Car Satellite Navigation: Sat Nav Systems Explained

Satellite navigation has gone from an expensive dealer option to something almost every driver carries in their pocket. What has changed most is not the accuracy, which was already good enough twenty years ago, but the live data layered on top of it: traffic, roadworks, incidents and rerouting.

This guide covers how the technology actually works, how the three types of system compare in real use, what the UK law says about operating one, and the practical fixes for the problems drivers most often run into.

How does car satellite navigation work?

A sat nav works out where you are by measuring how long signals take to arrive from several satellites at once. Each satellite broadcasts its position and a very precise time stamp. Your receiver compares those time stamps against its own clock, works out the distance to each satellite, and where four or more of those distances intersect is your position.

Three things happen after that:

• Map matching. The raw fix is accurate to a few metres, which is not always enough to tell one carriageway from the slip road beside it. The software snaps your position to the nearest plausible road on its stored map.
• Dead reckoning. When the signal drops in a tunnel or a multi storey car park, better systems keep estimating your position using wheel speed, steering angle and a gyroscope until the satellites come back.
• Routing. The device searches its road network for the best path to your destination, weighted by road type, speed limits, turn restrictions and, if it has a data connection, live traffic.

GPS is strictly the American constellation, but most modern receivers are GNSS receivers and can also use Europe's Galileo, Russia's GLONASS and China's BeiDou. Seeing more satellites means a faster first fix and better accuracy in built up areas, which is why a modern phone locks on far quicker than a sat nav from 2010.

Built in sat nav, phone app or standalone unit?

For most UK drivers a phone running Google Maps or Waze through CarPlay or Android Auto gives the best live routing, while a built in system gives the best integration with the car. Standalone units still make sense for vans, caravans and lorries.

A few practical points that the spec sheets do not tell you:

• Phone apps reroute better. Crowd sourced traffic from millions of handsets picks up a queue within a couple of minutes. Built in systems relying on broadcast traffic data are slower and coarser.
• Built in systems are better integrated. Directions in the instrument cluster or head up display keep your eyes closer to the road, and the audio ducks the radio properly rather than fighting it.
• Standalone units earn their keep on specialist routing. If you drive a Luton van, tow a caravan or run a horsebox, a unit that knows your height, width and weight will keep you away from low bridges and weight limits. No phone app does that reliably.
• Battery and heat. A phone navigating with the screen on in summer will get hot and may throttle. A decent vent mount and a proper cable help.

What sat nav map updates cost and why they matter

Map data goes stale quickly, which is the single biggest cause of a sat nav giving bad directions. New housing estates, changed junction layouts, revised speed limits, new one way systems and low emission zones all appear faster than most built in systems get updated.

• Phone apps update in the background at no cost. This is their clearest advantage.
• Built in systems vary enormously. Some manufacturers include several years of updates with the car and push them over the air. Others charge for a USB or SD card update, and older cars can end up with maps that are effectively frozen.
• Standalone units typically offer either lifetime maps or a subscription. Read the small print on what lifetime means, since it usually refers to the life of the device rather than yours.

If your car's built in maps are years out of date and updates are not economic, running the phone through CarPlay or Android Auto is usually the better answer than paying for a refresh.

Is it legal to use a sat nav while driving?

Using a sat nav is legal in the UK, but holding a phone while driving is not, and the rules changed significantly in March 2022.

• It is an offence to hold and use a phone or similar device while driving for almost any purpose, including entering a destination, changing music, scrolling or taking a photo. The penalty is six penalty points and a fixed fine. Anyone within two years of passing their test loses their licence at six points.
• A phone used as a sat nav must be in a cradle or mount and must not be held. Touching it in the cradle to accept a route is still risky ground, so set everything before you move.
• The device must not obstruct your view of the road. A large unit stuck in the middle of the windscreen can result in a charge for driving without a full view of the road, and it can also fail an MOT if it is in the swept area of the wiper on the driver's side.
• You remain responsible for driving safely and legally regardless of what the screen says. A sat nav telling you 40 does not override a 30 sign, and the maps are frequently wrong about limits.

