# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 2 Internal Calibration System
### Calibration Pulses  
Based on the LT-1 satellite internal calibration system configuration and considering the high-precision inter-satellite synchronization requirements, a novel internal calibration scheme is proposed. This scheme includes the design of chirp non-delay calibration loops and chirp delay calibration loops. Moreover, considering better calibration accuracy, synchronization calibration is designed as non-delay calibration. The specific design of the calibration loops is shown in Table 2. All calibration loops cover the entire system's instruments that the chirp or synchronization signals pass through during transmitting and receiving. By combining multiple calibration loops, the system achieves functions such as fault detection, calibration between H-polarimetric and V-polarimetric, also amplitude and phase calibration between channels. LT-1 has independent synchronization calibration loops to correct amplitude and phase variations during synchronization operation. The synchronization calibration loops are jointly implemented through the synchronization transceiver and other imaging equipment. Figure 1: Composition and principle of internal calibration system of LT-1. There are two methods for implementing internal calibration: delay internal calibration and non-delay internal calibration. The difference between delay calibration and non-delay calibration lies in the delayed calibration signal passing through the optical delay line in the internal calibrator, which temporally separates the calibration signal from the high-power transmitting signal, achieving high isolation. However, the delay module includes active components such as optical delay lines and microwave amplifiers, resulting in poor calibration phase stability, significant temperature drift, and poor link cancellation, making it challenging to meet high-precision internal calibration requirements. In contrast, the non-delay internal calibration method adopts a fully passive design, achieving better amplitude and phase stability. To ensure calibration accuracy, LT-1 primarily uses the non-delay calibration method. In some cases, the delay calibration method is selected. For example, when the single T/R module chirp transmitting calibration loop operating, output signal level is low and susceptible to transmitting signal interference, then delay calibration with high-isolation can be chosen. Additionally, the chirp transceiver calibration loop must use delay to temporally separate the transmitting and receiving operations. According to different calibration requirements, the LT-1 system offers various internal calibration schemes. Controlled by the monitoring computer, these calibration methods can be flexibly combined and selected as needed. As shown in Table 2, five non-delay calibration loops comprehensively cover all system instruments that the chirp and synchronization signals pass through during transmitting and receiving. The signal paths of the five calibration loops are listed in Table 3, with parameter definitions provided in Table 4. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Definition** & **Parameters** & **Definition** \\ \hline \(CR_{1}^{i\ast}\) & Internal calibrator chirp reference loop & \(CE_{cal}^{i}\) & Chirp reference calibration loops \\ \(CR_{2}^{i}\) & Internal calibrator chirp transmitting loop & \(TX_{cal}^{i}\) & Chirp transmitting calibration loops \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters Definition Table.