What ADAS calibration is, and why a new windshield needs it

If your car has lane keep assist, adaptive cruise, automatic emergency braking or a lane departure warning, it has an Advanced Driver Assist System. On most vehicles the main eye of that system is a camera bracketed to the inside of the windshield, usually tucked behind the rear view mirror.

That single fact is why a windshield replacement on a modern car is not the job it was fifteen years ago.

Move the glass and you have moved the camera

The camera does not measure the world in feet. It measures angles, and it converts those angles into distances using an assumption about exactly where it is mounted and exactly which way it is pointing.

Replace the windshield and every one of those assumptions is now approximate. A new bracket sits a millimetre or two differently. The glass has a slightly different thickness or curvature at that point. The camera is looking through a different piece of optical material than the one it was taught on.

A couple of millimetres at the mirror becomes a large error out at the distance where the system is trying to judge a car in front of you. That is the whole problem in one sentence.

It is not only glass

Windshield replacement is the most common trigger, but it is not the only one. Calibration is also called for after:

• Repairs involving steering and suspension, because they change the vehicle's ride height and thrust angle, and the camera's idea of level goes with it.
• Wing mirror replacement, on vehicles with blind spot or surround view cameras in the mirror housing.
• Headlight replacement, on systems that steer or shape the beam based on what the forward sensors report.
• Anything that disturbs a radar unit behind the grille or a sensor in a bumper.

Static and dynamic calibration

There are two methods, and the manufacturer decides which your vehicle needs, not the shop.

Static calibration happens in the workshop. The car is set on a level floor and precise targets are placed at measured distances and heights in front of it. The system is told what a known pattern should look like from a known position, and it corrects itself until what it sees matches what it should see.

Dynamic calibration happens on the road. The vehicle is driven at a steady speed on a stretch with clear lane markings while the system learns against real road furniture. It sounds looser than the static method and it is not: the vehicle is running its own procedure and it will refuse to complete if the conditions are not good enough.

Plenty of vehicles need both, in a specific order. Getting that order wrong means starting again.

Why an uncalibrated system is worse than no system

This is the part that matters and the part that is easy to miss, because nothing looks wrong. There is no warning light for a camera that is confidently pointed slightly wrong. The dashboard looks normal. The system reports itself as available.

What actually happens is subtler. Lane keeping nudges the wheel toward a line that is not quite where it believes it is. Automatic emergency braking judges a closing distance it has slightly off, so it reacts a little late or a little early. Adaptive cruise holds a gap that is not the gap you set.

A driver who knows their assist features are switched off drives accordingly. A driver who believes the system is working, when it is quietly wrong, does not. That is why this is a safety item rather than a convenience one.

How to make sure it actually gets done

Ask before the work starts, not after. Two questions cover it: does my vehicle need calibration after this repair, and will it be done here or sent out?

Sending the car on to a second shop is common and it means another appointment, another day, and a gap where the car is back on the road with the glass replaced and the camera not yet reset. We calibrate in house, in the same visit as the glass, which is the main reason we brought it under the same roof in the first place.

Advanced Driver Assist is meant to prevent accidents. It can only do that from the right position.