"Similar trials carried out for the Government, by the MOD have already taken place in a number of areas including Lincolnshire, Wales, Scotland and Cornwall have had an effect over a range of approximately 11km. Details of the trials are "restricted", but the government has said that the trials are intended to research "vulnerabilities in a range of military applications" of satellite navigation, although is has also been noted that the Defence Science and Technology Laboratory PR team have denied that such trial are taking place! For previous trials the Coastguard have locally broadcast details of the trial with timings and a Notice to Airmen was also published at least for the trial in Portreath, Cornwall.
GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. All radio navigation systems are susceptible to interference and environmental effects, which can adversely affect their availability. Because of the low power and Ultra High Frequency GPS is particularly susceptible to interference and even hostile jamming which is why the General Lighthouse Authorities are establishing a new Enhanced Loran (eLoran) station in Cumbria, UK (coordinates: Latitude 54° 54' 40.35'' N. Longitude 3° 17' 14.21'' W). It is hoped that this will be fully operational by the end January 2008.
The RYA strongly advises that no single aids to navigation system should be used in isolation and that users should use all alternative means available to cross check the information received."
So keep those paper charts and sextants handy!