ISM Operating Requirements for GPS-based telematics systems
ISM specifies six operating requirements that ensure telematics systems accurately measure and report vehicle speed using GPS.
- The resolution of the stored latitude/longitude position calculated by the In-Vehicle Unit (IVU) GPS receiver shall be to 0.00001 degrees or better. This level of resolution is adequate for calculating a positional accuaracy of 10 metres.
- Vehicle speed shall be derived from GPS Doppler measurements. Doppler is a method to calculate vehicle speed based on the change in frequency or timing of signals caused by movement of the difference GPS satellites relative to the IVU GPS receiver.
- Vehicle speed shall be recorded to a resolution of at least 0.1km/h.
- The IVU shall generate a Speed Record no less frequent than every 3.0 ± 0.1 seconds.
- Vehicle speed, in excess of 60 km/h for heavy vehicles, should be accurate to within 3.0 km/h for at least 99.9% of observations when using at least four GPS satellites and a Horizontal Dilution of Precision (HDOP) of < 4.
- Reliance on a single GPS speed observation is not appropriate; owing to localised interferences which may not necessarily be identified and corrected by the IVU GPS receiver. An approach which analyses an array of speed observations for integrity is the most optimal.