## 1 Introduction
analyzed the inter-satellite ranging performance using 30 days of measurement data from the complete BDS-3 constellation, revealing that the ISL ranging accuracy was better than 5 cm [26]. [PERSON] et al. conducted an  
Figure 1: The development timeline of the BeiDou satellite navigation system. experimental analysis using one month of raw observation data from 29 BDS-3 satellites, consistently demonstrating that the time synchronization accuracy of each satellite was better than 0.4 ns [6]. The BeiDou ISL communication and measurement system employs a time-division half-duplex mode, establishing narrow-beam directional links based on a phased-array mechanism. Each satellite, following a time-slot schedule uploaded from the ground, establishes links with other satellites (or ground anchor stations) in a polling manner. The BDS-3 ISL system divides the timeline into superframes (1 min), with each superframe further subdivided into 20 time slots (3 s). Within a single time slot, the linked satellites alternately transmit signals to each other during their respective 1.5 s intervals, completing two-way inter-satellite measurement and communication [26; 27; 28]. Currently, ISLs in major global navigation systems predominantly employ a time-division half-duplex (TDHD) mode. While numerous scholars have focused on analyzing link performance based on raw inter-satellite observation data, such as orbit determination accuracy, time synchronization precision, and routing transmission efficiency, research on the operational mechanisms of ISLs remains limited. The TDHD mode, which prevents simultaneous transmission and reception on the same frequency, results in unidirectional transmission rates being only 50% of the total channel capacity. Additionally, the link establishment process is complex, and timestamp calculation and error correction are prone to significant inaccuracies. These limitations lead to issues such as low communication efficiency, high latency, and restricted inter-satellite measurement accuracy [3]. In contrast, Co-frequency Co-time Full Duplex (CCFD) technology, a key innovation proposed during the 5G era, has matured significantly in recent years [29; 30; 31]. Compared to traditional time-division and frequency-division systems, CCFD technology enables wireless devices to achieve simultaneous signal transmission and reception on the same frequency. Relative to time-division systems, CCFD technology allows concurrent signal transmission and reception, theoretically doubling the information throughput within the same time interval. In contrast to frequency-division systems, CCFD technology can effectively conserve bandwidth resources, theoretically improving spectral efficiency by a factor of two. Currently, numerous researchers have begun dedicating efforts to the study of CCFD technology, covering a wide range of fields such as massive multiple-input multiple-output systems, low-power wide-area networks, relay communication networks, Wi-Fi systems, Long-Term Evolution systems, and cognitive radio [29]. Introducing CCFD into ISL measurement and communication processes could effectively overcome the constraints of TDHD systems, which are unable to simultaneously transmit and receive signals on the same frequency. Moreover, it would simplify the link establishment process and enable continuous inter-satellite transmission and precise measurement. In terms of transmission, CCFD could double the inter-satellite transmission rate, measurement frequency, and data update rate. For measurement, CCFD could effectively mitigate ranging errors caused by relative motion, clock bias, and Doppler effects between satellites. This would eliminate the need for complex models to improve ranging accuracy and enhance the performance of inter-satellite measurements. In this paper, we propose a novel inter-satellite two-way measurement method based on the CCFD system to enhance the overall performance of the ISL network.