# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 3 Calibration Equipment
### Antenna Calibration Network  
In this section, we discuss the composition of the antenna calibration network and its temperature compensation design. The calibration network of the phased array antenna is used to achieve transmitting calibration and receiving calibration, consisting of calibration power dividers and RF cables. The calibration signal is sampled and output or input via couplers in the T/R modules, with H-polarimetric and V-polarimetric sharing a single  
Figure 5: Diagram of the six calibration loops in the internal calibrator. (**a**) The chirp reference calibration loop is labeled with a green solid line, while the chirp receiving calibration loops is labeled with a blue solid line, and the synchronization receiving calibration loop is labeled with a blue dashed line. (**b**) The chirp transmitting calibration loops is labeled with a red solid line, while the synchronization transmitting calibration loop is labeled with a red dashed line. (**c**) The chirp transmitting and receiving calibration loop is labeled with an orange line.  
Figure 6: Block diagram of the optical delay circuits.  
calibration network. Each module contains 22 calibration signals, with two ports of a central 1:8 power divider connected to 50 \(\Omega\) loads. Figure 7 shows the connection relationship of antenna calibration networks of a single-wing. The calibration signal passing through the 1:8 power divider, 1:3 power divider, and 1:4 power divider, then the 176 combined signals are transmitted to the internal calibrator.  
The L-band dual-polarimetric phased array antenna includes two single-wing apertures. The calibration network is a part of the SAR internal calibration system, and its stability significantly enhances the imaging quality of the BiSAR system. The calibration network consists of calibration power dividers and calibration RF cables, forming a passive RF power division network. In on-orbit operation, environmental temperature is a crucial factor determining its stability, making it essential to analyze the calibration network's operational temperature.  
According to the results of thermodynamic simulation, the temperature of the antenna is divided into three areas on the range direction, as shown in Figure 8. The average temperature of the red areas in the upper and lower parts of the figure is higher than that of the blue area in the middle. It can be seen from the figure that the 1-to-8 power divider is close to the T/R modules, resulting in a significant increase in operating temperature, while the 1-to-3 power divider is far away from the T/R modules, resulting in a smaller increase in operating temperature. By arranging more temperature telemetry sensors in the area with a larger temperature rise and considering the temperature change distribution globally, it is beneficial for accurately extracting the phase (amplitude) temperature dependence of instrument drift during ground temperature experiments.  
To ensure the synchronization performance of the BiSAR system, the internal calibrator employs non-delay calibration methods, while also conducting thermal analysis and reasonable telemetry point configuration for the large-scale phased array. Subsequently, temperature experiments are conducted to investigate the temperature compensation performance of the calibration equipment.  
Figure 7: Block diagram of the antenna calibration networks of a single wing.