to measure the emissions during the on-road test, vehicles are equipped with a portable emissions measurement system (pems) that monitors pollutants and co values in real time. the pems consists in a complex instrumentation that includes: advanced gas analyzers, exhaust gas flowmeters, an integrated weather station, a global positioning system (gps), as well as a connection to the network. the protocol does not indicate a single pems as reference, but indicates the set of parameters that its equipment has to satisfy. the collected data are analyzed to verify that the external conditions under which the measures are taken satisfy the tolerances and guarantee a legal validity.