# Analysis and Demonstration of First Cross-Support Interferometry Tracking in China Mars Mission
## 1 Introduction
More baselines and longer baselines could acquire a more accurate angular position of the spacecraft [29]. The benefit of combining tracking assets of multiple agencies to provide cross support has been generally recognized by the world community, not only by space agencies but also the astronomy and geodesy communities. Successful joint DeltaDOR interoperability has been tested and validated in several projects by NASA, ESA and other agencies, while excellent results are also obtained as expected in some cross-support operational scenarios [30; 31; 32]. For example, NASA and ESA conducted the first interoperability validation, in 2007, with Venus Express. Following the successful test, DeltaDOR cooperation has been undertaken for the operational support of Phoenix during cruise and prior to Mars Orbit Insertion. The first three-agency DeltaDOR cooperation was conducted in 2008 by JPL, JAXA and ESA. During the cooperation, spacecraft Hayabusa was observed with four ground antennas, one from ESA, one from JAXA, and the other two from DSN. These successful applications testified that DeltaDOR capabilities could be used for future operational cross support on all interplanetary missions. China's first Mars mission, Tianwen-1, is China's first independent interplanetary mission, which is combined with a series of complex instruments to meet scientific expectations. The probe, a combination of orbiter, lander and rover named Zhurong, launched from Earth aboard a Long March 5 rocket on 23 July 2020. After its seven-month journey to Mars, with a few necessary Trajectory Correction Maneuvers (TCM), Tianwen-1 captured into orbit around Mars in February 2021 (see the flight orbit of Tianwen-1 in Figure 1), and Zhurong successfully landed on 14 May 2021 [33]. So, Tianwen-1 became the first Mars exploration mission in the world which has completed orbiting Mars planet, soft landing, and Mars surface measurement on the planet within one launch. Before the Tianwen-1 mission, CLTC had no previous project experience on precision interplanetary navigation and orbit insertion in deep space at Mars distance. While arriving at Mars and capturing in orbit require a high degree of precision, it sets a challenge for the tracking and navigation system. Earth-based tracking stations within CLTC are adopted to support navigation and communication of Tianwen-1, mainly including JMS, KSH and ARG deep space stations and China VLBI network stations belonging to Chinese Academy of Sciences. Until now, restricted by the distribution of deep space stations, only the baseline of JMS-KSH has a longer common tracking arc of Tianwen-1, besides, since the tracking antenna must point away from the spacecraft during the time of the quasar observation in DeltaDOR measurements, which causes interruption of the transmission of telemetry data to the ground station. Therefore, in order to make sure the accuracy of orbit determination is good enough for navigation, especially during critical stages of the mission, such as the case on performing an insertion into Mars orbit, CLTC and European Space Operations Center (ESOC) entered into an agreement to conduct cross-support interferometric tracking under a commercial contract. Deep space station KSH of CLTC and deep space antennas CEB and NNO of ESA are involved in conducting the cross-support DeltaDOR measurements. As in this case, telemetry communication could still be supported by the JMS deep space station; furthermore, the baseline CEB-KSH (east-west direction) is approximately perpendicular to baseline NNO-KSH (south-north direction), which will increase confidence to the conventional orbit solutions. This paper mainly focuses on the techniques developed and applied in CLTC during this first formal cross-support DeltaDOR campaign. In total, there are eight cross-support DeltaDOR measurements in the mission, and six of them are conducted successfully because of an unexpected problem that happened with the CEB station during two campaigns.