On the Orbit Constellation Assessment for the Next-Generation Mars Telecommunications Orbiters

The Next-Generation Mars Telecommunications Orbiters, a.k.a. Mars ComBirds, (MCB), are intended to serve as a deep-space relay hub that provides high-performance links to Earth at extreme data rates and to increase data return from a variety of Mars rovers, landers, aerobots, and science orbiters. As the current science orbiters used for relay are aging, the needs for these MCBs are becoming realistic, justified, and increasingly urgent. The message is further echoed when demands for Earth return data for next-decade missions to Mars, such as ExoMars, Mars Ice Mapper, Mars Sample Return, and future human exploration missions to the Red Planet, continue to increase. In addition, by communicating directly to these MCBs instead of Earth, communications systems for future science missions can be reduced. Thus, the costs can be lowered, or more science equipment can be added. Furthermore, MCBs' fields of view with Earth are much longer; therefore, an appropriate choice of orbits, a network of MCBs with cross-link capability can connect any users at Mars with Earth almost continuously. In this paper, we primarily provide a trade study on the design of the MCB orbits, which include the number of orbits, sizes, shapes, and orientations. Special attention is also given to a class of orbits that provides daily repeating ground tracks. These orbits can facilitate surface operations because they rise and set daily over a specific area at constant revisiting times. In addition, there is another class of orbits where a spacecraft would tug a science orbiter to a sun-sync Mars orbit and then raise its altitude and serve as a relay orbiter. More particularly, we will consider different orbit types such as (1) circular equatorial, (2) circular sun-sync, (3) Apoapsis at Constant time-of-day Critically Inclined (ACCI), (4) Apoapsis at Constant time-of-day Equatorial (ACE), and (5) SEP- Tugs. Mars surface users are assumed to be global and of any longitude and latitude. For users in orbit, we assume their orbital parameters similar to the typical low-Mars sun-sync orbits such as Mars Odyssey and Mars Reconnaissance orbiters. JPL-developed Telecom Orbit Analysis and Simulation Tool (TOAST) software is used to compute the contacts between the orbiters and users. The performance of these orbit constellations can be assessed through several metrics of interest, which include the maximum latitude, number of contacts per sol, contact duration, total contact time per sol, and maximum communication gap. Recommendations for the optimal orbital constellation choices (3-planar and coplanar variations) will be provided based on comparing the weighted means of each metric calculated at latitude-longitude coordinates during a simulation duration of 1 sol. The chosen orbits will then be further investigated in greater depth to weigh the pros and cons regarding a satellite's operational capabilities and limitations at that orbit.