GNSS is not the only useful input to aid inertial navigation. For different environments, different sensors can also be added to aid the solution. A common external sensor for ground vehicles is the addition of a Distance Measurement Instrument (DMI) such as an odometer. This provides another independent measurement of displacement and velocity that can aid the GNSS+INS navigation solution. This is mainly of use when the GNSS signal is denied, for example, when travelling through a tunnel.