Thread: Traffic Receivers

some GPS need cord to receive traffic signals

And by the way if anyone knows how do they do it, from the engineering perspective? I mean what's the interface, protocol, data formats, etc. Perhaps it's possible to design a box sitting between a radio receiver and any usb-enabled Garmin GPSr like the GPSMAP series

I have exactly same question.

One of the older devices I have has a traffic transponder in the cable, tiny plastic brick and without it I don't get traffic data (It has never been correct though, but thats whole lotta another issue). Another device I got claims, it has FM receiver of some kind in the device and no such transponder in the cable. So I am wondering the same, what if I use standard charging cable connected to a phone charger? Will it work?

The ability of a GPS device to receive live traffic information is dependent not only on hardware but also the firmware run by the device.

Hardware: Some Garmin devices have the hardware (FM receiver only or dual FM and digital radio receivers) as part of the device itself and those use the OEM vehicle charging cable as the signal antenna in much the same way as a cell phone receives radio signals via the earplug cable so without the correct cable fitted they don't receive a proper signal. Other devices which don't have the receivers onboard and rely on a special charging cable which contains the traffic receiver either in the ciggy plug (eg early ones like GTM 20) or in a little plastic brick built into the cable (eg GTM 25/26/35/36/60/70). Some devices which use an external receiver need to use a specific one or two models and if an incompatible GTM receiver is plugged in there will be a message displayed on the screen to that effect.

Software: Unless the device firmware (unit software) supports traffic then regardless that it may be physically capable of using a traffic receiver then it cannot work. An example of that is the 2nd generation nuvi 2x5W released in ~2009 which can be modified to receive traffic. Some handhelds mean for mainly off-road and hiking cannot receive traffic nor can they be modified to do so.

How the system operates: Traffic Message Channel (TMC) is a type of Radio Data System (RDS), a special application of the radio band for sending small packet transmissions of additional information. Most car stereos support FM RDS signals, which is how you can see radio station call letters or artist and song title information on your display when tuned to certain stations. HD Radio/DAB signals can broadcast traffic data in a similar fashion, but since digital radio signals are able to carry more information than FM they can provide real-time updates quicker, up to four times faster than FM RDS as well as supply more detailed info. In some countries the service is free while generally it requires either an upfront subscription lifetime license as part of the hardware purchase or previously commonly it was a yearly subscription. Technically it may be possible to build a traffic receiver yourself however the cost of doing so would be far in excess of any you can buy. Wikipedia actually has a very good article on how the TMC systems works:

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Originally Posted by Butters

The ability of a GPS device to receive live traffic information is dependent not only on hardware but also the firmware run by the device.

You're dead right. Christmas is over and now it's safe moment to reveal the truth by saying that santa does not exist. Yes the hardware and firmware depends on the application. But the topic implies the vehicle sat nav application and in that application it's not the best approach to pack all the hardware in a one toy-looking glossy box for use in cars. Indeed a GPSr with internal FM receiver looks ridiculous because all cars now are already equipped with FM radios. It's the radio unit's job to receive a live traffic data and to provide it to the navigation unit. The navigation unit's job is to plot the chart using internal cartography data base, a location information provided by the GPS receiver unit, a speed data from the car's speed sensor, and a road traffic data from the TMS receiver unit (which can be an FM radio unit). Not every car radio can provide a road traffic data but anyway it's not a new thing, I've decades old Blaupunkt FM radio that has dedicated digital output (so called TMS bus) for use with compatible navigation units. To my memory, some of that units were from Garmin. So this technology is by no mean new to Garmin either. In the automotive world, it's all very fragmented and I don't know if it's something Blaupunkt-specific or there is some kind of industry standard. What I'd tried to ask is if someone did reverse engineered an interface like this