A long-standing TVL campaign has been the use of "detector vans", which claim to be able to identify the location of television sets and the programmes they are receiving:

A map of recent detector van sightings can be found here.

Unfortunately the BBC have consistently refused Freedom of Information requests to determine how these devices work, and have not allowed evidence collected from them to be tested in court.

Although detector evidence has been used to obtain a warrant to enter a premises, this requires a much lower standard of proof than evidence presented in open court (e.g., the calibration or accuracy of the detector is not examined).

How do they work?

The most common hypothesis is that detectors, either hand-held or van-based, detect a signal given off by a TV component known as a "local oscillator".

In order to improve the reception of the signal being received by the TV, a TV uses this component to generate its own signal which it combines with the received signal.

This locally generated signal can be detected some distance away from the TV itself, and can be used to obtain the direction from the detector to the TV (after which, a few readings several metres apart can be used to triangulate the position).

How effective are they?

Despite several FOI requests, the BBC have consistently refused to provide information on the equipment used by detector vans (or how that equipment is calibrated or tested).

Interestingly, some apparently innocuous questions have also been refused on the grounds that they would reduce the effectiveness of the license collection system:

• How many detection devices the BBC possesses.
• The technical specification of these devices.
• How often are they deployed.

Since the answers to these questions should not affect the effectiveness of a television detector (it either works or it does not), it is difficult to understand why they would be refused.

The most likely explanation is that, like TVL's letter campaign, their effectiveness is largely based on the popular perception that they are effective (some estimates are that there are only 26 vans in use, which may explain the recent claim that detector vans are now unmarked).

This view appears to be shared by the Information Commissioner's Office, who have confirmed that "the BBC state that to release information which relates to the number of detection devices and how they are used will change the public's perception of their effectiveness".