Using CTCSS to mute GPS data noise
Posted: Thu Jul 31, 2008 2:53 pm
At the request of those teams who do not use GPS tracking but can hear the data noise produced by those who do and as a suggestion only...............
The attached Simoco configuration files are a variation on the generic ones already posted on this site. However, there is now an additional channel programmed, 'CTCSS_53a', channel 13, that has a muting tone of 218.1Hz programmed on both receive and transmit. A few other small modifications have been made to the config. If any of the other channels are used then performance will be as for the standard, generic configs. If channel 13 is used then the GPS data received by both the base radio and the sending portable will be completely silent to you but will still be processed by the base radio and the position report will appear on MRMap. Voice audio is not affected, only the sound of the GPS data.
It works because there is a new function included in the recent versions of the Simoco programming software, currently v5.44. This is available from the TMC Radio website, NOT the UK Team Simoco one. The GPS data sent by a portable or vehicle radio on release of the press to talk key (PTT) is part of the Automatic Numeric Identifier (ANI) data for that radio. This is normally used to tell base who is calling in but the GPS position report can be tagged onto this if selected in the programming. This is how we send the position reports. The new function allows the previously unavailable lead-in delay to be added to the PTT release so that the GPS data is sent so many milliseconds after you release the PTT instead of instantaineously as it has been to date.
The reason we need it to do this is that the CTCSS muting system (Continuous Tone Controlled Squelch System) has a built in hang time after the received signal has been lost. This can be anything up to 200 milliseconds where the mute gate remains open so anything sent immediatly the PTT is released, gets through the hang time and is heard in the speaker. It follows then that if the lead-in delay for the GPS data, is made longer than the hang time then the CTCSS mute gate will have closed before the GPS data is sent. You don't now hear it but the radio still does and processes it correctly but in silence.
A single CTCSS tone, (218.1Hz) is now permitted on the UKSAR channels and this is a means by which the GPS data noise can be removed. The decision as to whether you use this method, and it works for any make of radio capable of handling CTCSS, is yours. However if any team does elect to use it then their radios become deaf to any transmisson made on the same channel that doesn't also have the correct tone attached. To prevent this happening, we would suggest that the 218.1Hz tone is added to the transmit channel of all team radios regardless of whether you use GPS or not. There is no need to add it to the receive channel which will then remain open to all incoming signals. However, because you are now transmitting a tone, any team using full Rx/Tx CTCSS can still hear you normally. Your own radios without a tone on receive, will still hear the GPS data noise but that decison rests with yourselves. Also a sound idea, is some way of defeating the CTCSS in emergency. The exact way of doing this will depend on your make of radio but for the Simoco users it's a button needs programming as 'CTCSS On/Off'.
Please note also that the current version of the programming software for Simoco radios also includes the requested option to allow GPS data to be sent on a channel-by-channel basis. It can and should be removed from channels where the recipient of the transmission couldn't reasonably be expected to be monitoring your location. Helicopters and Coastguard etc come into this catagory. Under the 'Selcall' options page for the individual programmed channels, there's a check box called 'No FFSK ANI'. If you check this for any channel then no GPS or any other kind of FFSK data will be sent on that channel.
Please also see the documentation page for upto date info on this topic.
The attached Simoco configuration files are a variation on the generic ones already posted on this site. However, there is now an additional channel programmed, 'CTCSS_53a', channel 13, that has a muting tone of 218.1Hz programmed on both receive and transmit. A few other small modifications have been made to the config. If any of the other channels are used then performance will be as for the standard, generic configs. If channel 13 is used then the GPS data received by both the base radio and the sending portable will be completely silent to you but will still be processed by the base radio and the position report will appear on MRMap. Voice audio is not affected, only the sound of the GPS data.
It works because there is a new function included in the recent versions of the Simoco programming software, currently v5.44. This is available from the TMC Radio website, NOT the UK Team Simoco one. The GPS data sent by a portable or vehicle radio on release of the press to talk key (PTT) is part of the Automatic Numeric Identifier (ANI) data for that radio. This is normally used to tell base who is calling in but the GPS position report can be tagged onto this if selected in the programming. This is how we send the position reports. The new function allows the previously unavailable lead-in delay to be added to the PTT release so that the GPS data is sent so many milliseconds after you release the PTT instead of instantaineously as it has been to date.
The reason we need it to do this is that the CTCSS muting system (Continuous Tone Controlled Squelch System) has a built in hang time after the received signal has been lost. This can be anything up to 200 milliseconds where the mute gate remains open so anything sent immediatly the PTT is released, gets through the hang time and is heard in the speaker. It follows then that if the lead-in delay for the GPS data, is made longer than the hang time then the CTCSS mute gate will have closed before the GPS data is sent. You don't now hear it but the radio still does and processes it correctly but in silence.
A single CTCSS tone, (218.1Hz) is now permitted on the UKSAR channels and this is a means by which the GPS data noise can be removed. The decision as to whether you use this method, and it works for any make of radio capable of handling CTCSS, is yours. However if any team does elect to use it then their radios become deaf to any transmisson made on the same channel that doesn't also have the correct tone attached. To prevent this happening, we would suggest that the 218.1Hz tone is added to the transmit channel of all team radios regardless of whether you use GPS or not. There is no need to add it to the receive channel which will then remain open to all incoming signals. However, because you are now transmitting a tone, any team using full Rx/Tx CTCSS can still hear you normally. Your own radios without a tone on receive, will still hear the GPS data noise but that decison rests with yourselves. Also a sound idea, is some way of defeating the CTCSS in emergency. The exact way of doing this will depend on your make of radio but for the Simoco users it's a button needs programming as 'CTCSS On/Off'.
Please note also that the current version of the programming software for Simoco radios also includes the requested option to allow GPS data to be sent on a channel-by-channel basis. It can and should be removed from channels where the recipient of the transmission couldn't reasonably be expected to be monitoring your location. Helicopters and Coastguard etc come into this catagory. Under the 'Selcall' options page for the individual programmed channels, there's a check box called 'No FFSK ANI'. If you check this for any channel then no GPS or any other kind of FFSK data will be sent on that channel.
Please also see the documentation page for upto date info on this topic.