# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 6 Discussion  
Due to the large aperture of LT-1 phased array antennas, the calibration networks are distributed throughout the entire aperture, and the temperature difference between power dividers and RF cables at different positions is relatively large, which has a significant impact on the phase and amplitude of the whole antenna calibration networks. While the temperature variation range within the satellite cabin for the internal calibrator is relatively small, it is necessary to comprehensively analyze the temperature characteristics of the transmitting calibration loops, receiving calibration loops, and reference calibration loop. Additionally, the temperature variation characteristics of active components, such as microwave amplifiers and optical delay lines, within the internal calibrator are more pronounced compared to passive components. The environmental temperature change of the coaxial cables passing through the cabin is the most significant. Considering the impact on radar signal transmission and reception, temperature compensation must be carried out. In temperature compensation experiments, considering the risk of system hardware overstress damage, the number of temperature test measurements for radar instruments  
Figure 17: (**a**) The DSM result of LT-1, (**b**) TanDEM data, (**c**) the error distribution histogram with ICESat, (**d**) the difference between LT-1 and TanDEM [33].  
is limited, resulting in a relatively conservative analysis for temperature compensation accuracy. However, by acquiring temperature compensation data during the module-level circuit screening tests process and appropriately increasing the number of tests during the unit-level temperature tests, it is feasible to effectively enhance the accuracy of the temperature compensation data.  
According to the experimental and analytical results of the LT-1 ground verification system, the phase history of system internal calibration exhibits a negative slope, and the residual phase after internal calibration compensation is optimized from 0.28\({}^{\circ}\) to 0.16\({}^{\circ}\), demonstrating the significant effect of internal calibration compensation on improving synchronization accuracy. Considering the stable ambient temperature and good heat dissipation of the equipment in the laboratory, the residual phase after internal calibration compensation corresponds to \(\varphi_{H_{\text{sys}}}\) in Formula (11). By substituting \(K_{T}\) into Formula (8), the calibration accuracy of the auxiliary satellite echo compensation can be calculated. Based on the analysis conducted in temperature compensation and ground validation experiments, it can be seen that the performance of auxiliary satellite echo compensation is good.  
The design of specialized calibration equipment ensures the effective hardware integration for system internal calibration of LT-1. Obtaining temperature-dependent data from calibration equipment on-ground supports high-precision in-orbit calibration work. As in-orbit internal calibration and synchronization performance test data accumulates, further analysis and verification of the BiSAR system's internal calibration performance becomes feasible.