Thanks, we were looking at using a preamble signal for exactly the reasons
your stated. I've looked at the Keymarks, and the range seems a little
under what we are hoping for. The accuracy you quoted is far better than I
expected though, I must have missed this in the datasheet. I shall
certainly give then another look.

Mat

>> The main issue is how to ensure that all the radio's have the
>> same latency recieving the pulse through the circuitry, to
>> ensure they are all in sync
>
> I can comment on the Keymarks but I guess this applies to
> others as well. Without any 433MHz present, the output
> of the receiver is random noise, because of the AGC, and
> it does take a little while (several milliseconds) for the o/p
> to settle down on reception of RF
>
> If this is also the case with whatever receiver you use, you'd
> want to have the receiver powered up for a short time before
> the expected pulse transmission, and the transmission itself
> may need to include a preamble (like a balanced bit pattern
> such as a series of 0xAA or 0x00FF) to stabilise the receiver.
> You'll have to test the latency of modules for yourself
>
> In my experience with the Keymark modules, the falling edge
> of a '1' bit is within 20us of expected, ie a 1.000ms pulse
> comes out of the Data pin as 1.000ms +/- 0.02. The bit-length
> inacuraccy rends to be in a '0' - '1' sequence. Although the
> '0' and '1' bit add up to 2ms, the rising edge in the middle is
> +/- more than the falling edge
>
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