GaoFen-3, the first polarimetric SAR satellite of China, carried out polarimetric calibration experiments using C-band polarimetric active radar calibrators (PARCs), trihedral corner reflectors (TCRs), and dihedral corner reflectors (DCRs). The calibration data were firstly processed referring to the classic 2 × 2 receive R and transmit T model for radar polarimeter systems, first proposed by Zebker, Zyl, and Held, and Freeman’s method based on PARCs, but the results were not good enough. After detailed analysis about the GaoFen-3 polarimetric system, we found that the system had some nonlinearity, then a new imbalance parameter was introduced to the classic model, which is equivalent to the γ proposed in Freeman’s paper about a general polarimetric system model. Then, we proposed the calibration data processing algorithm for GaoFen-3 based on the improved model and obtained better results. The algorithm proposed here is verified to be suitable for GaoFen-3 and can be applied to other spaceborne and airborne fully-polarimetric SAR systems.

The parameters for polarization distortion of spaceborne polarimetric synthetic aperture radar (SAR) have range-dependence (or look-angle-dependence), which requires a polarimetric calibration to be performed at any look-angle. It is a huge endeavor to rely solely on ground experiments to obtain a polarimetric calibration at all look-angles. For SAR with phased array antennas we deduce, based on the model for the general polarimetric system, the model for fine polarization distortion described by the parameters of the radar device under the condition of high polarization isolation. We point out the mechanism that causes both variable and constant polarization distortions, and we deduce the correction algorithms for the two types of polarization distortion. Then we propose a polarimetric calibration scheme combining internal and external calibrations to calibrate the two types of polarization distortions for SAR with phased array antennas. The scheme uses the internal calibration data of the radar and the model of the antenna pattern established before satellite launch to invert the in-orbit antenna patterns to correct for the variable polarization distortion, and it needs only a small amount of calibration equipment to solve for the parameters for constant polarization distortion. The scheme no longer depends on the distributed target and improves the polarization precision of the data. It is applied to the calibration experiment for the data processing of the GaoFen-3 satellite and has achieved good results in applications.

No abstract

The paper proposes a new method for measuring the azimuth pointing of spaceborne synthetic aperture radar (SAR) antenna beams based on the ground receiver, which can receive and record complex sampling data of the pulse signals transmitted from the spaceborne SAR. The center of the antenna pattern is extracted from the complex sampling data amplitude envelope to obtain the time when the beam main lobe center irradiates the ground receiver, and the range migration information is extracted from the complex sampling data to obtain the time when the satellite is over the top of the ground receiver. The results of Chinese civilian remote sensing GaoFen-3 SAR satellite experiment data processing show that the measurement accuracy of this method is better than 0.002°, which can be applied to the accurate measurement of azimuth pointing of various low Earth orbit (LEO) SAR antenna beams.