The Science of Astrogation: Using GMAT to Simulate and Optimize Satellite Missions

One of my tasks at Spacemanic entails simulating our future satellite missions. Recently one of my colleagues asked me how exactly I prepare the simulations. And I thought that maybe more people would be interested in hearing about what astrogation entails. Author: Ondřej Hladík

To begin we need to introduce software that is being used. GMAT, the General Mission Analysis Tool, is our main tool for simulating satellite trajectories. It is an open-source software developed primarily by NASA. Many different organizations and companies have used it to prepare their space missions, for example, the well-known OSIRIS-REx and TESS missions. Within GMAT you can set all parameters of your spacecraft and prepare a mission tree, a set of commands for the simulation to perform.

Depending on the purpose of the simulation, different parameters of the spacecraft are needed. As an example for this article, we want to simulate an active deorbit maneuver. That means that we will need to specify the propulsion of our spacecraft. For a more general simulation, we will mainly want the specific impulse, which tells us what the effective exhaust velocity is and subsequently how much fuel is needed for the maneuver. We also need to somehow define thrust, in more specific simulations we will get this value from the datasheet of our selected propulsion system, but in a general simulation, this would usually be a function of estimated available power.

Once we have created our propulsion system we need to set parameters of the spacecraft. Deorbit maneuvers are a bit trickier than some others in this regard. We need to not only set the proper mass of our spacecraft but also the drag coefficient and drag area. Drag on the spacecraft is especially noticeable at lower orbits where it alone is strong enough to deorbit the satellite in a few years. The speed of orbital decay is, sadly, hard to predict as the density of the thermosphere varies wildly depending on solar activity. For our purposes right now, a qualified estimate is more than enough. Finally, we set the starting orbit of our satellite. In this case, we choose a LEO orbit, specific parameters are not very important in this case.

Once we have set up our spacecraft, we head over to the mission tree to prepare the mission itself. Here we use the available commands to create a mission that will mimic the real maneuver as closely as possible. The next steps will diverge slightly depending on the purpose of the simulation. We can simply be verifying that a certain maneuver is within the capabilities of the spacecraft, or we can be looking for a solution to a specific question, for example, how much propellant is needed for a certain maneuver with a specific propulsion system. In our case, we are investigating how effective the engine is at deorbiting the satellite. For that, we will want to perform multiple simulations with varying altitudes of the orbit and mass of the satellite to see for what satellites this propulsion system provides sufficient total impulse.

Some of you could be asking why we need to perform the simulations when all of this can be computed using a few simple equations. Firstly, a properly set up simulation can easily have its parameters adjusted to find what needs to be changed for a viable solution. The other answer is that while under some circumstances those equations do work, they represent a considerable simplification and omit many details that can considerably impact the accuracy of the prediction. Using simulations allows us to predict the necessary parameters of spacecraft with greater accuracy, allows us to ensure the satellite’s safe operations and gives our customers more available payload mass and volume.

Cover photo: Finished simulation of deorbit maneuver