# Analysis and Demonstration of First Cross-Support Interferometry Tracking in China Mars Mission
## 1 Introduction  
Very-Long-Baseline Interferometry (VLBI) was firstly used in radio astronomy, and since the 1970s, it has been introduced in deep spacecraft navigation by differential measurement of angular nearly calibration quasar, which is known as Delta-Differential One-Way Ranging (DeltaDOR) [1, 2]. The year 1977 is a keystone in DeltaDOR history, a proposed system that featured a custom signal generator onboard was published, the spacecraft could produce a series of tones for differential one-way ranging [3]. Conventional earth-based tracking technique, ranging and Doppler could precisely measure the distance and velocity along the line of sight between the spacecraft and station; however, they suffered from a poorer ability to take angular measurement. While DeltaDOR is a technique that is highly efficient in determining spacecraft angular position in the plane of sky, it could be used in conjunction with conventional Doppler and ranging data to improve spacecraft navigation [4, 5, 6]. Taking planet orbit insertion maneuver as a typical scenario, DeltaDOR measurements could ensure more reliability and accuracy in orbit determination than only relying on conventional ranging and Doppler data. The Deep Space Network (DSN) developed the capability for VLBI measurement, and both the implementation performance and importance to flight projects have evolvedover the past several decades [7; 8]. Nowadays, the DeltaDOR measurement has matured into the most accurate system of DSN for precise planetary navigation, and DSN has been providing DeltaDOR measurements to a large number of deep space navigation missions with validated performance. The accuracy of routine DeltaDOR by DSN could be about one nanoradian, which represents the state-of-the-art performance of all of world-wide space agencies [9; 10; 11; 12; 13]. Other space agencies, including the European Space Agency (ESA) [14; 15; 16] and Japan Aerospace Exploration Agency (JAXA) [17; 18], also developed their own DeltaDOR capabilities and systems. In particular, China Satellite Launch and TT&C General (CLTC) adopted the VLBI measurement in the China Lunar Exploration Project (CLEP) and other deep space exploration projects. Deep space navigation accuracy is obviously improved when combining Unified S/X Band and VLBI. The China VLBI Network (CVN), which was mainly developed for radio astronomy, was firstly induced to support ChangE-2 project, and CVN played an import role in China deep space exploration projects [19; 20; 21; 22]. In order to support China deep space exploration missions, China developed deep space stations Jiamusi and Kashi in 2013, forming a baseline more than 4000 km. Another deep station at Argentina was put into application in 2017 for providing continuous telemetry, tracking, and command (TT&C) and precise navigation services. Since then, China has owned the third-largest worldwide distributed deep space network after NASA and ESA [23; 24]. Both DOR and DeltaDOR measurements, with expected accuracy, were adopted in CLTC to support deep spacecraft tracking and navigation [25; 26; 27]. In principle, the angular position accuracy of DeltaDOR is proportional to baseline length. In addition, from the point of view of orbit determination, navigation accuracy is affected by baseline diversity. For example, in order to determine both components of angular position, a measurement by at least two baselines is required; in the case of orthogonal baselines, the best two-dimensional coverage is provided, so navigation accuracy could be improved [28]. Usually, a single space agency does not own enough tracking stations to provide orthogonal baselines by itself; moreover, multi-tracking and communication missions for different spacecrafts make this limitation worse. Performance of DeltaDOR could be enhanced by intra-agency cooperation, because stations from different agencies can be used as data collectors under the Council of the Consultative Committee for Space Data Systems (CCSDS) standards for deep space navigation purpose.