My old sat-nav (a fat TomTom Go 300) has served me well. Yes, it's an old chunky model, eerily like a small CRT TV. The battery life is degrading all the time, so I need to keep it plugged in most of the time. But it still gets me from A to B, and occasionally to C.

Yet I find myself wanting a new one. Especially as TomTom recently unveiled its new x40 Live sat-nav models.

It's not that they're slimmer, lighter and have a bigger screen than my Go 300. But these next-gen nav-boxes represent a new wave of 'connected' sat-nav systems, boasting more frequent traffic updates (HD Traffic Live) and improved routing algorithms (IQ Routes).

Revolutionary new tech? Or just a clever sales gimmick? I spoke to TomTom's Chief Technical Officer Mark Gretton to see what HD Traffic and IQ Routes add to in-car navigation.

Improving traffic information

If you already use a sat-nav system then you'll know that the real-time traffic information services are (a) not delivered in real-time and (b) often completely wrong.

"All sat-nav systems to date use a TMC (Traffic Message Channel) service," Gretton told me. "TMC is patchy in its road coverage and patchy in its update frequency. The data is typically 20 minutes out of date, so we can't use the information the way we want to - I.e. to automatically and dynamically route you around problems."

TomTom says that its new HD traffic service is designed to get around this problem. Unlike TMC, which is broadcast over FM radio channels, HD Traffic is updated every three minutes. The new x40 models have a SIM card inside, which can receive over-the-air traffic and content updates via an always-on GPRS connection. It's an obvious plus. The downside? These connected sat-nav services will also sting you for a subscription.

There's some clever technology under the hood. "First we take data from existing news services and traffic reports," says Gretton. "We also use extremely clever software that exists on the Vodafone network. It looks at all the call traffic going on and it roughly knows how far each call is from a base station, where the base station is and when calls are handed over from one base station to another. It takes all this data and calculates the velocity of the caller. If it's above a certain threshold, it assumes that they're probably in a car."

"We do this lots and lots of times with the volume of calls on the Vodafone network. What you get out is an average real-time speed estimate of every road signal. This data is then merged with the other traffic data. We also run a traffic and tracing service for fleets, which provides a third set of high-quality data."

Connected sat-nav systems

Because the new TomTom x40 models feature a SIM card, route-specific data can be regularly broadcast to your sat-nav as you drive. This not gives you access to rapidly-updated traffic information and accident hotspots, but opens up the prospect of accessing other dynamically delivered content - safety camera locations, fuel prices, POIs and so on.