Deep-space navigation with intersatellite radio tracking

In the past six decades the navigation of interplanetary space- craft has been accomplished through ground-based radio tracking only [1,2]. Deep-space probes have been equipped with sophisti- cated onboard radio subsystems to communicate with Earth’s sta- tions (e.g., NASA’s deep-space network [DSN] [3], ESA’s tracking network [ESTRACK] [4]) enabling Telemetry, Tracking, and Com- mand (TT&C) functionalities. Recent development and design of this instrumentation led to significant enhancements of the quality of the radio tracking data that have been used for precise orbit determination (POD) of interplanetary spacecraft [5]. Future space missions will require extremely accurate knowledge of spacecraft trajectories, and the intrinsic limitations of deep-space radio tracking data could not be fully adequate to fulfill those challenging operation goals. Alternative instruments have then been studied including optical systems that are expected to provide orders of magnitude improvements in the precision of probes positioning over ground-based radio [6]. Laser systems have been used so far in space applications (i.e., satellite laser ranging and lunar laser ranging) with passive corner cube retroreflectors [7]. These well-established passive techniques, however, would not be well-suited to enable spacecraft navigation over deep-space distances. Active optical systems may be possible in the near future by developing laser tran- sponders that would provide few centimeters interplanetary ranging accuracies [8]. An alternative technique of interplanetary orbit determination is based on satellite-to-satellite tracking (SST) with a multispacecraft configuration. Instrument architectures have been extensively inves- tigated for both radio (e.g., [9,10]) and laser (e.g., [11,12]) intersa- tellite systems. The missions Gravity Recovery and Climate Experiment (GRACE) [13] and Gravity Recovery and Interior Labo- ratory (GRAIL) [14] successfully used radio science systems for intersatellite tracking between a pair of spacecraft to precisely deter- mine the gravity fields of the Earth [15] and the Moon [16], respec- tively. Interferometric laser ranging system has also been designed to demonstrate the feasibility and the benefits of this technology. GRACE Follow-On (GRACE-FO) mission includes a laser ranging interferometer (LRI) as a demonstrator experiment with the goal to compare LRI data with microwave ranging data that are acquired by GRACE-FO intersatellite radio tracking instrument [17]. First LRI measurements have been collected in-orbit between GRACE-FO spacecraft, showing range biases comparable to those obtained through the microwave ranging instrument but also demonstrating a substantial improvement in the accuracy of the intersatellite mea- surements, thus confirming expectations [18]. The processing of these extremely accurate range and range-rate data between satellites orbiting the same celestial body strongly constrains the accuracies of the reconstructed trajectories [19]. These data types have a significant advantage compared with ground-based data because SST observations can also be processed by autonomous navigation systems onboard spacecraft. The analysis of intersatellite data only, however, leads to the determination of absolute orbits of two or more spacecraft only if one of the probes is in an orbit with unique size, shape, and orientation [20]. The Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAISON) method dem- onstrated the benefits of SST observations by studying libration orbiters [21], and spacecraft orbiting a celestial body with an asym- metric internal gravity field [22]. Therefore, the combination of SST and deep-space tracking is fundamental to provide absolute orbit determination of spacecraft with unconstrained orbit configurations. The radio and laser intersatellite systems based on GRACE and GRAIL technologies are high-mass payloads with a significant power demand. Future space robotic missions will only be able to host these instruments as mission-unique equipment devoted to gravity investigations. Deep-space navigation with highly accurate ground-based and SST observations will require a more compact instrument scheme. This work is based on a radio system architecture that enables intersatellite measurements few orders of magnitude more precise than deep-space tracking with significant mass and power savings with respect to the GRACE and GRAIL radio science instruments architecture [23,24]. This new intersatellite tracking system will be well-suited to dual- or multispacecraft configurations with SmallSats in the solar system.