Sat Nav Speedo - How accurate?
Discussion
I know it's commonly accepted that the average Sat Nav unit's estimation of your speed is more accurate than your car's speedo, but exactly how accurate are they?
Two points make me question the accepted lineof thought:
1) GPS based performance dataloggers tend to have a sampling frequency of at least 10hz and are expensive because of this. I would imagine that the average Sat Nav unit samples significantly less often than this and would therefore be inaccurate for fractions of a second at a time leading to perhaps significant inaccuracies when accelerating or braking hard.
2) I understand that military spec. Sat Nav is much more accurate than the Sat Nav units we use in our car, which are only accurate to within a few yards. If this is the case, how accurate can a Sat Nav estimation of your speed be when it can only pinpoint the distance you've travelled to a factor of twice it's accuracy (start distance error plus finish distance error)?
Surely a combination of both of the above would make the Sat Nav units have a considerable margin for error in their calculation of your speed in certain conditions?
Two points make me question the accepted lineof thought:
1) GPS based performance dataloggers tend to have a sampling frequency of at least 10hz and are expensive because of this. I would imagine that the average Sat Nav unit samples significantly less often than this and would therefore be inaccurate for fractions of a second at a time leading to perhaps significant inaccuracies when accelerating or braking hard.
2) I understand that military spec. Sat Nav is much more accurate than the Sat Nav units we use in our car, which are only accurate to within a few yards. If this is the case, how accurate can a Sat Nav estimation of your speed be when it can only pinpoint the distance you've travelled to a factor of twice it's accuracy (start distance error plus finish distance error)?
Surely a combination of both of the above would make the Sat Nav units have a considerable margin for error in their calculation of your speed in certain conditions?
Point 2 is no longer as true as it once was. The SA (Selective Availability) was turned off in 2000 -- this was a deliberate error introduced into the civilian GPS signal (C/A code). Furthermore, the P (precise) code is now broadcast unencrypted on the L1 frequency. However, L2 (P code only) is still only broadcast encrypted. If you have the codes from L1 and L2 you can measure the ionospheric delay. If you only have L1, you have to guess the ionospheric delay.
The overall answer is that if you're travelling at a fairly steady speed in straight 'n' level flight, pretty damn accurate.
The overall answer is that if you're travelling at a fairly steady speed in straight 'n' level flight, pretty damn accurate.
I can understand that at a constant speed it should be very accurate, but is the 0.1 mph in the right ball park?
After all, if it's sampling once per second and using the distance travelled to calculate speed, even a metre's difference in actual vs measured position could give rise to a difference of a few mph, and that would bring it into the region of the car's speedo's inaccuracy?
After all, if it's sampling once per second and using the distance travelled to calculate speed, even a metre's difference in actual vs measured position could give rise to a difference of a few mph, and that would bring it into the region of the car's speedo's inaccuracy?
youngsyr said:
I can understand that at a constant speed it should be very accurate, but is the 0.1 mph in the right ball park?
After all, if it's sampling once per second and using the distance travelled to calculate speed, even a metre's difference in actual vs measured position could give rise to a difference of a few mph, and that would bring it into the region of the car's speedo's inaccuracy?
I think that the idea is that if you stay at a steady speed for, say 1000m, it could be accurate to 0.1mph, right?After all, if it's sampling once per second and using the distance travelled to calculate speed, even a metre's difference in actual vs measured position could give rise to a difference of a few mph, and that would bring it into the region of the car's speedo's inaccuracy?
If you are also changing altitude significantly then accuracy is seriously affected.
kambites said:
GPS's should be very accurate at constant velocity but can be all over the place if you're changing speed and/or direction.
LOL, when we used to load our GPS simulator with 'fighter dynamics' it did require an INS to keep the GPS locked. But I don't see you pulling 9g turns in your car...kambites said:
GPS's should be very accurate at constant velocity but can be all over the place if you're changing speed and/or direction.
