Sensitive Microwave Radiometer for Remote Sensing of Soil Moisture and Sea Surfaces

A XRM type of X-band modulation radiometer was developed by utilizing satellite receivers’ parts under the Bulgarian-Vietnamese government program framework for collaboration in technology and science known as ‘Design and development of a Dicke-type microwave X-band radiometer and its utilization for environmental investigations’ research project’s component between the Hanoi-based Vietnamese Academy of Science and Technology’s Institute of Space Technology and the Institute of Electronics of the Bulgarian Academy of Sciences (IE-BAS).

The IE-BAS had created several low-priced microwave transmitters for Ku and C bands utilizing satellite TV receivers’ commercially available components in the past decade. The IE-BAS team was the pioneer in investigating this domain during 1997. This research was driven by the requirement for reducing the radiometer’s cost, and rendering them more affordable for the developing nations’ researchers.  The field and laboratory results in Vietnam and Bulgaria have verified the suitability of these radiometers for natural objects’ investigations under controlled situations. These radiometers are low cost and offer good technical parameters as compared to traditional radiometers.

The radiometer is a very sensitive microwave receiver of noise emitted by different natural objects. It consists of an info and a microwave receiver unit. A pyramidal horn antenna at the radiometer’s input of the radiometer is focused on the investigated object.  The front-end of the radiometer in the microwave unit incorporates customary X-band waveguide parts. The modulated signal’s down-conversion is done by using satellite TV receivers’ parts, namely a 30MHz tuner and 0.5 dB noise figure low noise block converter for the Ku-band.

Particular focus is given to reducing the effects of radio frequency by radiometer center frequency tuning and a band pass filter. The law frequency signal is processed by special electronic circuits in proportion to object’s brightness sensor.

A microcontroller forms the info unit’s main block. Special software has been created for data acquisition and processing, besides generation of different commands based on the operational mode selected.

A common thermo-stabilizer that maintains a constant 50°C temperature helps to realize the microwave unit’s temperature regime. A reference temperature component to maintain a nearly 55°C temperature independent of the microwave unit’s temperature regime variations is also implemented. The recommended temperature regime enhances both short and long term stability of the radiometer. The measured resolution of the radiometer complies with monitoring and remote sensing needs of natural ecosystems and objects. The developed radiometer is a highly sensitive microwave receiver of the noise emission from different natural objects.