GPS Used As Defence In Radar Speeding Case

James Thigpen writes "There is an article over at Ars Technica about an accused speeder contesting his speeding ticket based on his car's built-in GPS system's records. According to the article his car says he was going slower than the radar gun clocked him at. Contesting a ticket based on GPS data has never before been tested in court."

Comments

Re:Video Evidence

[ls -la]

I'm pretty sure that's not what the OP is advocating. If you set up a camera in your car, YOU control it and all the tapes. If you do something illegal or that you don't want taped, you can either turn the camera off beforehand or destroy the tape after. The only place the government comes into this is if you turn the tapes over to the government/court to prove your innocence.

Instantaneous velocity

[hummassa]

No, my friend, you are wrong.
You see, Instantaneous velocity is the limit of the average velocity where the time of averaging tends to zero.
In other words, the value of f'(t0), where the position x is x = f(t) at a given time t0.
Or in other words, angle of the tangent of the curve x = f(t) in the given time t0.

Now, if your argument is that "a GPS device cannot give the measure of the instantaneous velocity because it does not sample fast enough to get a really good approximation of the curve x = f(t) and hence, the value of f'(t0)", then you could be right because 1Hz is not really a high sampling rate. But you could have said so ;-)

The (analog) speedometer in most cars measure speed by measuring the RPMS of the gear box and multiplying by gear ratios and tire size: they normally do that with a continuous measuring (springs and coils), and what they measure is a good approximation of the instantaneous velocity of the vehicle. A good analog speedometer is somewhat reliable, especially if the scale is correct(*)

(*) their scale is not linear like you see in a normal car:

0 .... 20 .... 40 .... 60 .... 80 .... 100

but exponential, so it should be like:

0 . 20 .. 40 .... 60 ........ 80 .............. 100

and this is why they have a "sweet calibration spot" (normally near the top of the dial; have you already thought about why they make 1.2l-engine cars with 220 km/h marking in the speedometer [a speed they usually don't achieve even in freefall :-)] ?? ) -- in my GM Celta, the sweet spot is at 100 km/h [~60mph], so speeds lower than 100 km/h are usually reading HIGHER than real and speeds higher than 100km/h are usually reading LOWER than real. The speed limit in our highways is 110km/h.

DISCLAIMER: I was a software developer for a road engineering company for one and a half year.