While discussing with my Grandfather my concerns about relying solely on cell towers to track the location of my balloon, he, being a former pilot, suggested attaching a radar reflector to the balloon so that air traffic control could track it. It seemed like a good idea, and he offered to make it, so I decided to use it.
It looked like this:
The idea is that it is a corner reflector, that it reflects incoming rays directly back at the source.
We picked a day to go to the nearest radar tower, and went out to visit them. They were very friendly and their instruments were very cool, though I don't have any pictures to post. We agreed to take our reflector up on an adjacent mountain to test it out.
We drove up one day and wandered around old forest service roads until we found a decently clear spot toward the airport.
They couldn't see it, though the guy on the other end didn't seem as friendly or motivated as the ones we met.
Not ones to be deterred by difficulties, we went to the only more clear spot on the top of the mountain. The only reason we didn't try there at the beginning is that, at first, we didn't realize their fence went only 200ft from the gate.
The view from this spot was much better; we could even literally see the airport, but they still claimed they couldn't see it. Something about a mountain being too close or something like that.