# Feasibility, Design, and Deployment Requirements of TCR for Bistatic SAR radiometric Calibration
## 1 Introduction  
The radiometric calibration theory and technique of the monostatic SAR system have been considerably improved in the past, and the relevant problem has been extensively studied and well understood [1; 2; 3]. Numerous canonical passive reflectors have been proven to be suitable as monostaticcalibration targets such as spheres, dihedrals, and trihedrals. From the first decade of this century, great interest in the bistatic synthetic aperture radar (SAR) system has been arisen rapidly given its special merits in various microwave remote sensing applications [4; 5; 6]. However, for the bistatic SAR system, the calibration problem is significantly more complex, and limited bistatic SAR calibration theory and experiments have been reported in the literature. The main reason may be due to the lack of suitable calibration targets introduced by the bistatic SAR observation geometry. Therefore, the design of appropriate calibration targets and the investigation of their performances in the SAR image are the first and foremost issues in bistatic SAR radiometric calibration. At present, several bistatic calibration methods based on point targets have been proposed, which can be roughly divided into two categories. One idea is to use an active calibration target with a high radar cross-section (RCS) over a large frequency range that can be easily configured for different bistatic angles [7; 8; 9]; however, active calibration targets are expensive and complex to manufacture. The other view is to utilize passive calibration targets. In 2003, a passive cross-polarization bistatic calibration target was designed for RCS measurements [10]. Although this device performed well over a wide frequency range, it was very complicated to design and manufacture. Trihedral corner reflectors (TCRs) are generally considered not suitable for bistatic SAR calibration with large bistatic angles [11]. One bistatic SAR experiment has been reported using TCR, but failed to perform bistatic radiometric calibration [12], which may be due to the lack of an exact bistatic RCS pattern and not considering the design and deployment requirements. Theoretically, spheres can be considered as good calibration targets at arbitrary bistatic angles, but their relative low RCS restricts their practicability. The TCR is perhaps the most practical device for calibrating monostatic SAR systems. The monostatic RCS pattern of TCR has a 3-dB beamwidth of approximately 40\({}^{\circ}\) in both the azimuth and elevation dimensions, which means that it is much more tolerant to field alignment errors [13]. It is also relatively inexpensive to manufacture, and can be produced in large quantities. Consequently, the TCR have been used as the standard calibration target in monostatic SAR radiometric calibration for many years. However, knowledge on the bistatic calibration performance of TCR is inadequate or questionable; in particular, its feasibility for bistatic SAR with different bistatic angles is not so clear. Therefore, it is important and meaningful to analyze the TCR RCS characteristics, reexamine their feasibility, and subsequently analyze the design and deployment requirements of this classical passive calibration target in bistatic SAR radiometric calibration. The TCR has a very narrow bistatic pattern when the bistatic angle is used as an independent variable [8]. Therefore, it should be noted that the bistatic pattern discussed in this paper used the incident angle as an independent variable, rather than the bistatic angle, and we just analyzed the RCS characteristics of the TCR for bistatic SAR tandem mode radiometric calibration. The bistatic SAR tandem mode, whose transmitting and receiving antennas are spatially separated by a fixed distance (i.e., baseline) along the flight track of the SAR platform, can ensure that the bistatic angle is unchanged during the movement. One example for the tandem mode is the interferometric carstwheel [14].