GPS Data, silent or not?

[Rob_Brookes]

It's long been known that those teams not using Simoco radios can hear the full data burst when a position report is sent and not the truncated version that Simoco users hear. For this reason we argued for permission to use CTCSS on the UKSAR channels in order that the data could be fully muted. This works because the ctcss mute gate on our radios, any make, has closed off the speaker audio not more than about 200 milliseconds after the PTT is released. If the GPS position report is timed via the programming software to be sent say, 320 ms after PTT release then it follows that the receiving radio won't hear it because the mute gate has closed and GPS data doesn't carry the necessary ctcss tone to open it. The receiving radio however can still 'hear' the GPS data even if you now can't and so it's processed in silence. At least, that was the idea.

Kendal MRT for instance, have elected to program thier team working channel for ctcss on both receive and transmit and they are not alone in doing this, a growing number of teams have opted to use tone muting mainly to reduce the impact of interference on the working channels.

However, there's a potential problem with this that was raised recently by a Lakes team and confirmed by our colleagues in Scotland, where Simoco radios are not used.

If 'Data CTCSS' is programmed either with a tone value or by using the 'Unchanged' option then a ctcss tone is sent when the PTT is released and the GPS position report is being transmitted. This results in the tone mute gate of the receiving radio hearing the required tone and connecting the speaker back into circuit. The upshot is that you hear both the voice signal and, 320ms later, the GPS data burst.

To retain the GPS data-muting effect of ctcss then the radio should be programmed with Data CTCSS set to 'Undefined' ie, don't send a tone with the data.

Whilst we can't in any way even suggest what others might choose to do when programming their radios, it's been pointed out that for those who want to fully mute their radios and not hear the GPS data, this is defeated by those teams with Data CTCSS set to 'Unchanged'.
It's really for MREW to rule on this one if indeed a ruling is seen as necessary but in the spirit of neighbourliness it might be good practice where your area or region borders with teams who don't use Simoco radios to not set the Data CTCSS option to either a value or to 'Unchanged'. Leave it as 'Undefined' and data will be inaudible to those teams not wishing to hear it and fully audible to those who do.

Data will now be completely muted in your own radios of course and maybe this is the price we pay but I'll look into other ways of duplicating what the GPS data burst has come to be used for by team members on the hill but was never actually intended to do.

With the potential arrival of a generic GPS microphone that sends data in a new format and not the one sent by Simoco radios, we will ALL soon be hearing full GPS data bursts so maybe it's time to consider whether silencing it via ctcss is for the common good.

If anyone has a view on this one then we'd like to hear it as I just report the issues.