Hornbuckle Group Research Infrastructure

A microwave radiometer is essentially an AM radio with very high amplification that is tuned to a single frequency. In our group we use radiometers to measure the natural emission of electromagnetic energy from the land surface. Wet soils appear "dark" or emit low amounts of energy, while dry soils appear "bright" and emit more energy. Vegetation alters emission from the surface and masks changes in soil radiation.

Even though these instruments have been in use for about four decades, microwave radiometers can not be bought "off the shelf" but must be custom built. In cooperation with the University of Michigan, we have built a dual-polarized L-band radiometer system. L-band is a group of frequencies near 1.4 GHz (gigahertz) that have a nominal wavelength of 21 cm. The radiometer system consists of a single dual-polarized horn antenna, a microwave receiver, control electronics, and power supplies. A picture of the radiometer system in the lab is shown at left. The radiometer's antenna and two electronics boxes attach to an axle that is rotated by a motor. We can deploy our radiometer on a variety of lift systems (see below) and eventually with two 10 m towers.

Cihan Erbas, a Ph.D. student in electrical engineering, spent January through May, 2005, at Michigan and assisted with hardware fabrication. She tested microwave hardware, including amplifiers, filters, and a high-speed A/D board, and tested low-speed hardware that control heating/cooling of the radiometer. She also assisted with characterization of the pattern of the horn antenna. A picture of the front-end electronics inside one of the boxes is shown at right. The system is controlled by a microprocessor and by a computer user interface. Nick Olson, a 2005 graduate in electrical engineering, designed and built the user interface with National Instruments LabWindows/CVI, a C programming environment with many pre-written GUI functions. Work on this user interface was completed Summer 2005. Cihan has now added components to this user interface that allow communication with the radiometer microcontroller

The system is now maintained by Matt Nelson, a staff member here at ISU how also works in the Spacecraft Systems and Controls Laboratory (SSCL). Matt also coordinates the work of both graduate and undergraduate students who also contribute to the radiometer system. At left, Matt is debugging a problem with the radiometer in the field.

We can deploy our microwave radiometer system in the field on a variety of lifts, and eventually on two 10 m towers. When integrated with a mobile boom lift, the lift becomes a research platform that can support the entire radiometer system during field experiments and point the antenna at any combination of azimuth and elevation degree. Additionally, the lift will allow us to make measurements at several points within a field. This is a new capability: in the past, only fixed points have been observed in ground-based microwave radiometry.

We have designed the system so that a number of different kinds of boom lifts that are available from rental companies can be used. One deployment configuration (lift with basket) is shown at right. Erik Kabela, a 2006 masters graduate in agricultural meteorology, is shown in the picture with the radiometer system.

Ryan Burnley, a 2005 graduate in mechanical engineering, led a team of undergraduates that designed a mechanical structure that connects our radiometer system to a lift. The design allows the radiometer to rotate from 0 to 180 degrees so that all incidence angles of interest are available, and the sky can be used to calibrate the radiometer. Ryan completed fabrication of the design during Summer 2005. Nick Olson, a 2005 graduate in electrical engineering, designed and built a system to electronically measure the three-dimensional position of the antenna so that measurements taken at many different points in a field during an experiment can be geo-referenced. At present the system consists of a GPS receiver with WAAS technology, along with a magnetometer and an inclinometer that measure the azimuth and elevation angle of the antenna.

We have obtained a trailer to transport our equipment and to function as a mobile field laboratory during experiments. The trailer has a desk/workspace, an internal power supply (a gasoline generator), external power connections, and an air conditioning system. Erik Kabela coordinated the effort needed to obtain this important piece of equipment. Cihan Erbas is pictured with the trailer.