How do engineers measure satellite drag, and how do they track it over time?

Question

I’ve been slowly getting myself familiar with astrodynamics and understanding satellite orbits. I’m familiarizing myself with the concept of drag experienced by the satellites. I am curious about how do GNC teams measure it and then respond to it?

Specifically:

• Do gnc teams / astrodynamicists monitor drag on their missions continuously? If so, how do they monitor it?
• How does the change in drag get tracked by the team? For example, drag today in comparison to what it was same time yesterday?
• If there is a sudden change in drag due to changes in the atmosphere, is there a certain limit that the engineer waits until acting on the information? e.g. if there is a space weather event (like during spacex launch in 2022), where there is a loss in altitude, do they wait until the event passes before acting or will they start boosting the satellite as soon as they see the drag has changed significantly? Or do they take preemptive action and bump it up even before the drag has changed?

Answer 1

For most operations drag is continuously monitored, at least for LEO orbits where it affects the most. This is usually done by measuring the spacecraft speed, since drag will have a noticeable effect on it. Depending on what you want to do you usually analyze historical data to have an idea of the remaining lifetime of the mission, or if you have thrusters, to estimate how long do you need to thrust to counteract or mitigate the drag effect.

In regards to if there is a limit that triggers an action regarding drag, well, that depends on the mission, you could have programmed maneuvers from the beginning an throughout the mission lifetime to try to maximize the time the satellite will remain in the ideal orbit, and once its out of fuel its up to physics. Or you could have an alert triggered if it goes below certain altitude, depends on what you want to accomplish.

Other mitigation techniques are to reorient the satellite to provide the smaller cross section possible to drag when the main payload is not in use.

Space weather definitely affects and has to be taken under consideration, a warmer atmosphere expands, which increases particle density at higher altitudes, increasing drag, so solar activity will have a direct impact on it. You can usually use solar activity models to try an predict this effect, but real data is provided by sun monitoring spacecraft and in the best case scenario you have a couple of days to prepare for it.

This is what happened to a Starlink satellites batch in 2022 (Starlink), due to a solar storm the satellites were put into safemode and reoriented to provide less crossection (kind of edge on), however the drag increased so much that prevented the satellites from leaving safemode, and thus use their propulsion. This was very bad luck due to the timing of the event, since the satellites were freshly deployed and hadn't had a chance to raise their orbits.