# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 2 Internal Calibration System
### Instrument Hardware  
The dedicated equipment for internal calibration in the LT-1 SAR includes the internal calibrator, antenna calibration network, and coaxial cables connecting them. The principles and components of the internal calibration system are shown in Figure 1, where the red lines represent the entire internal calibration link. The internal calibrator contains multiple calibration paths and switches among them through a group of switches. The antenna calibration network is divided into the left-wing calibration network and the right-wing calibration network, as well as the H-polarimetric calibration network branch and the V-polarimetric calibration network branch, according to the deployment requirements. The parts that cannot be calibrated are indicated by the yellow coils in Figure 1, including the H/V antenna radiation array and the RF cables connecting the H-polarimetric T/R and V-polarimetric T/R to the radiation array.  
In the design of the internal calibration loops, to truly reflect the changes in the characteristics of the radar instrument during operation, these calibration signals follow the nominal radar signal path as closely as possible and cover all active components, ensuring the calibration signals undergo the same variations as the nominal radar signals. They also have the same pulse width, operational bandwidth, operation frequency, and pulse repetition frequency as their respective SAR imaging modes. The calibration signals  
\begin{table}
\begin{tabular}{c c c c c} \hline
**SAR Satellites** & **Launch Year** & **Type** & **Schemes** & **Performances** \\ \hline RADARSAT-2 & 2007 & monostatic & \begin{tabular}{c} The internal calibration loops \\ exclude the antenna, limiter, \\ and LNA \\ calibration, temperature \\ compensation \\ \end{tabular} & \begin{tabular}{c} Moderate accuracy, acceptable \\ complexity \\ High accuracy, acceptable \\ complexity \\ \end{tabular} \\ TaraDEM-X & 2007 & monostatic & \begin{tabular}{c} Non-delay calibration, temperature \\ compensation \\ \end{tabular} & \begin{tabular}{c} High accuracy, acceptable \\ complexity \\ Moderate accuracy, periodically \\ interrupts SAR imaging \\ \end{tabular} \\ Sentinel-1 & 2014 & monostatic & \begin{tabular}{c} Non-delay calibration, did not \\ include calibration networks \\ Delay calibration and non-delay \\ calibration \\ \end{tabular} & \begin{tabular}{c} Moderate accuracy, excessive \\ complexity \\ Complexity \\ \end{tabular} \\ TH-2 & 2019 & bistatic & \begin{tabular}{c} Non-delay calibration, interrupted \\ phase synchronization scheme \\ \end{tabular} & \begin{tabular}{c} Moderate accuracy, periodically \\ interrupts SAR imaging \\ \end{tabular} \\ LT-1 & 2021 & bistatic & \
\begin{tabular}{c} Non-delay calibration, \\ noninterrupted phase \\ synchronization technology \\ \end{tabular} &
\begin{tabular}{c} High accuracy, SAR imaging \\ with no interruption \\ \end{tabular} \\ \hline \end{tabular}
\end{table}
Table 1: Comparative analysis of the internal calibration schemes and performance of typical SAR satellites.  
are sampled through the coupler in T/R modules or the coupler in the synchronization transceiver, gated and adjusted to appropriate levels by the internal calibrator, and then transmitted to the receiver via microwave combination for data acquisition. In addition, the synchronization transceiver and quadrifilar helix antenna are used to establish inter-satellite synchronization link [8; 9].