To make this improvement the wheel speed state has been added to the Kalman filter in the Inertial+. Previously the Inertial+ only used the wheel speed to update the Kalman filter when the external GNSS was not available. Now the wheel speed is used all the time and this lets the Kalman filter remove more of the poor GNSS measurements. The older processing platform in the original RT products was not fast enough to have the wheel speed state as part of the main Kalman filter but the new, faster processor used in the Inertial+ product does not have a problem with the extra processor load caused by this new state.
Wheel speed is an excellent addition to the Inertial+ since it makes a significant difference to the drift. Our tests show that the Inertial+ drifts by about 2.5m per kilometre (or minute) in “ideal” conditions when using wheel speed. When there are significant errors in the GNSS position and velocity before an obstruction, like a tunnel, then the drift can be higher. Using the new wheel speed this is reduced so that we should be closer to our specification in non-ideal conditions. Quantifying this exactly is very difficult.