Flights that take place at lower altitudes require more transmitters, due to reduced visibility over the horizon. We see this particularly with helicopter flights between drilling platforms. For multiple transmitters in accordance with the ED137 standard, the supplier preferably supplies hardware that converts IP transmissions to analogue signal. The time setting of these IP transmissions is timed based on a GPS signal. At the transmitter a delay will be added, so all transmitters will transmit the same audio at the same time. In practice the audio will be delayed on purpose by 1,5 – 2,5 milliseconds between two different locations, to increase the audibility of the signal. Working with absolute time via GPS is preferred. In principle, in addition to a GPS time measurement, one can also work with a network delay measurement. Changing network delay (jitter) could cause incorrect transmission timing.
Also possible on FM
In marine communication FM modulation is used, instead of AM modulation. Audio should than transmitted with an accuracy of 8 microseconds, instead of milliseconds. Timing via GPS is required here.. The areas of application are: waterways and harbours.
As mentioned before, you need Climax (Simulcast) when an aircraft receives two transmitters at the same time, to avoid audio quality problems in the transition area.
