People in Cornwall have been warned satellite navigation systems will not work later this week as the Ministry of Defence carry out a jamming exercise.
The aim of the GPS blocking exercise is to find out how interference could affect military personnel. It will take place on Thursday and Friday.
The MoD said its Portreath base would be the base.
The effect will extend for a radius of 11 km (7 miles) which would cover Camborne and Redruth.
"Although GPS provides highly accurate information, the radio signals from the satellite are extremely weak and are susceptible to both jamming and unintentional radio interference," said the MOD in a statement.
"The trials are taking place to better understand these effects on military equipment and therefore will help to protect our forces."
An MOD spokesman said all the emergency services which might use the systems have been informed and if an emergency is under way, the MOD can suspend the trial.
All ambulances are fitted with satellite navigation and in a statement, South Western ambulance service said it was aware of the jamming exercise and that key staff had been informed.
Falmouth Coastguard said it had also been told.