Issues of technical implementation of ground-based microwave radiometric system calibration

The theoretical bases of a method of carrying out calibration of microwave radiometric system on a noise signal of the external noise generator are analysed, the parameters setting value of gain of antenna temperature at introduction in structure of system of such external source of a calibration signal are determined. The questions of technical realization of an external source of a calibration signal for microwave radiometric system are considered, the structural scheme is presented, the place of its arrangement in structure of the antenna is determined. The necessity of using a horn antenna to create directional radiation towards the mirror antenna emitter is shown. The data on the design of such a calibration signal source for the case of its use in a three-band microwave radiometric system with the reception of a noise signal at the common antenna aperture and a sequential separation of the input signals of the three bands in the feeder path of the common mirror irradiator are presented. The influence of the design and, accordingly, the directional properties of the horn antenna of a microwave radiometric system calibration signal source on the coefficient characterizing the radiation divergence created by the noise generator horn, as well as the frequency properties of this coefficient are analysed. The results of measurements of radio-thermal radiation by a three-band microwave radiometric system when two calibration periods with reception of a radio-noise signal from an external calibration signal source located at the base of the antenna mirror are presented.

Calibration issues for a three-band microwave radiometric system with background noise compensation

For microwave radiometric remote sensing systems in natural environments, the important issue is calibration, i.e. determination-one correspondence of values of the output signal value of the brightness temperature of the sensed region, the largest of which evaluated physical parameters. Microwave radiometric systems operate under the influence of external interference, for example, background noise – radiothermal radiation of the space surrounding the system, received through the scattering region of the antenna pattern. This article examines the influence of background noise on the external calibration of a microwave radiometric system (based on an external “deterministic” noise source) and analyzes the conditions for performing such calibration in a system with compensation for the influence of background noise. The analysis of the output signal of the microwave radiometric system showed a significant influence of background noise on the parameter determined during calibration of the system – the beginning of the receiver scale, due to a change in the value of the contribution of background noise to the output signal when forming two reference levels during calibration from two angular directions. The possibility of reducing the interference effect of background noise on the results of measurements in a microwave radiometric system with a special two-channel antenna operating on two modes of a circular waveguide, with the formation of an additional compensation signal at the antenna output, is shown. The proposed version of the analytical evaluation of the degree of compensation of the influence of background noise on the output differential signal of the system and presents results of its application to numerical analysis of error in external calibration a tri-band microwave radiometric system with compensation of background noise when receiving thermal radiation onthe total aperture of the antenna.