IR beacons have the downside of blatantly broadcasting their positions, but they might be less jammable than radio waves? OTOH, pointing an IR laser at your camera isn't that hard, so maybe not.
You could also take a page from the SR-71 and correct your inertial guidance system's error using star observations. Only needs to see up, so you can shield it against blinding attempts from below. Can a drone-weight camera pick up stars?
Horizon tracking would be super cool. You'd have to open your cameras to blinding attacks again to make that work, but still. Even as a project unrelated to getting around jamming, that would be a neat piece of tech.
Another approach would be to do what insects do and use optical flow. Essentially look at how fast the ground is moving to figure how fast the UAS is moving and integrate to figure out where you are. This is exactly how optical mice work, in fact optical mice sensors have even been used to do this. Below is an example of a quadcopter that employs such system.[2]
[0] https://en.wikipedia.org/wiki/TERCOM [1] https://en.wikipedia.org/wiki/Optical_flow [2] https://www.youtube.com/watch?v=bbS1N--bT0w