Mount the device low on the windscreen towards the passenger side, on a vent mount, or use the car's own screen. It should sit within your natural field of view without covering any part of the road.

Getting a better GPS signal

Poor signal almost always comes down to something between the receiver and the sky. The fixes are usually simple.

• Give it a clear view. Tunnels, deep cuttings, tall buildings and multi storey car parks all block or reflect signals. Reflections off buildings are why your position sometimes jumps a street sideways in a city centre.
• Check for a heated or metallised windscreen. Many cars have a thin metallic coating in the glass for heating or heat rejection, and it blocks satellite signals effectively. Manufacturers usually leave a small unmetallised window behind the rear view mirror, marked by a faint dotted rectangle. Put the device or its aerial there.
• Swap the charger. Cheap USB chargers are a common source of radio interference that swamps a weak satellite signal. Try a different one before blaming the device.
• Keep the software current. Firmware updates improve satellite acquisition, particularly on older units that predate Galileo.
• Allow time for a cold start. A device that has been off for weeks, or has travelled a long distance since it was last used, needs a minute or two of clear sky to download fresh satellite data.

Choosing a sat nav: what actually matters

Screen size gets the marketing attention, but three other things matter more day to day.

• Map quality and update policy. More important than any hardware feature. Check how updates are delivered and what they cost before you buy.
• Traffic data source. Crowd sourced live traffic is worth more than any other single feature for UK commuting.
• Clear voice guidance. Lane guidance at complex junctions and clear instructions well before the turn are what stop you making a late decision. A system that says the road name and lane, rather than just left in 200 metres, is measurably easier to follow.
• Ease of destination entry. Postcode entry that accepts a full UK postcode and takes you to the right house, rather than the middle of the postcode area, saves genuine time.
• Speed limit display. Useful, but treat it as a prompt rather than an authority, because the underlying data is often out of date.

For walking or cycling, use a phone app rather than a car unit. Car systems route only on roads suitable for vehicles and will happily send you the long way round.

Where sat nav goes wrong, and the damage it causes

Sat navs are routing engines, not judges of whether a road suits your vehicle. They cause a predictable set of expensive problems.

• Narrow lanes and single track roads. Shortest route settings send cars down farm tracks and unclassified lanes with hedges either side. Hedge scratches down a full flank are common, and they usually need proper panel work rather than polishing out.
• Low bridges and weight limits. Standard car mapping ignores height and weight restrictions. Every year vans and lorries hit railway bridges that a specialist device would have routed around.
• Tight urban rat runs. Traffic apps divert drivers down residential streets with parked cars either side. Kerbing a wheel or catching a bumper on a bollard in an unfamiliar street is the usual outcome, and a scuffed alloy can normally be refinished without replacing the wheel.
• Car park and multi storey damage. Signal drops indoors, attention shifts to the screen, and pillars and ramps do the rest. Scraped bumper corners are the most common result and are usually repairable in a few hours.
• Fords, flooded roads and closed routes. A sat nav has no idea how deep water is. Never drive into standing water on the strength of a routing decision.

The practical answer is to keep the screen in your peripheral vision and the road in your central vision, and to override the device the moment the road in front of you stops looking sensible.

What3words and giving your location in an emergency

UK emergency services accept what3words locations, which divide the world into three metre squares each identified by three words. If you break down or witness a collision on a road with no obvious landmarks, it is often the fastest way to give an exact position.

Motorway and A road driver location signs, the small blue markers every 100 metres with a road number and distance, do the same job and are the preferred reference on the strategic road network. Either is far more useful to a control room than describing a stretch of dual carriageway.

Where car satellite navigation is heading

Two developments are already reaching production cars. Augmented reality guidance overlays the turn arrow onto a live camera view or projects it into a head up display, so the instruction appears to sit on the road itself. It removes most of the ambiguity at complex junctions, which is where people make late lane changes.

High precision positioning combines satellite signals with correction data to fix position to centimetres rather than metres. That is what driver assistance and eventual automated driving need in order to know which lane a car is in, not just which road. As those systems arrive, the map itself becomes a live, continuously updated model of the road rather than a static file you refresh once a year.