Sat Nav

I have recently bought myself a TomTom Go 720 with the TMC (Traffic Message Channel) receiver add-on. I’ve used it on a few longer journeys already so I’ve had a good opportunity to assess it.

I’m not new to Sat Nav, I’ve been using TomTom Routeplanner on my HP iPaq with a Bluetooth Emtac GPS receiver, but TomTom no longer support this package fully, so the maps are now very out of date and I would find myself apparently driving across fields according to Routeplanner when on new Autoroutes in France.

The TomTom 720 is a very well developed receiver. It has a long list of features:

  • GPS navigation system for Western Europe
  • 4.3 inch touch screen
  • Spoken instructions
  • Built-in FM transmitter
  • Text to speech,
  • Bluetooth car kit
  • Safety camera database and TomTom Map Share
  • iPod control, using the optional iPod connection cable

So there is a lot in such a small package. Once set up it seems very easy to use.

Some technical observations. Whilst GPS is very accurate, ensuring that the mapping is just as accurate is just as difficult to acheive. It would appear that Tom Tom has achieved this using some form of algorithm that always puts the pointer on to the nearest road.

The receiver is very sensitive as it will gain lock even indoors. Again the TomTom uses a downloaded quick fix file to predict what satellites it should be seeing at whatever time of day.

The traffic alert system is great when it is receiving a strong enough signal. There are a lot of comments about this aspect on the many UK SatNav forums. However, using my knowledge of radio propagation I think I’ve worked out why the performance in UK is not quite as good as expected.

Here in UK the TMC data is carried only on ‘Classic FM’s transmitters. Whilst these give nearly 100% coverage, this is achieved only for VHF FM receivers with an outdoor antenna at 10 metres above ground. Receivers in cars are of course more sensitive these days so areas of poor music/speech coverage might not be so obvious.

Having used the TMC receiver around the M25 recently and noting where it is receiving data and when it’s retuning constantly. Then comparing it to the coverage maps. It would appear that the TMC receiver is possible less sensitive than the average FM receiver. The field strength required for satisfactory TMC reception seems to be in the region of 70 dBuV/m, if you then look at the coverage around UK at this level there are significant holes in the coverage area in UK.

I will soon be able to report how the UK users experience compares to that of France. In France the TMC transmissions are all carried on 107.7 MHz the Autoroute Info channel. They have on average a relay transmitter every 8 kms so hopefully the traffic receiver should work better over there.

It is not possible to use the iPod connection kit at the same time as the TMC receiver, as they use the same socket on the TT Go 720. But if you sacrifice the Traffic alerts for playing your iPod via the TomTom then you gain a very integrated system.

It is possible to separately output the speech instructions and the music from your iPod to either an external line output jack, via Bluetooth, via the FM transmitter or via the internal speaker. Both the speech and music have their own independent volume controls as well. When ever there is a speech announcement or instruction the TomTom fades down the music and then fades it back in again, very useful…

The Tom Tom displays the track details and includes icons to control the iPod. This seems to work very well and as it is on a bigger display it is easier to view.

The bluetooth to your phone also integrates your phone in to the system as well. letting you use the TomTom as a hands-free kit for your phone. It will also read out incoming text messages.

I’m bound to have missed something out in my description, such is the huge list of things that this package can do.

Of course I needed a case for the TomTom, There is one from TomTom, but it only holds the TomTom receiver itself. I wanted something that would also contain the connection cables as well. So I’ve bought a Lowepro Apex AV Photo case, it is designed for digital storage units, but it’s a perfect fit for the Tom Tom and there is space for the cables as well.

I will post further views after my next trip to France.