample of soil and measure its water content in one place, but for the LSP models to be useful, data is needed continent wide. This makes a remote sensing application desirable
Remote-sensing hydrology takes advantage of the fact that upwelling microwave radiation from the Earth is reflected by moisture in the top layer soil. This happens because water has such a high dielectric constant compared to the soil. As a result, measured radio brightness at specific frequencies decreases. With a sound understanding of the physics, the radio-brightness can be correlated to soil surface moisture content.
A microwave radiometer is a device, much like an expensive AM radio, that receives the radiation at a certain frequency and measures it to determine the radio-brightness. MWGP builds a number of different microwave radiometers to measure various land surface characteristics used to create and verify land surface processes models and to calibrate airborne and satellite radiometer instruments. Different frequencies and polarizations can used to make different measurements, such as investigating the effects of vegetation.
One of the ongoing projects of MWGP is STAR-Light. STAR-Light is a 1.4GHz Scanning Thinned Array Radiometer that is to be used on a light aircraft for studying land-surface hydrology. The STAR technique uses multiple receivers with a low noise homodyne RF front end with a direct sampling digital back-end. The digital signals are cross correlated and used to generate a 2-D radio brightness map. Each radiometer receiver is contained in a small hex-wing case roughly 6"x9".
The other ongoing project, which I was hired for, is the repackaging of the Truck Mounted Radiometer System and Tower Mounted Radiometer System into TMRS-3. TMRS-3 is currently in the field in Frasier, Colorado as part of the Cold Lands Processes Experiment (CLPX). The TMRS-3 system is quite comprehensive:
- L-band (1.4 GHz) Dual-Polarization Dual-Receive Radiometer with STAR-Light Front-ends
- C-band (6.7 GHz) Dual-Polarization Dual-Receive Radiometer with STAR-Light Front-ends*
- 19 GHz Dual-Polarization Single-Receive Radiometer
- 37 GHz Dual-Polarization Single-Receive Radiometer
- IR Radiometer/Pyrometer
- Black & White Video Camera
- Norstar utility truck with articulated boom
- Data collection/System control computer
* The C-band radiometer is currently undergoing an upgrade to a digital correlating radiometer.
The L-band and C-band radiometers use STAR-Light front-end components but have no direct digital sampling in place. When the funding for STAR-Light becomes available to continue development, the analog sections will be replaced. Then, the L and C-band radiometers will be fully polarimetric.
In addition to the radiometers required by the full TMRS-3 system, one more C-band radiometer is being constructed. This radiometer is Dual-Polarization but only Single-Receive using only one STAR-Light front-end. It is being constructed under contract for Jasmeet Judge at the University of Florida. Unlike our TMRS-3 system, which needs to survive arctic conditions to -40, this radiometer must survive the Florida heat.