GTM 12 traffic alert receiver

morvern · Occasional Visitor Joined: Apr 05, 2006 Posts: 6

Hi,
Just recently bought a Garmin GTM 12 to go with my Nuvii 300.
It works fine around the A1 in North Yorkshire with the main signals obviously coming from the vicinity of the TrafficMaster roadside sensors.

My problem is that although the unit appears to be working Ok, when I press the "Traffic" button in "Settings" it comes up with a dialogue box indicating that the subscription is not activated. It will be activated automatically when the GPS is working and a signal is received from the traffic alert service (TrafficMaster/RAC Alive). Despite driving past a few sensors and receiving traffic news from around the country I can't get rid of the activation notice.
A bar containing the words RAC Alive - life time subscription is visible under this box also "New Subscription" Neither are filled in just black bars containing the wording. Nothing happens if one presses them either???

If anyone has managed to activate their Nuvii's correctly I would be pleased to know how they did it. (manual on line not much use - none supplied with receiver)

Thanks

swing

morvern · Occasional Visitor Joined: Apr 05, 2006 Posts: 6

Hi Swing,
Thanks for the info.
Re the source of the data received.
My observations were based on the simultaneous reception of data from the GTM 12 and the beeping of an old TrafficMaster receiver I had in my car at the same time. This was as I approached a TM roadside sensor.
I have noticed that there are two aerials (base loaded dipoles) on each roadside sensor. One appears to be of the length associated with the VHF FM radio band and the other is quite a bit shorter...top end of the PMR VHF band (175mhz - 200mhz) I would say. As TrafficMaster data is collected from these roadside sensors, the data will only be applicable to those roads where they are sited. So surely it would make sense to rebroadcast the processed data (traffic alerts) from the same masts but using a different frequency from the original raw data transmissions (PMR VHF range)
I accept that there is also commercial radio transmission of the data but I think the intermittant reception that people have complained about may be due to being both out of range of the roadside transmitters and the commercial radio ones. This may be because of terrain signal blocking or just weak signal strength (distance from transmitter = out of the signal footprint). There is also the problem of signal attenuation from the metalwork of the car (all ready well discussed in this forum)

The current drawn by the GTM 12 is probably negligible so it is unlikely that reason causes a significant reduction in signal strength.
Garmin have a tendency to acquiesce to customer demands so would provide almost any low cost solution to keep folk happy until they solve the problem or bring out a more refined product.
:D
Morvern

swing · Posted: Thu Sep 21, 2006 11:48 am Post subject:

The 2 radio masts are for the transmission of the data from the mast to and from their HQ, and for the transmission of the traffic data on the dedicated frequency TM have rights to for broadcasting to some of their own traffic display units.

These masts neither have the technology, nor the license to broadcast on the VHF FM frequency band. Also, because of the way that their units work, the transmission is only for the very local area, and does not have the information to broadcast the data for the whole country, which is available via RDS-TMC.

There are other technical reasons why they don't and won't use these sensors to broadcast the data.

It's not the current drawn by the GTM that causes the problem (although there are rumours it's related to voltage), however, Garmin have stated that the problem is that the car charging cable introduces electrical noise which prevents the GTM units getting sufficiently good signal to be able to lock onto and receive the traffic data.

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