I have just recieved the following on another reflector
Subject: Ofcom Update: New Ofcom notification service - advanced notice ofpossible interuption to Global Positioning Systems
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 has today launched a new email update notification service to give advanced notification of these tests.
In reply to GW0DSP:
These tests are often done in Cornwall/Devon areas. In the years I been in the GPS industry we have never seen an issue that has caused a problem.
Well in theory there aren’t too many military receivers in civilian hands which means that people who need to have their GPS jammed by the MoD are very much more likely to use civilian GPS. The people with military GPS receivers are probably “our friends”. Given the sensitive nature of what is being tested you can never be sure if they are doing what they say they are doing. And of course if you jam one and not the other system then anyone with one of each receiver will know something is happening. Lots of bluff, double bluff and tripple bluffs take place!
Ground based jamming using brute force needs some quite big brute force! Most of the GPS receivers are looking up for their signal and of course the system is using spread spectrum so any ground based system has to put out a considerable signal strength over a wide bandwidth to jam the real signal. The effect has quite a limited range but in some scenarios would be sufficient. Drone aircraft which have a long endurance would make a good platform to carry a airborne jamming signal but they may not be able to carry a big enough transmitter and power supply for brute force jamming. You’d probably want to carry out spoofing though from one of these. You need to transmit a signal that appears to be from the satellites but is giving the wrong data so the ground based receiver calculates the wrong position.
If you just jam it so it doesn’t work at all then ground based users immediately know something is wrong. By jiggering about sufficiently with the spoof signal you can destroy the user confidence in their GPS as it appears to be working but it takes them sometime to realise it can’t be trusted.
Of course it’s much easier to jam ground based receivers than airborne ones. In the air you can have an antenna on top of the wing shielded from the ground and one on the bottom of the wing shield from the sky. A signal that comes up and differs from one coming down is a jamming signal and can be ignored.
Given the limited range it’s not surprising you’ve not really been aware.
In reply to MM0FMF:
Portreath MoD base is the place they do most tests and the range can be up to 12 miles away.
A warning will be given such as "GPS SIGNAL JAMMING TRIALS. JAMMER LOCATED WITHIN 0.5 [nautical miles] OF 5016N 00516W (PORTREATH, CORNWALL). ACTIVITY MAY AFFECT AIRCRAFT WITHIN 6NM RADIUS FLYING BELOW FL300 [30,000 feet] … DURING TRIAL PERIODS, GPS RECEIVERS MAY SUFFER INTERMITTENT/TOTAL FAILURE, OR GIVE INCORRECT POSITION INFO …"
They also do test in Sennybridge Training Area in Wales, Lincolnshire and Scotland
