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Radar Beats GPS In Court — Or Does It?
TechnologyResource writes "More than two years ago in California, a police officer wrote Shaun Malone a ticket for going 62mph in a 45-mph zone. Malone was ordered to pay a $190 fine, but his parents appealed the decision, saying data from a GPS tracking system they installed in his car to monitor his driving proved he was not speeding. What ensued was the longest court battle over a speeding ticket in Sonoma county history. The case also represented the first time anyone locally had tried to beat a ticket using GPS. The teen's GPS pegged the car at 45 mph in virtually the same location. At issue was the distance from the stoplight — site of the first GPS 'ping' that showed Malone stopped — to the second ping 30 seconds later, when he was going 45 mph. Last week, Commissioner Carla Bonilla ruled the GPS data confirmed the prosecution's contention that Malone had to have exceeded the speed limit and would have to pay the $190 fine. 'This case ensures that other law enforcement agencies throughout the state aren't going to have to fight a case like this where GPS is used to cast doubt on radar,' said Sgt. Ken Savano, who oversees the traffic division. However, Commissioner Bonilla noted the accuracy of the GPS system was not challenged by either side in the dispute, but rather they had different interpretations of the data. Bonilla ruled the GPS data confirmed the prosecution's contention that Malone had to have exceeded the speed limit."
The problem of this calculus you mention wasnt the speed at the end, nor even the beginning. we're missing a piece of information to properly go through this. distance. it says at a stop light, he was 0, then the next ping was 45. but the problem becomes distance covered in that 30 seconds. tie in the math, etc. if it says 45 on the ping, thats worthless. we need to know how far he traveled in 35 seconds to get an average speed, and, for the sake of argument, his vehicles 0-60 speed as well to get the stats on how quickly he could have possibly gone up to 60, nearly where they "clocked" him. obviously, his average speeds worthless, and his speed 30 seconds after his initial of 0 is worthless. we need the distance traveled in that 30 seconds. And TFA says "virtually" the same location. For all we know, he spotted the cop, hit his brakes and was doing 45 when he was pinged. Distance is key ... notice how TFA forgets that wonderful detail.
And, I'm sure as a teenager, with a GPS, he knew that if he hit 70, theyd get an email alert. Heck, he probably knew that if he wanted to, he could go 69, wait for a ping, if he had timed them right, speed up to 100 and brake to 69 again, all before the second ping... I guess the parents forgot that Teenager + Technology is generally > Parents + technology
No, it does not. GPS only tells you your average speed between two GPS pings. Ping 1 - you are at X, ping 2 - you are at Y, your current speed is how fast you must move in order to get from X to Y in time between ping1 and ping2.
A GPS typically calculates velocity from Doppler shift of the D-band signal. This give higher accuracy since the position reading is somewhat unreliable. It also means you can (in principle) get the velocity information virtually instantaneously without having to sample two locations. However, in reality a lot of averaging and filtering is going on, and I think many receivers weighs in both position deltas and Doppler shift in the equations, so the reading is going to have at least some lag.
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