Intern Paper: ODTS for the Future Mars Relay and Navigation Constellation

The current Mars relay network (MRN), which utilizes the combination of NASA and ESA orbiters to transfer data from Mars surface assets, greatly contributed to the success of past scientific missions. However, these orbiters are now operating beyond primary design lifetimes, and were not principally designed for relay services. To enable future human exploration on Mars, it is necessary to develop a dedicated network of satellites that provide both communication relay and positioning, navigation, and timing (PNT) service in Mars' orbit and on its surface. Additionally, it is desirable to operate the constellation semi-autonomously and reduce reliance on the oversubscribed Deep Space Network (DSN). In the paper, we first address the design of the next-generation Mars constellation with a focus on the following three objectives: coverage between +-30 degrees latitude, communication data volume, and surface user PNT performance. Previous studies on Mars constellation design have addressed portions of these objectives, but none have addressed all of the three objectives simultaneously. To address this gap, our paper conducts an analysis on the coverage, communication data volume, and PNT performance for a notional constellation comprising five satellites: two orbiters at areostationary orbit and three orbiters in inclined repeating ground track orbits at lower altitudes. We propose a candidate constellation that can balance these three objectives effectively. We also analyze the sensitivity of altitude and inclinations of the inclined orbits to the three objectives. There is a trade-off because increasing the inclination enhances the navigation accuracy but reduces coverage, while increasing the altitude improves coverage but diminishes data volume. Second, we propose a semi-autonomous orbit determination and time synchronization (ODTS) framework for the MRN orbiters. Our proposed approach integrates three different methodologies: DSN tracking, inter-satellite links (ISLs), and links with a Mars surface station (MSS). The orbiters will utilize either dual one-way links or two-way radio-frequency or optical links to other orbiters and the MSSs to obtain range, range-rate, and bearing angle observables. Within this network connected by ISLs, the surface stations serve as anchor nodes since their position and clock offset are known very precisely. Therefore, this approach minimizes the use of DSN, limiting its monitoring to a portion of the network at shorter time windows. In this paper, we investigate two approaches for processing the obtained measurement data for semi-autonomous ODTS: 1) downlinking the collected ISL measurement data to MSS and processing them in batches, and 2) processing the measurements in a distributed manner at each node (orbiters and MSS). The first approach can provide a more accurate solution, while the latter approach can lower the communication burden. We demonstrate the effectiveness of the proposed ODTS approach in the proposed constellation in the first part of this paper.

当前的火星中继网络（Mars Relay Network, MRN）整合美国国家航空航天局（NASA）与欧洲空间局（ESA）的轨道器，用于传输火星表面载荷的数据，为过往科学探测任务的成功发挥了关键作用。然而，这些轨道器现已超出初始设计寿命，且最初并非专为中继服务设计。为支撑未来火星载人探测，亟需构建专用卫星网络，可同时在火星轨道及表面提供通信中继与定位、导航与授时（PNT）服务。此外，理想情况下该星座应具备半自主运行能力，以降低对资源过载的深空网络（Deep Space Network, DSN）的依赖。

本文首先开展下一代火星星座的设计工作，重点聚焦三大核心目标：±30°纬度区间的覆盖能力、通信数据量与表面用户PNT性能。过往火星星座设计相关研究仅覆盖了部分目标，尚未有研究同时兼顾全部三项指标。为填补这一研究空白，本文针对由5颗卫星组成的概念星座开展覆盖能力、通信数据量与PNT性能分析：其中2颗部署于火星静止轨道（areostationary orbit），剩余3颗为低高度倾斜重复地面轨迹轨道卫星。本文提出了一款可有效平衡三大目标的候选星座方案，并分析了倾斜轨道的高度与倾角对三项指标的敏感性。由于存在性能权衡：增大倾角可提升导航精度却会降低覆盖范围，而提升高度可改善覆盖能力却会削弱通信数据量，因此需进行折中设计。

其次，本文提出了面向MRN轨道器的半自主轨道确定与时间同步（ODTS）框架。该方案整合了三类不同方法：深空网络跟踪、星间链路（ISLs）以及与火星表面站（MSS）的链路。轨道器可通过双向射频或光链路，或双单向链路与其他轨道器及火星表面站建立连接，获取距离、距离变化率与方位角观测值。在星间链路构成的网络中，火星表面站作为锚节点，其位置与时钟偏移精度已知，因此该方案可减少深空网络的使用量，仅需在较短时间窗口内对部分网络节点进行监控。

本文研究了两种用于半自主轨道确定与时间同步的观测数据处理方案：1）将采集到的星间链路观测数据下行传输至火星表面站并批量处理；2）在各节点（轨道器与火星表面站）分布式处理观测数据。前者可提供更高精度的解算结果，后者则可降低通信负担。本文第一部分将在上述候选星座中验证所提轨道确定与时间同步方案的有效性。