4 ms·

Officer: I clocked you going 40 miles per hours. Driver: I wasn't going to be out that long. GPS measures average speed (delta position vs delta time). Radar g

by kbutler 16y ago

Officer: I clocked you going 40 miles per hours. Driver: I wasn't going to be out that long. GPS measures average speed (delta position vs delta time). Radar guns measure instantaneous speed (doppler shift). GPS also tends to be less accurate in a partial Faraday cage (like a metal vehicle). Bottom line: don't expect GPS to get you out of your speeding ticket with a tech-savvy judge.

pilom 16y ago
But for some reason many courts accept speedometer calibrations done with GPS instead of on a dynamometer (treadmill). I never got that.
ghurlman 16y ago
My GPS, right now, has me clocked at a top speed of 273mph. I haven't a clue how that happened, but I'm sure as hell not bringing it to court to prove I wasn't speeding.
- sahaskatta 16y ago
  I no longer have the smart phone I used when I got caught, but my HTC EVO is actually quite accurate. I've pulled up my current speed while in a car (not at the wheel). It was spot on with the number from the car's dashboard. On that note, my father has a BlackBerry smart phone and his Endomondo GPS tracking app states all sorts of weird things. He went for a few mile long hike and it thought he traveled a few hundred miles across the San Francisco Bay Area. (And yes across the bay.)
thalur 16y ago
Some GPS units can measure speed by doppler shift. However, I wouldn't be surprised if the receivers in smartphones can't.
- CWuestefeld 16y ago
  Can you show an example? I can't imagine what signal it would be calculating a doppler shift on, that could be more accurate than the positional data it computes. Most of the satellite signals -- certainly the strongest -- will be high overhead. Thus the signal path is essentially perpendicular to your direction of travel, and doppler shift wouldn't be significant.
  - Bud 16y ago
    Seems to me it won't be long til GPS units simply include a six-axis accelerometer, since acceleration can be directly measured without need of GPS and since such tech is cheap and already in both smartphones and game systems like the PS3 and Wii. This will allow for far more precise tracking of your speed at any given moment.
    - zb 16y ago
      Actually, an accelerometer is far, far less precise because something like going up a slight incline results in a pretty significant change in the horizontal acceleration but a comparatively small change in the vertical acceleration (which is easily lost in the noise). The rotation caused by starting to go up a hill is also so tiny that it is completely lost in the noise.
    - colanderman 16y ago
      Except that integrating any real world signal tends to introduce drift due to noise in the input signal. If you have a smartphone, try writing an app that does exactly what you propose; you'll soon find that it's impossible to get the "speed" to zero out. (Either that or you have to filter the data, which will cause other fun errors like your speed drifting toward zero.)
      - Bud 16y ago
        I thought about this as a complication, but my assumption was that this would be a solvable technical problem. Is it really not feasible to add an accelerometer to the GPS data (which does include altitude), and end up with at least somewhat more precise data?
rmc 16y ago
The GPS recorded his maximum speed aswell, which was under what the radar gun said.
- CWuestefeld 16y ago
  The parent post already noted that the GPS doesn't know your instantaneous speed, just the average speed since your last reading. If your signal was blocked by trees, buildings, bridges, (not to mention your car itself), you can go at least several seconds without a reading, and what happened in between those points is unknown to the GPS, except for total ground covered. And that's even before you account for GPS inaccuracy, which is ~3m in optimal conditions.
  - Bud 16y ago
    In my experience, those outages don't tend to be frequent on main roads, where there is usually an unobstructed sight-line to the GPS satellites. Sure, it's possible that this dude floored it, hit 50-60mph for a second or two, slammed on the brakes leaving rubber all over the pavement, slowed to 15mph for a while to make the averages all work out, and proceeded on his merry way at 25mph. But it doesn't seem that likely.
zb 16y ago
Technically, GPS uses both pseudorange and doppler data as an input. These are usually combined with the previously measured position and speed using a Kalman filter. Speed is generally more accurate than position (to the extent that it makes any sense to compare them) because pseudorange errors tend to change comparatively slowly. Of course how the receiver outputs the speed is up to it, but since it falls out of the position calculation for free it's unlikely many will be storing straight delta position/delta time. Having said that, it's not especially strong evidence since it's pretty trivial to fake. However, in this case the title is correct: it contributed to him getting off because it gave him the confidence to challenge the ticket.
- pkulak 16y ago
  Sure it's easy to fake, but you are under oath. You can stand up there all day and say that, to the best of your knowledge you were not speeding and not perjure yourself. If you get up there with forged documents and get caught, that's something else entirely. So, they are not iron-clad evidence, but they should be pretty good.
jellicle 16y ago
This is silly and dumb. GPS doesn't measure average speed. GPS measures location. Speed is calculated. It's calculated over the entire run - average speed - and it can be calculated between any two location measurements - which can be very close to instantaneous speed. Radar guns measure location too, for that matter - distance from the gun is measured at two instances, and speed is calculated. There's absolutely no inherent reason why GPS-based speed calculations are any less reliable than a radar gun. The better GPS systems update their position 5 times per second - far more precision than is necessary for good vehicle speed readings.
- yummyfajitas 16y ago
  Radar guns measure location too, for that matter - distance from the gun is measured at two instances, and speed is calculated. No, they use a single pulse and measure it's Doppler shift.

