# An Innovative Internal Calibration Strategy and Implementation for LT-1 Bistatic Spaceborne SAR
## 1 Introduction  
Synthetic Aperture Radar (SAR) is an active remote sensing instrument with all-weather, all-day, and multi-dimensional detection capabilities [1; 2], widely applied in earth observation, military reconnaissance, deep space exploration, and other fields. Since the 21 st century, bistatic and multistatic SAR technology has gained increasing attention from researchers due to its unique advantages compared with monostatic SAR [3]. Bistatic and multistatic SAR is a highly flexible remote sensing technology with adaptable baseline configurations, allowing for multi-dimensions, ultra-wide swath, and high-resolution imaging simultaneously [4; 5]. However, the separation of transmitters and receivers in bistatic and multistatic SAR systems introduces new technical challenges. Taking the bistatic SAR (BiSAR) interferometry system as an example, the system operates by transmitting signals from the primary satellite and receiving signals simultaneously with both the primary satellite and the auxiliary satellite. Due to the different oscillators used in the two satellites, phase synchronization errors are introduced by the frequency inconsistency of the oscillators, accumulating over time and affecting both imaging focus and interferometric phase accuracy [5; 6; 7]. LuTan-1 (LT-1) BiSAR mission, consisting of two full-polarimetric L-band SAR satellites, uses the baseline formed by the formation satellites to obtain surfaceelevation information through interferometric processing [8]. The phase synchronization adopts a noninterrupted phase synchronization technology, achieving high-precision synchronization without interrupting the SAR imaging process [9; 10]. Innovative and effective calibration techniques are essential for SAR systems to ensure detection accuracy and adapt to the complexity of future spaceborne SAR systems. Internal calibration of SAR system involves calibrating the amplitude and phase variations in both the transmitting and receiving channels. These variations, superimposed on the echo signal, result in phase and amplitude errors in the target echo signal [11]. On the one hand, changes in transmitting power and receiving gain impact the radiometric calibration accuracy of SAR images; on the other hand, variations in amplitude-frequency and phase-frequency characteristics within the transmission and reception bands affect the pulse compression effect in the range direction of the images [12; 13; 14; 15; 16]. There are two major methods for internal calibration: delay internal calibration and non-delay internal calibration. In delay internal calibration, the gain stability of the microwave amplification chain is mainly influenced by environmental temperature changes, while the optical link suffers from poor phase stability and increased insertion loss due to space radiation. In non-delay internal calibration, a fully passive design is used, achieving better amplitude and phase stability; however, issues such as isolation between the echo link and the calibration link need to be addressed [17; 18]. We have conducted a detailed investigation into the internal calibration schemes of typical SAR satellites that have been successfully launched globally in recent years. Furthermore, a comparative analysis of the performance is carried out, focusing on both the internal calibration accuracy and the complexity of the calibration methods, as summarized in Table 1. The in-orbit internal calibration loops design of RADARSAT-2 covers the transmission and reception signal paths within the instrument but excludes the antenna, limiter, and LNA (Low Noise Amplifier) [19; 20]. TerraSAR-X employs onboard temperature measurement and ground calibration methods to enhance internal calibration accuracy, compensating for temperature-related drifts and hardware changes [21; 22].