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Experiments with old DECCA Equipment

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DECCA hyperbolic navigation basic principles
Rather than using a pulse system, the DECCA system made use of a chain of
continuously transmitting stations on different frequencies but phase coherent locked at their reference oscillator.
In a stationary situation the resolved signals would be phase coherent as well.
However when moving the phases slowly would shift along the position of the vessel.
These shifts where counted using a DECOMeter offering a numbered hyperbolic line on a map.
Using triple meters, one could find at least two hyperbolic lines which intersections would represent the position.

a decometer a decometer

Generating DECCA signals using a FPGA
In order to get the old DECCA devices to live, i needed the original signals.
However all the DECCA transmitters have been dismantled long ago.
So i needed to generate the signals myself, using FPGA hardware i did use for some other projects as well
I decided to use a FPGA for this project as dedicated hardware would become to bulky not knowing the pitfalls waiting for me on this road.
As digital logic can't multiply clock signals i had to divide.
So instead of using a 14.2 kHz common clock, i did use a 5.112 MHz clock,
derived from one of the PLL's of the FPGA.
I also added a transmission scheme scheduler as used in the later DECCA chains,
and even a Orange slave, which differs from the others, is not phase coherent
and is used for measuring distance rather than position.

The  DECCA signal generator concept The DECCA signal generator concept

The DECCA signal generator hardware The DECCA signal generator hardware

Next to generating the RF signals i had to generate the MultiPulse transmission scheduler
as shown in the block diagram above.
The resulting output is as shown here.

The output of the generator The output of the generator

The 20 second MultiPulse schedule The 20 second MultiPulse schedule

Getting old DECCA equipment to run
There are several DECCA devices that i was able to purchase on sites like EBay.
One of them is an original DECCA MK21. However it does not really work which is a bummer
as it is the easiest device to test my generator on.
Although not utterly complex, without schematics it is a tough job to get it back to live.


Most of the less vintage devices are from the nine-tees and need the scheduled transmission scheme in order to properly function.
This scheme initially was created to confuse unofficial DECCA receivers.
As the DECCA signal was available unencoded and de DECCA devices where expensive and needed a annual registration like telephone systems in some countries required at the time.
Oddly enough most of the pirated DECCA-devices had no trouble with the newly coded signal as they all used a microprocessor eliminating the need for a special hyperbolic navigation map such offering a way more navigation-aid than the genuine DECCA equipment.

Still i had to develop the signal generator from information found on the internet, mainly thanks to the excellent website of:
Jerry Proc