On-board Calibration Of Microwave Aperture Synthetic Radiometers Based On External Methods

As a new array interferometric microwave radiometric imaging system, microwave aperture synthesis radiometer (ASR) forms a large physical observation aperture by a thinned array of small-aperture antennas. ASR has the advantages of high resolution, reduced antenna size and weight, and can instantaneously imaging for the entire field of view (FOV) without scanning. This makes the ASR a promising technique in the measurement from the space of some geophysical parameters of crucial importance such as ocean salinity and soil moisture. The concept has aroused wide concern in spaceborne applications of earth observation.For the obtaining of high quality image, the calibration of ASR is a major problem and crucial technique. So far the on-board calibration relies on internal noise injection; for example, the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) and GeoSTAR which is a microwave sounder for weather temperature and humidity; this method obtains receiver parameters in real-time and has high accuracy. However, noise injection needs a distributed network. It is difficult from the viewpoint of mass, volume and system complexity, and unlikely feasible for ASRs with large arrays with high resolution. Moreover, it can only deal with receiver imperfections and can not deal with the imperfections of the antenna array. For example, the displacement of the ideal antenna positions due to the mechanical distortion of antenna arms. Therefore, the idea of external calibration is proposed in this paper.According to the radio regulations from International Telecommunication Union (ITU), to avoid the interference between satellites, the emission at the frequency of microwave band is not allowed. Thus, the absence of normal external calibrator is the main difficulty and challenge we are facing and the reason that external calibration has not been taken in on-board calibration. In this paper, novel external calibration methos have been proposed to deal with the specific condition.(1) The external calibration method based on celestial body. Stars can be treated as a kind of natural souces, they have the advantage of known direction and high radiation intensity; moreover, its cooperation with the background of sky can deal with the problem of absolute temperature calibration in microwave images of ASRs. The capture of the star is the difficulty this method may be encountered.(2) The self calibration method based on antenna array rotation. This method makes use of specific antenna placement before and after the rotation to construct redundant baselines and carry out the calibration. It does not rely on any reference sources and uses only the observation scene itself. The implementation is easy and costless. In addition, reasonable array configuration can reduce the number of antenna elements as well. In this way, the system complexity of ASR can be further reduced. The loss of integration time and the accumulation of phase error are the costs this method would pay for.(3) A combinational calibration method based on various scenes. In this case, the satellite carrys near field artificial cource on its own for the purpose of calibration, the position of the source is precisely calculated by the measurement to Earth scene and by source rotation. The method makes full use of available external reference signals in calibration and has high calibration accuracy.The on-board calibration methods proposed in this paper have been proposed though rigorously deduction and tested in simulations and on-ground experiments. They are promising in practical utilization.