The dragy uses a technique called integrated doppler(carrier phase) to essentially count the number of gps carrier wavelengths between the start and finish. Carrier phase can be measured to about 1% accuracy of the 20 cm wavelength.This is also what the vbox uses, and this is the description that they gives for their distance accuracy.
GPS satellites are equipped with an atomic clock, which ensures timing stability to less than one-millionth of a second. By integrating Doppler-derived speed with this level of time signal reliability, an extraordinarily accurate distance measurement is achieved.
There are a number of tests which we have carried out over the years to verify and improve our measurement algorithms for applications in vehicle testing.
One such test is to place two reflective strips on the road at a known distance apart. Using a laser sensor connected to the trigger input of a VBOX, the vehicle is then driven between these two points a number of times, and the distances compared. In such tests, the VBOX 3i will always be within 3cm in 1000m, which is about the same as the measurement uncertainty due to the slight deviation of the vehicle during the driving.
Accelerometers are used but in a kalman filter to basically interpolate between the 0.1 second spacing of the gps data. The kalman filter uses the gps data to calibrate the accelerometers.
Thanks for the info but does Dragy have one antenna or two? It is only with the two antenna version that VBOX3i is able to achieve such precision. Otherwise the resolution is 80cm even with a DGPS base station which is far longer than the <1ft resolution needed to analyze what is happening in that critical first foot. See below copied and pasted directly from the VBOX website:
Velocity and heading data are calculated from Doppler Shift in the GPS carrier signal, providing users with unparalleled accuracy. For example, over 10 tests between two light barriers placed exactly 1000m apart, the VBOX will report a distance within ±2-3 cm, on every test, and the average comes out to within less than 1cm. Impressive for a device using satellites 18,000 km away!
Brake distance accuracy is equally impressive, using a brake pedal mounted trigger input, VBOX 3i will measure the braking distance to within ±1.8 cm for a typical 100-0 kph brake stop. More about GPS accuracy.
VBOX 3i comes in three versions, single antenna, dual antenna and (dual antenna) RTK. In conjunction with a DGPS Base Station (Option 1) VBOX 3i Single Antenna can achieve a positional accuracy of 80 cm."
It would be interesting to find out if the 1ft rollout time subtraction is simply calculated from the acceleration essentially averaged over a longer distance (than 1ft). It would be interesting to analyze Dragy or VBOX results vs a high speed camera aimed at the wheel during launch. Has this been done or are there any volunteers?