UK comms regulator Ofcom are offering a new service sure to interest the electronic warfare buff and/or tinfoil-titfer'd conspiracy connoisseur. One can now sign up with the pipes'n'airwaves patrolmen for email alerts giving details of military GPS sat nav jamming trials.
According to Ofcom:
The Ministry of Defence conduct occasional tests on military systems which may result in some loss of service to civilian users of the Global Positioning System (GPS) including in-car navigation devices and networks which rely on GPS signals. Ofcom seeks to provide citizens and consumers with information about possible interruptions to these services via these email updates giving advanced notification of these tests. It must be emphasised that this notification process only warns of future jamming excercises that are brought to the notice of Ofcom and may not cover all jamming exercises. It cannot be assumed that any loss of service is due to jamming exercises.
You can sign up for alerts at the link above - that is, if you don't mind giving the black jammer helicopter pilots your email address.
Also, given the caveats in the Ofcom annoucement it might be better to rely on other channels. Last time we looked into this sort of thing, the NOTAM (Notices to Airmen) aviation safety warning was a lot more informative. ®