by kbutler 16y ago

pilom 16y ago

But for some reason many courts accept speedometer calibrations done with GPS instead of on a dynamometer (treadmill). I never got that.

ghurlman 16y ago

My GPS, right now, has me clocked at a top speed of 273mph. I haven't a clue how that happened, but I'm sure as hell not bringing it to court to prove I wasn't speeding.

sahaskatta 16y ago
I no longer have the smart phone I used when I got caught, but my HTC EVO is actually quite accurate. I've pulled up my current speed while in a car (not at the wheel). It was spot on with the number from the car's dashboard. On that note, my father has a BlackBerry smart phone and his Endomondo GPS tracking app states all sorts of weird things. He went for a few mile long hike and it thought he traveled a few hundred miles across the San Francisco Bay Area. (And yes across the bay.)

thalur 16y ago

Some GPS units can measure speed by doppler shift. However, I wouldn't be surprised if the receivers in smartphones can't.

CWuestefeld 16y ago
Can you show an example? I can't imagine what signal it would be calculating a doppler shift on, that could be more accurate than the positional data it computes. Most of the satellite signals -- certainly the strongest -- will be high overhead. Thus the signal path is essentially perpendicular to your direction of travel, and doppler shift wouldn't be significant.
- Bud 16y ago
  Seems to me it won't be long til GPS units simply include a six-axis accelerometer, since acceleration can be directly measured without need of GPS and since such tech is cheap and already in both smartphones and game systems like the PS3 and Wii. This will allow for far more precise tracking of your speed at any given moment.
  - zb 16y ago
    Actually, an accelerometer is far, far less precise because something like going up a slight incline results in a pretty significant change in the horizontal acceleration but a comparatively small change in the vertical acceleration (which is easily lost in the noise). The rotation caused by starting to go up a hill is also so tiny that it is completely lost in the noise.
  - colanderman 16y ago
    Except that integrating any real world signal tends to introduce drift due to noise in the input signal. If you have a smartphone, try writing an app that does exactly what you propose; you'll soon find that it's impossible to get the "speed" to zero out. (Either that or you have to filter the data, which will cause other fun errors like your speed drifting toward zero.)
    - Bud 16y ago
      I thought about this as a complication, but my assumption was that this would be a solvable technical problem. Is it really not feasible to add an accelerometer to the GPS data (which does include altitude), and end up with at least somewhat more precise data?

rmc 16y ago

The GPS recorded his maximum speed aswell, which was under what the radar gun said.

CWuestefeld 16y ago
The parent post already noted that the GPS doesn't know your instantaneous speed, just the average speed since your last reading. If your signal was blocked by trees, buildings, bridges, (not to mention your car itself), you can go at least several seconds without a reading, and what happened in between those points is unknown to the GPS, except for total ground covered. And that's even before you account for GPS inaccuracy, which is ~3m in optimal conditions.
- Bud 16y ago
  In my experience, those outages don't tend to be frequent on main roads, where there is usually an unobstructed sight-line to the GPS satellites. Sure, it's possible that this dude floored it, hit 50-60mph for a second or two, slammed on the brakes leaving rubber all over the pavement, slowed to 15mph for a while to make the averages all work out, and proceeded on his merry way at 25mph. But it doesn't seem that likely.

zb 16y ago

Technically, GPS uses both pseudorange and doppler data as an input. These are usually combined with the previously measured position and speed using a Kalman filter. Speed is generally more accurate than position (to the extent that it makes any sense to compare them) because pseudorange errors tend to change comparatively slowly. Of course how the receiver outputs the speed is up to it, but since it falls out of the position calculation for free it's unlikely many will be storing straight delta position/delta time. Having said that, it's not especially strong evidence since it's pretty trivial to fake. However, in this case the title is correct: it contributed to him getting off because it gave him the confidence to challenge the ticket.

pkulak 16y ago
Sure it's easy to fake, but you are under oath. You can stand up there all day and say that, to the best of your knowledge you were not speeding and not perjure yourself. If you get up there with forged documents and get caught, that's something else entirely. So, they are not iron-clad evidence, but they should be pretty good.

jellicle 16y ago

This is silly and dumb. GPS doesn't measure average speed. GPS measures location. Speed is calculated. It's calculated over the entire run - average speed - and it can be calculated between any two location measurements - which can be very close to instantaneous speed. Radar guns measure location too, for that matter - distance from the gun is measured at two instances, and speed is calculated. There's absolutely no inherent reason why GPS-based speed calculations are any less reliable than a radar gun. The better GPS systems update their position 5 times per second - far more precision than is necessary for good vehicle speed readings.

yummyfajitas 16y ago
Radar guns measure location too, for that matter - distance from the gun is measured at two instances, and speed is calculated. No, they use a single pulse and measure it's Doppler shift.