The trident is fitted with doppler. A trick with those was that if you were flying a single track for miles, ie the Atlantic, is was not neccessary to have a map.
You selected the track on the doppler, put a chinagraph spot under the pen on the backplate and turned left/right to keep the pen on the spot.
Simple.
Information for those that have not used Decca roller maps. The map can never be true to scale because the Decca signals are curved and the map requires straight inputs to roll it up/down and the pen left/right. The result is that the map gets more distorted the further one gets from the master and slaves station. The London area would look reasonably normal but the South of France would look like a crushed sausage. To have a reasonable presentation would require dozens of chains and VOR was a lot simpler.
They had a go at creating a topographically correct map but it was impractical. I remember flying a trials unit that gave a digital readout instead of deccometers but it was too prone to lane shift and faded away.
Decca was OK when used by aircraft and ships in the marine envirionment. SAR, oil support helicopters used it until the end and then it closed on the 31st Mar 2000. No more would there be a runaway map pouring yards of paper over the cockpit floor.
Last edited by Fareastdriver; 10th Jun 2010 at 12:06.