Its recording which cell phones are in the area talking. While they can do this on the ground later. Its easy now to figure out who is talking and where, without a warrant using this plane. One of these planes can, and do fly around foreign countries of interest and record or otherwise detect all transmissions. They can even simulate local phone use. And they can bounce internet traffic into the area. The real work is being done in Florida. The plane is there like a local router. The plane can even connect into the local network of the counting center.
This plane had to be flying fairly high to be above the restricted airspace of Sky Harbor Intl Airport. Phoenix is UTC-7 so the track is from roughly 8 am until noon. The flight track came from the direction of Phoenix Deer Valley Airport where the Phoenix PD keeps their PC-12. The Phoenix PD PC-12 has long range video capability and likely has Stingray cell intercept technology which can mimic a cell tower and make all of the cell phones connect through it (as in the Dec 6 Capitol surveillance). If they flew directly over the stadium it would be obvious what they are doing. If they flew this pattern there is plausible deniability. Not saying they are spying on the vote audit, only that it is suspicious.
I forgot to add that with the Stingray cell intercept technology they can see the GPS location of each phone and tell which ones are inside the stadium, record their conversations or their data and tell phone number or IP address they are connecting to.