Autonomous Station Keeping of Satellites in Areostationary Mars Orbit: A Predictive Control Approach

The continued exploration of Mars will require a greater number of in-space assets to aid interplanetary communications. Future missions to the surface of Mars may be augmented with stationary satellites that remain overhead at all times as a means of sending data back to Earth from fixed antennae on the surface. These areostationary satellites will experience several important disturbances that push and pull the spacecraft off of its desired orbit. Thus, a station-keeping control strategy must be put into place to ensure the satellite remains overhead while minimizing the fuel required to elongate mission lifetime. This paper develops a model predictive control policy for areostationary station keeping that exploits knowledge of non-Keplerian perturbations in order to minimize the required annual station-keeping $Δv$. The station-keeping policy is applied to a satellite placed at various longitudes, and simulations are performed for an example mission at a longitude of a potential future crewed landing site. Through careful tuning of the controller constraints, and proper placement of the satellite at stable longitudes, the annual station-keeping $Δv$ can be reduced relative to a naive mission design.

💡 Research Summary

The paper addresses the growing need for continuous communication between Earth and the surface of Mars by proposing an autonomous station‑keeping solution for satellites placed in an Areostationary Mars Orbit (AMO). After reviewing past and current mission concepts that rely on AMO relays, the authors identify the primary non‑Keplerian disturbances affecting such satellites: Mars’ non‑spherical gravity field (high‑order spherical harmonics), solar radiation pressure, atmospheric drag at high altitudes, and solar‑wind induced electromagnetic forces. These perturbations cause drift in latitude, longitude, and radius, demanding frequent corrective thrusts that consume valuable propellant.

To minimize propellant usage, the authors develop a nonlinear Model Predictive Control (MPC) law that directly optimizes the annual Δv while respecting a prescribed station‑keeping window (±6° in both latitude and longitude). The control input is a low‑thrust electric propulsion system, modeled with realistic thrust limits (≈0.1 N) and specific impulse (≈3000 s). The MPC cost function penalizes both the integrated thrust (i.e., Δv) and any violation of the station‑keeping bounds. A prediction horizon of six hours is used, with a receding‑horizon update every ten minutes, allowing the controller to incorporate up‑to‑date predictions of the gravitational potential and solar position.

High‑fidelity simulations are performed using a 40‑order Mars gravity model, time‑varying solar radiation pressure, and an atmospheric density model for the thin Martian exosphere. The satellite’s dynamics are expressed in a Hill frame rotating with the desired longitude, and the MPC is solved as a constrained nonlinear optimization at each step. A comprehensive sweep of longitudes (0°–360° in 1° increments) yields a benchmark of annual Δv requirements. Results show that longitudes near the minima of the gravitational potential (approximately 90°, 180°, and 270°) benefit from natural restoring forces, achieving annual Δv as low as 25 m/s. In contrast, regions such as the Southern Meridiani Planum (~150°), where solar pressure and higher‑order gravity terms combine unfavorably, require up to 60 m/s without optimization. By widening the station‑keeping window, extending the prediction horizon, and exploiting the continuous thrust capability of electric propulsion, the MPC reduces Δv in these challenging zones to roughly 30–35 m/s, representing a 40–50 % improvement over naive, off‑line control strategies.

A mission‑design case study focuses on placing an AMO satellite over the prospective crewed landing site at Southern Meridiani Planum. The authors examine the effect of launch epoch, initial phasing, and window size on fuel consumption. Launching six months before the crewed mission and allowing an 8° drift window lowers the annual Δv to 28 m/s. Moreover, the paper proposes a “stable‑longitude transfer” concept: initially placing the satellite at a naturally stable longitude (e.g., 180°) and later performing a low‑Δv maneuver to the target longitude, further cutting total propellant use by about 15 %.

The study concludes that a well‑tuned MPC can autonomously maintain AMO satellites with substantially less propellant than traditional ground‑commanded schemes, even in the presence of significant communication delays (8–22 min) between Earth and Mars. The authors suggest future work on multi‑satellite cooperative station‑keeping, real‑hardware implementation of the MPC on electric thrusters, and integration with attitude‑control and momentum‑management subsystems.