However, measuring internal clock frequencies in an ECM is beyond the scope of this research. A secondary possible influence on the accuracy of speed data in an ECM is drift and jitter in the timing of the ECM. The accuracy of the computed values is affected by calibration parameters specifically the " pulses per mile " parameter relates the tail shaft rotational velocity to the forward velocity, and is based on tire size and gearing. The external GPS reference provides an assessment of the accuracy of the speed data computed by the Caterpillar ECM. There are three data sources that were gathered during the test: 1) the J1708/J1587 Road Speed from the network traffic recorded by the NeoVI Yellow, 2) the J1708/J1587 Road Speed from the network traffic sampled by the eDAQ and the Vehicle Bus Module and 3) the eGPS Speed calculated in hardware by fusing the GPS speed signal with the inertial measurement units in the HBM eGPS-200. The vehicle speed data from the school bus crash test is shown in Figure 5. Speed record comparison from different sources.
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