I found an interesting webpage on RSDs.
The page explains that remote sensing devices are used to measure the emissions of vehicles. It uses infra-red and ultra-violet beams to measure the gas emissions and can be placed both on fixed land or on the vehicle itself, from where the emissions have to be measured.
On the following image taken from Smogtips, the measuring and analyzing processes are explained.
As the picture explains, if the toxic emissions are too high, the car owner will receive an email to be able to join a free repair program.