| Popis: |
Radiometry is the science of measuring the intensity of electromagnetic radiation, and a radiometer is the instrument that makes these measurements. The amount of energy radiated by a material is dependent on several characteristics of the material, so the measured radiation can be inverted to remotely sense information about an observed scene. Radiometers are generally less expensive and less intrusive than other sensors because they do not require transmitting a signal to operate. However, widespread use of electronic devices (both wired and wireless) creates sources of radiation that interfere with making accurate measurements, known as radio frequency interference (RFI). Historically, radiometry has been performed in narrow bandwidths that are protected from RFI by governmental frequency allocation. Even these frequency bands see interference due to a combination of illegal and unintentional emissions, leading to the research and development of software algorithms that can identify and ignore radiometer data contaminated with RFI. Advances in RFI filtering algorithms and analog-to-digital conversion technology have made it more feasible to operate radiometers outside the traditional protected bands, enabling more information to be derived from radiometric measurements. The Ultra-Wideband Software-Defined Microwave Radiometer (UWBRAD) is a radiometer that operates from 500-2000 MHz and was developed at The Ohio State University to begin exploring the challenges and possibilities of performing radiometry in completely unprotected bands. This research covers several aspects of the UWBRAD instrument. UWBRAD uses a novel design that required analysis to determine how to calibrate the instrument and how to perform RFI mitigation without falsely flagging radiometric data. Three different flight campaigns were conducted in the polar regions of the Earth, and the techniques developed were used to calibrate and analyze the data. The brightness temperatures obtained demonstrated the potential of wideband radiometry and are presented here. Using the developed RFI mitigation algorithms, a characterization of the RFI environment encountered during the campaigns is presented, providing a key resource describing the noise environment that can be expected for future radiometers. Finally, an examination of how RF reflections internal to the instrument may contribute to calibration errors is presented. This research has demonstrated that 1) the calibration and operation of UWBRAD is sufficient to perform accurate radiometric measurements, 2) the RFI environment encountered in high latitude regions of Earth in the majority of the 500-2000 MHz portion of the spectrum appears to be sufficiently quiet to perform radiometry if appropriate RFI mitigation is included in the design, and 3) multiple precautions that are generally not applicable to narrowband radiometers must be used in the design and calibration of radiometers when operating in unprotected bands and over wide bandwidths. |
