Just recently it was in the papers that a certain scooby owner was convicted for causing death by dangerous driving if i recall, part of the conviction was secured by using telemetry data obtained.

now i have been using a few sat nav systems in my car recently, and according to them my speed is under what my actual odometer states. 5mph at all normal legal speeds to be exact.

the question is, which is correct, the speeds shown on the gps unit, or the car speedo.

now if i was to be pinged whislt driving to the correct gps speed, which in theory is correct ( otherwise there are soome wildy offtrack satellites) where would you stand if you used that as your indication of true speed?

would it work with you or against you


Mart

 

Car speedos generally read slightly higher than the actual speed being done.

 

Agreed, but would gps data be admissable in court.

 

Basically it's your word (GPS) against plod's (calibrated gun). And whereas he takes the reading at one point, how can you prove your GPS reading was at the moment he pointed the gun at you, and not after you dabbed the brakes?

 

I had 2 gps devices in my car and they read different speeds by 2mph, so I wouldn't believe GPS is the be all and end all in how fast you're going.

 

wouldn't believe the speed gun either

 

GPS is not infallible by any means. Whilst the speed report given by it may be a bit more accurate than your speedo most of the time it really depends on a number of factors such as how many satellites you are receiving from, what the dilution of precision is, etc.

My GPS has given some really wacky fixes on odd occasions too, bit disconcerting when up an mountain in the cloud, good job I can use a map and compass. So what if that happened and your speed suddenly jumped as a result. Not admissable in court I would say.

 

I have three speed readouts in my WRX, the speedo (obviously ), GPS (Origin B2 ) and a SECS monitor. I have found the reading on the SECS to be close to GPS speed possibly due to it's direct connection to the ECU. The SECS and GPS readings are always lower than speedo reading. The ECU also stores a maximum speed attained reading in its memory ( probably used to help convict driver in case mentioned ).
A GPS calibrated speed reading can be very accurate due to the nature of the technology ( satellites in precise orbits, atomic clocks etc) but I would have thought that the quality of the receiver/antenna being used in the car could affect how quickly that data is displayed, hence 2 GPS devices giving different speed readings ( Origin used to quote 3 seconds to update speed reading for the Origin Blue i ).

 

I had 2 tom tom go's at the same time and they were exactly the same also always the same out in differnt cars and van.

 

I used navigator (medion software) & tom tom both on the same pda and gps antenna, bot consistantly gave an under reading of 5mph

Mart

 

I gave my take on GPS speed calculation a while back on this thread:

http://bbs.scoobynet.co.uk/showthread.php?t=466988

I believe this to be correct but am awaiting a better take on it. The bottom line is that I think it is very accurate and uses doppler shift, though there may be some which use other methods and so may have problems.