In AC 150/5220-24 FAA mentions radar equipment to detect foreign object debris (FOD) on runways, based on stationary and mobile radars with an astronomical precision, e.g. able to detect an object as small as 3 cm at a distance of 1 km.
Looking at their patent it appears it uses a multi spectrum system (radar & electro optical) to track all object: birds, aircraft, and FOD.
It uses a radar to detect objects
The potential foreign object source identifier subsystem preferably includes potential foreign object source detectors such as a tower-mounted millimeter wave sensor (MWS) detector
And a specialized light sensor to determine the makeup of the object
FOD identification functionality may be added to the FOD detectors, such as functionality which identifies the material which constitutes the FOD. Remote spectrometry functionality, such as that used in mineral prospecting satellites, may be employed for this purpose.
A neat function too is the ability to determine the where the FOD came from and send pictures of the FOD and the offending source to airport officials
The ascription output may be presented to an airport official in an airport control center on a display console which may show on a first portion of a screen, an image of the detected FOD and its location, both preferably in a zoomable format, along with a time stamp, and on a second portion of the screen, an image or other representation of a most probable source of the detected FOD, preferably in a zoomable format, along with its time stamp.
It appears it also fuses sensor data with aviation data to augment it's determination
flight database data indicating recent aircraft movements and optical identification data indicating vehicle movements is supplied to data fusion functionality which also receives ASMGCS data and weather data and interfaces
If it works as advertised it not only replaces visual inspection but also provides realtime feedback for bird control and communicating with aircraft falling apart.
There are a number of different runway inspection radars on the market - xsight (shown above) Tarsier or FodFinder.
What they have in common is exceptionally small wavelength - a few millimetres - which in radar terms is tiny. If you want to detect small pieces of debris (nuts, bolts, fragments of metal...), then you need a wavelength about the same size as the objects you are looking for.
However much airports might want to replace visual inspections (nobody wants to close their runway for an inspection, if instead they can bill someone for landing a plane on it) radars have their limits, they sometimes miss things, and don't always work in inclement weather.
A number of major airports have inspection systems, but the majority don't. Runway inspection is a business of radars, cameras and the Mark I human eyeball.
