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Sky High Links The airborne data repeater is fitted to a fixed wing aircraft that also carries all the major teams' v.h.f voice repeaters. Whilst the cars are on the special stages the aircraft flies overhead, providing a reliable line-of-sight between the cars and the ground based stations. Using airborne repeaters is the only option as the terrain and distances encountered on some WRC special stages can be very hostile to ground based radio communications. Creating a reliable terrestrial system for every stage would be prohibitively expensive. Putting all the repeaters in one aircraft not only reduces cost, it's a lot safer than having a squadron of light aircraft flying around quite close together. As well as links from the cars to their team HQs and the service areas, some teams have additional links, which results in the aircraft sometimes being fitted with close to 30 repeaters. Like the u.h.f data link, the frequencies will vary from event to event as dictated by the radio authority of the country where the rally is taking place, but they are usually somewhere within the standard PBR bands. Some of the cars maybe fitted with mobile phones as a second string of voice comms should the v.h.f link fail, or as a secure alternative should the information that needs to be passed be too sensitive for a clear voice channel. A separate GPS based system collects other data about the cars' place on the stage at any given time are downloaded from cars via a wireless Local Area Network (LAN) at the start and finish points. The data is used to produce images for the Virtual Spectator system, which overlays images of the vehicles over graphical representations of the stage. Cars can be made to appear on screens as if they are on the same part of the stage at the same time, enabling visual comparisons of their progress. Great for the television fan and sports pundit ! Last but not least are the wireless LANS that are used to convey mechanical and engine management data collected by sensors on the car during the stages to a technician's laptop as the car drives up to the service area. This is only a very short range system, but it allows data to be collected just that little bit earlier than it could be if the car had to stop to enable a hard link to be made, and that's important in a sport where hundredths of a second count.
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