It's been speculated that advanced spy satellites carry thrusters and fuel to conduct frequent orbit changes. I think it's pretty telling that the DoD just did a SBIR competition for software to detect evasive behavior in space junk: https://www.sbir.gov/sbirsearch/detail/1319195

> It's been speculated that advanced spy satellites carry thrusters and fuel to conduct frequent orbit changes.

That's not really surprising; I'd imagine most long-term satellites carry thrusters and at least some amount of fuel for stationkeeping, especially in geostationary orbit. It'd definitely make it much more difficult to track, unless you keep something pointed at it 24/7 so you notice the change in velocity.

And nice find with the SBIR competition! Does seem to point towards satellites being much more difficult to track than I thought if they're constantly moving.

The X-37B has been known to vanish from being tracked from time to time. We've got some clever stuff.

Might that be just because it changed orbit and had to be reacquired? Might take a while since you have no clue what the parameters for the new orbit are or when the burn(s) took place. Some kind of new stealth tech would be pretty cool, though.