# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 5 Experiment  
### Ground Validation System for LT-1  
To evaluate the effectiveness and accuracy of the internal calibration strategy, a ground verification system of LT-1 was constructed based on imaging equipment and synchronization devices, which simulates the bistatic mode of LT-1, and specifically analyzes the inter-satellite synchronization performance. The key parameters of LT-1 mission are shown in Table 6 [8]. The schematic diagram of the ground verification system is illustrated in Figure 12.  
The ground verification system comprises a primary satellite, an auxiliary satellite, and an echo simulation unit. The echo simulation unit internally consists of a circulator (to achieve duplex function), an attenuator and an optical delay line (to achieve echo delay and signal level control functions), a power divider, and two horn antennas (to achieve radiation of echoes to the radar antennas). Given the large aperture size of the phased array antenna, two horn antennas are used to illuminate the geometric centers of the two apertures. The primary and auxiliary satellites are designed consistently, with the SAR system including a signal generation unit, an echo reception unit, a synchronization unit, a calibration unit, and a control unit. Among them, the signal generation unit comprises a reference frequency source and a frequency modulation signal source; the echo reception unit comprises a microwave combination, a receiver, and a data former; the synchronization unit comprises a synchronization transceiver and synchronization antennas; the calibration unit comprises an internal calibrator, an antenna calibration networks, and through-cabin coaxial cables; \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Carrier frequency & 1.26 GHz \\ Polarization & single/quad/compact \\ Channel number & 4 \\ SAR payload mass & 1250 kg \\ Peak power & 16,000 W \\ Antenna length & 9.8 m \\ Antenna width & 3.4 m \\ PRF & 1400 Hz\(-\)4500 Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 6: LuTan-1 mission parameters. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Equipment** & **Amplitude** & **Phase** \\ \hline Internal calibrator & 0.045 dB & 0.735\({}^{\circ}\) \\ Antenna calibration networks & 0.05 dB & 0.85\({}^{\circ}\) \\ Through-cabin coaxial cables & 0.08 dB & 0.80\({}^{\circ}\) \\ The internal calibration system (\(K_{T}\)) & 0.10 dB & 1.38\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The compensation error of SAR internal calibration system. and the control unit is a radar computer. Additionally, the GNSS disciplining module provides a 90 MHz reference signal for the reference frequency source and a PPS signal for the radar computer. The data recorder is used for echo data recording and analysis. The signal generation unit generates local oscillator signals for the mixers and clock reference signals from the reference frequency source, and the frequency modulation signal source generates chirp signals with corresponding bandwidth, pulse width, and frequency modulation slope according to the radar operating parameters. The echo reception unit realizes low-noise amplification, filtering, and digital processing of the echo. The synchronization unit realizes power amplification of the synchronization signal and radiates it to another satellite, also receiving, filtering and amplifying the synchronization signal from the other satellite. The calibration unit realizes the generation and level control of the calibration signals for various calibration pulses of the system. The control unit is primarily the radar computer, which completes the control and monitoring of the SAR system. The system workflow of the ground verification system is as follows: the primary satellite transmits a chirp signal, which is received by horn antenna 1 and then enters the ground delay equipment through the circulator and attenuator. After a period of delay, the chirp signal is divided into two paths by the power divider and radiated by horn 1 and horn 2.