As well as 81FM, which is the primary safety channel, there are generally two administration channels available for use by officials in order to keep the MSA 81FM safety frequency clear from unauthorised traffic. These can be duplex channels using repeaters with outputs around 86MHz, with either two separate channels being used, or all the traffic using one channel depending on the size of the event.
If both are used, the first channel, designated 'A', is used by the Clerk of the Course to communicate with various officials such as Stage Commanders and Chief Spectator Officer. The 'B' channel is used by radio cars located at the start, finish and at intervals (typically 5km) along a special stage. The information passed on the 'B' link concerns the locations of competitors on the stage.
Two reporting systems are used, positive and negative. In both cases the start and finish cars report the number and times of competitors passing their locations. When positive reporting is used the radio cars' crews along the stages report the number of each car as it passes them. For negative reporting, the numbers of the passing cars are noted by the crews of the radio cars and only 'missing' cars are reported over the channel.
A competitor stopping on a special stage is reported by marshals to the nearest radio car, which then inform control. If repeaters are not used, or cannot be accessed from particular points along a stage, one or other of the simplex colour channels maybe used to pass information.
A short but extremely important radio link is between the finish and stop line officials. Whilst this may be on the 'B' channel, it may on occasions use PMR446 or some other ad hoc channel. As a car crosses the finish line, usually at considerable speed, its time is recorded by a timekeeper.
Before the event the finish line clock used will have been synchronised with the clock used at the start line. Electronic devices connected to the clocks are used to indicate the start time to the driver and the finish time to the timekeeper. having crossed the finish line, the car has to pull-up at the stop line, which is likely to be some way further along the stage. As the car is arriving at the stop line, the finish line timekeeper radios the car's time to the stop line official who records it on the competitor's time card.