Indirect TPMS (iTPMS) systems do not use physical pressure sensors; they measure air pressures using software-based systems, which by evaluating and combining existing sensor signals such as wheel speeds, accelerometers, and driveline data to estimate and monitor the tire pressure without physical pressure sensors in the wheels. First-generation iTPMS systems are based on the principle that under-inflated tires have a slightly smaller diameter (and hence higher angular velocity) than a correctly inflated one. These differences are measurable through the wheel speed sensors of ABS/ESC systems. Second generation iTPMS can also detect simultaneous under-inflation in up to all four tires using spectrum analysis of individual wheels, which can be realized in software using advanced signal processing techniques. iTPMS systems are sometimes referred to by other names, such as ‘Deflation Detection System (DDS)’ (e.g. Ford) or ‘Deflation Warning System (DWS)’ (e.g. Honda). iTPMS cannot measure or display absolute pressure values; they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly. The reset is normally done either by a physical button or in a menu of the on-board computer. iTPMS are, compared to dTPMS, more sensitive to the influences of different tires and external influences like road surfaces and driving speed or style. The reset procedure, followed by an automatic learning phase of typically 20 to 60 minutes of driving under which the iTPMS learns and stores the reference parameters before it becomes fully active, cancels out many, but not all of these. As iTPMS do not involve any additional hardware, spare parts, electronic/toxic waste, or service (beyond the regular reset), they are regarded as easy to handle and customer friendly. As mentioned however, the sensors must be reset every time changes are done to the tire setup, and some consumers do not wish to have this added responsibility. Since factory installation of TPMS became mandatory in November 2014 for all new passenger vehicles in the EU, various iTPMS have been type-approved according to UN Regulation R64. Examples for this are most of the VW group models, but also numerous Honda, Volvo, Opel, Ford, Mazda, PSA, FIAT and Renault models. iTPMS are quickly gaining market shares in the EU and are expected to become the dominating TPMS technology in the near future. iTPMS are regarded as less accurate by some due to their nature—given that simple ambient temperature variations can