Very interesting. I see this is rare. Assuming this is a current photo, it is interesting that it is operational, given the legacy engines on that older airframe. I suspect the USAF isn't going to tell anyone.

WRT to its role, IDK if it is a test aircraft for radar measurement, but I will say that airborne measurements for RCS have been a thing for awhile. Did some of that initial work a long time ago. Typically RCS testing is done on a range with a model where the model is adjusted (upside down sometimes) to get the right perspectives in the polar plots. Models can sometimes be the actual aircraft (usually stripped out). We would also get a lot of measurements by flying near the ground station where the subject aircraft would be required to maintain some crazy attitudes (reference to the ground) in order to get the right angles. Some of the most difficult testing I think I've ever done. But its actually a lot more accurate (and safe) to get it airborne in its actual environment, the way an airborne adversary would be looking.