Exactly. If I set my car's cruise control to 70mph on an open stretch of motorway (i.e. no bridges etc), my needle will be on 70mph on the clock. At that speed my Satnav seems to review every 10 secs; over a long test I find that three sets of 10 second reports will run 70-70-69 an then repeat. So my speedo is spot on! (i.e. less than 1 mph out)
have to be a rather crap GPS /NOT/ to cope with altitude changes...
sample rate will only affect how fast it tracks changes, the accuracy itself is not going to change, so assuming your not doing some zig-zag path, makes no odds if it's 1Hz or 10Hz, once you start to deviate from a constant speed or heading, then cleary sample rate then comes into it as to how accurately it tracks the route you take.
sample rate will only affect how fast it tracks changes, the accuracy itself is not going to change, so assuming your not doing some zig-zag path, makes no odds if it's 1Hz or 10Hz, once you start to deviate from a constant speed or heading, then cleary sample rate then comes into it as to how accurately it tracks the route you take.
Scuffers said:
have to be a rather crap GPS /NOT/ to cope with altitude changes...
I understood, though do not know hard facts, that GPS is poor at calculating altitude - it can do it - but it needs to lock on to more satellites to get a decent reading, and it has a greater degree of error than location.V8mate said:
kambites said:
GPS's should be very accurate at constant velocity but can be all over the place if you're changing speed and/or direction.
Exactly. If I set my car's cruise control to 70mph on an open stretch of motorway (i.e. no bridges etc), my needle will be on 70mph on the clock. At that speed my Satnav seems to review every 10 secs; over a long test I find that three sets of 10 second reports will run 70-70-69 an then repeat. So my speedo is spot on! (i.e. less than 1 mph out)
Joe911 said:
Scuffers said:
have to be a rather crap GPS /NOT/ to cope with altitude changes...
I understood, though do not know hard facts, that GPS is poor at calculating altitude - it can do it - but it needs to lock on to more satellites to get a decent reading, and it has a greater degree of error than location.kambites said:
That's extremely rare. Mine is reading 63 at 70mph which seems fairly typical for a modern car.
Aye, I get about 65-ish when the speedo says 70 and about 27-28 when it reads 30. Interestingly, hills don't seem to have much of an effect on it, though I've not tested it in extremely hilly areas, mainly motorway work.Scuffers said:
well, without knowing altitude, it can't work out possition if you think about it...
I'd rather not think about it - as there are too many complex issues to deal with - it's not just trig. calculations.People who know more than me said:
Generally, Altitude error is specified to be 1.5 x Horizontal error specification. This means that the user of standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP of 1 for 95% confidence. Altitude error is always considerably worse than the horizontal (position error).
http://gpsinformation.net/main/altitude.htmhttp://gpsinformation.net/main/gpsspeed.htm
GPS Speed said:
How accurate is it? How fast can I go? How HIGH can I go?
GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.
HOW ACCURATE IS THE SPEED READING?
From the NAVSTAR GPS User Equipment Introduction document Section 3.7:
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.
Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.
HOW FAST CAN I GO AND HAVE MY GPS READOUT MY SPEED?
All Currently manufactured CONSUMER GPS receivers we know about will measure speed to 999 miles per hour.
Garmin (uniquely as far as we know) manufactured the G-38, G-40, G-45, and the G-II (not PLUS) units which had a 90 mph speed readout limit. These units are manufacturing discontinued and we know of no other manufacturer who made a unit which would not go to 999mph.
HOW HIGH CAN I GO AND HAVE MY GPS READOUT ALTITUDE?
Defense department regulations prohibit standard consumer GPS receivers from functioning above 60,000 feet and 999mph (simultaneously). Most GPS receivers seem to set hard limits at EITHER 999mph or 60,000 feet.
GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.
HOW ACCURATE IS THE SPEED READING?
From the NAVSTAR GPS User Equipment Introduction document Section 3.7:
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.
Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.
HOW FAST CAN I GO AND HAVE MY GPS READOUT MY SPEED?
All Currently manufactured CONSUMER GPS receivers we know about will measure speed to 999 miles per hour.
Garmin (uniquely as far as we know) manufactured the G-38, G-40, G-45, and the G-II (not PLUS) units which had a 90 mph speed readout limit. These units are manufacturing discontinued and we know of no other manufacturer who made a unit which would not go to 999mph.
HOW HIGH CAN I GO AND HAVE MY GPS READOUT ALTITUDE?
Defense department regulations prohibit standard consumer GPS receivers from functioning above 60,000 feet and 999mph (simultaneously). Most GPS receivers seem to set hard limits at EITHER 999mph or 60,000 feet.
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