Somehow, I lived my whole life without knowing about the The Treaty on Open Skies.
It’s a 1992 treaty between NATO and Warsaw pact countries allowing mutual surveillance flights over sovereign territories.
I’m quite surprised that an agreement like this was possible to sign with Russia (but then again, the situation wasn’t quite symmetric in 1992, as was the case between France and Germany after World War II).
What kinds of sensors can be used?
Open Skies aircraft may have video, optical panoramic and framing cameras for daylight photography, infra-red line scanners for a day/night capability, and synthetic aperture radar for a day/night all weather capability. Photographic image quality will permit recognition of major military equipment (e.g., permit a member state to distinguish between a tank and a truck), thus allowing significant transparency of military forces and activities. Sensor categories may be added and capabilities improved by agreement among member states. All sensors used in Open Skies must be commercially available to all signatories. Imagery resolution is limited to 30 centimetres.
Speaking of radars, there is a whole interesting history of Bell Labs developing technology for the military in order to avoid anti-trust documented in the Idea Factory.
Back to the treaty. Are there enforcement mechanisms or does it rely purely on mutual benefit?