The distance accuracy quoted in VBOX units is generally 0.05 % (50 cm per km).
For example, a 100 m test result should be within 5 cm.
This is calculated by starting from 0 km/h and ending at 0 km/h, so that speed fluctuations at the start of the test don't influence the result and the reference distance is precisely known.
However, when a test is started at speed and distance is calculated from this (e.g. 100 km/h – 0 km/h), small fluctuations in the speed measurement at the start of the test can have an impact on the distance calculation for that test. At 100 km/h, a vehicle will travel 27.8 cm per 100 Hz sample, so the recorded starting location of the test may fluctuate depending how close the first sample of a test was to the starting speed condition. The difference between two stopping distances using identical conditions could be double the maximum error, i.e. 0.1 %.
For example, one test result could be 5 cm longer than the reference distance and another test could be 5 cm shorter than the reference distance, giving a potential difference of 10 cm between two tests.
If you are intending to use a set speed to start brake stop tests, it is then recommended to repeat the tests a number of times (e.g.10 times) and look at the average difference between them. This should provide a result within the 0.05 % (50 cm per km) stated.
A Brake Pedal Trigger could also be used, which is sampled at a much higher frequency than GPS signals. A VBOX unit will then be able to work out how far the vehicle has travelled before the next sample.
IMPORTANT - If you are wanting to compare the accuracy of results, the conditions must be the same, i.e. using the same GNSS feed or replaying a scenario using a LabSat GNSS Simulator Device.
A 100 km/h – 0 km/h brake test was recorded using a LabSat Unit. The identical scenario was then replayed and recorded to VBOX 3i unit on two separate occasions and the results compared within VBOX Test Suite Analysis Software.
One recording provided a distance of 36.88 m and the other 36.95 m.
Looking at the results in more detail, you can see in the graph below that after the 100 km/h start condition of the test is met, the first sample of the red recording occurs at approximately 99.90 km/h and first sample of the blue recording at approximately 99.83 km/h, providing a difference of roughly 0.002 seconds between both samples.
Due to these first sample differences, at the end of the test there is a total distance difference of 7 cm between the two recordings.
