Measuring Speed re Speedometer +/-

mikey72

Flyboy152 wrote: »

No it doesn't. What planet are you on? The average car satellite navigation system uses a far inferior systems to that which you have described above. It does not, for example, take account of vertical speed.

See above.

forgotmyname

Very accurate GPS gets within 2 metres. But i have not yet found a car GPS that could place me exactly in the right
spot whilst on the move.

Speed on these can not be trusted.

reeac

jaydeeuk1 wrote: »

GPS really isn't that accurate. Hence why this new galileo system is being built.

Garmin for example claim their devices are accurate to 5m.

Say you're traveling at 40m/s (about 89/90 mph).
Between 2 points at 5m accuracy in time of 2 seconds (about the interval mine measures speed), the gps system could think you've travelled either 70m or 90m in 2 seconds, ie between 78 and 100mph!

And taking the example of 2m given in a marine environment, the same 80m distance could be read as 76m or 84m (85mph or 93mph)


Not that accurate really is it? Granted thats an extreme example, but its based on manufacturers accuracy figures.

The US Govt use a more accurate system for their cruise missiles, and they're known to go astray by 25 m or so on a target!

Originally we were talking about accuracy at 30 mph. i.e. 13.4 m/s. and I guessed that you'd need to maintain a constant speed and direction for 100-200m. That's an interval of 7.5-15 seconds i.e several GPS update intervals. My 2m. positional accuracy would then yield 98-102m. or 198-202m. i.e. a spread of +-2% or +-1%.[My 2m. accuracy figure for GPS position is based upon readings of two independant and different make marine GPS s on my boat which give either exact agreement or differ by 0.001 minute i.e. 2m. which is the limit of the displays].

The cruise missile scatter may be due to the speed of those things, gusts of wind, speed of response to controls, etc., not necessarily GPS error.

Hammyman

reeac wrote: »

Can you justify this statement? A marine GPS is accurate to 0.001 minutes of latitude/longitude i.e. about 2 metres and its timing capability needs to be incredibly sensitive and accurate in order to translate satellite signals into position. It recalculates position every few seconds to give a sequence of positions over an interval of time and thus calculates speed over the ground. A car satnav uses the same satellite system.

It does use the same as marine but not the same as military. There are different "grades" of GPS. The one in your car Satnav is not as accurate as one in a ballistic missile. Civilian ones use the L1 frequency but only have access to the unencrypted C/A coarse acquisition codes. Military use L1 and L2 but unlike civilian ones, they have access to the Encrypted P "precision" Codes which give greater accuracy.


In the US, all GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 metres per second (1,001 kn)[48] are classified as munitions (weapons) for which U.S. State Department export licenses are required because they can be used to provide guidance for precision guided munitions.

mikey72

forgotmyname wrote: »

Very accurate GPS gets within 2 metres. But i have not yet found a car GPS that could place me exactly in the right
spot whilst on the move.

Speed on these can not be trusted.

reeac wrote: »

Originally we were talking about accuracy at 30 mph. i.e. 13.4 m/s. and I guessed that you'd need to maintain a constant speed and direction for 100-200m. That's an interval of 7.5-15 seconds i.e several GPS update intervals. My 2m. positional accuracy would then yield 98-102m. or 198-202m. i.e. a spread of +-2% or +-1%.[My 2m. accuracy figure for GPS position is based upon readings of two independant and different make marine GPS s on my boat which give either exact agreement or differ by 0.001 minute i.e. 2m. which is the limit of the displays].

The cruise missile scatter may be due to the speed of those things, gusts of wind, speed of response to controls, etc., not necessarily GPS error.

Hammyman wrote: »

It does use the same as marine but not the same as military. There are different "grades" of GPS. The one in your car Satnav is not as accurate as one in a ballistic missile. Civilian ones use the L1 frequency but only have access to the unencrypted C/A coarse acquisition codes. Military use L1 and L2 but unlike civilian ones, they have access to the Encrypted P "precision" Codes which give greater accuracy.


In the US, all GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 metres per second (1,001 kn)[48] are classified as munitions (weapons) for which U.S. State Department export licenses are required because they can be used to provide guidance for precision guided munitions.

Speed is calculated by doppler shift, not position.

It's accurate.

martinthebandit

mikey72 wrote: »

page 6

“an average of 10 consecutive 1-second Doppler readings
could produce an average speed over 10 seconds with accuracy
of 0.044 knot and 95% confidence”

95% confidence is not particularly confidence inspiring

mikey72

martinthebandit wrote: »

95% confidence is not particularly confidence inspiring

It's a statistical measurement.

Lilibee

Flyboy152 wrote: »

Quote: Originally Posted by Dave101t
my gps says im 4mph slower than reality. means i can do 74mph and know its only 70mph. They are supposed to be like that.

No they are not. I wouldn't recommend you test your theory.

According to a Police Speed Awareness Course I got sent on last week, they are supposed to be like that. The guy taking the course told us that, as speedos don't measure the car's actual speed but wheel revolutions (at least I think he said wheel revolutions, correct me if I'm wrong...), and those can be affected by what tyres you have and even what pressure the tyres are at - as others have pointed out - then the manufacturers always go for the safest option and use a higher setting than people will get in practice. From a road safety point of view, it's better that you think you're going faster than you actually are, than vice versa.

Flyboy152

Lilibee wrote: »

According to a Police Speed Awareness Course I got sent on last week, they are supposed to be like that. The guy taking the course told us that, as speedos don't measure the car's actual speed but wheel revolutions (at least I think he said wheel revolutions, correct me if I'm wrong...), and those can be affected by what tyres you have and even what pressure the tyres are at - as others have pointed out - then the manufacturers always go for the safest option and use a higher setting than people will get in practice. From a road safety point of view, it's better that you think you're going faster than you actually are, than vice versa.

Clearly, your theory tested wrong. :wall:

The car's speedometer is not allowed to underread. They are not supposed to overread either.

Lilibee

It's not my theory. Using it was not the reason I was on a speed awareness course, and I'm not saying it's right or wrong. Just that that is what I was told last week by a representative of the police, who teaches the course every week and presumably who's job it is to know these things. Just thought the info was relevant to the discussion.