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We used two GPS receivers in our measurement. One is a low cost 1 Hz receiver, and the second one is a high speed 20 Hz VGPS. I used two GPS visual controls - the left one has a high speed VGPS and the right one has a low speed GPS. I have also connected the CAN bus to show the wheel speeds (bottom left recorder). The recorder on the right has an orange line, displaying the high speed GPS velocity and the pink line shows the low speed GPS velocity. The web cam is just for reference.

 

Let's zoom in on a certain part of the trip. We tried braking on a dirt road to observe the blocking of the wheels. Notice on the GPS maps that when we zoomed in, the map changed to the satellite picture. When we zoom in on the recorder, part of track is shown with a thick line while the other part is shown in thin red.

The braking reveals the difference between the high and low speed GPS. The lower right side recorder clearly shows half second delay from the real speed. This renders it useless for any evaluation purposes.

The orange curve is also shown on the left recorder, displaying that there is virtually no delay between the measured wheel speed and GPS velocity. However, since the wheels are locking up, the speed is dropping. But here we see the action of the ABS system. Without the ABS, the wheels would totally lock up leaving us without the control of the vehicle. At 50 km/h and on a wide road this wouldn't be a problem, but we can think of many different situations where this is not desirable. However, that graph also shows that we can't trust the measurements from the wheels, especially under such conditions.

The third thing worth noticing here is the end of braking when the vehicle stops. Since the car body moves on its suspension, we see a small jump at the end. This is where the vehicle moves slightly back and then settles down. If we measure brake distance, this is not wanted and needs to be removed from the calculation.

 

The next screenshot clearly shows the effect of high surrounding buildings where the high speed GPS velocity has dropouts which are surely wrong. The low speed GPS shows a better behavior in this case since it has much more smoothing in the calculations. The path also shows that the signal was disturbed.

 

 

So what would be the conclusion?

The low speed GPS is very useful when we have slow changes, like with trains, or if we just want a reference of where the vehicle was driving.
The high speed GPS is needed when we require high precision speed measurements, but we need a clear sky view to have enough satellites to be able to trust the measurements.