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John Dekker
Guest
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 Posted:
Wed Dec 22, 2004 8:16 am Post subject:
speed accuracy compared to car |
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Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John. |
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Joe Parsons
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 17:46:51 +1300, "John Dekker" <jdekker@xtra.co.nz> wrote:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
It's a pretty safe bet your car speedo is more than a little optimistic. The |
GPS speeds are *very* accurate, from my understanding.
Joe Parsons |
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Robertwgross
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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John wrote:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
|
If you are traveling in a relatively straight line with good satellite view,
then a GPS receiver will be good to roughly 0.1 mph. Car speedometers are
notoriously bad, especially as the accuracy changes with tire tread wear. I've
never had to change tires on my GPS receiver.
---Bob Gross--- |
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gomez
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 17:46:51 +1300, "John Dekker" <jdekker@xtra.co.nz>
dropped the following oil-slick:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
|
The GPS is the more accurate if you travelling at a steady speed. It
will lag to catch up with braking and acceleration though.
--
gomez
(not is hot to reply)
"The progress of the kart is more important than its direction" |
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bw
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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"John Dekker" <jdekker@xtra.co.nz> wrote in message
news:S_6yd.371$mo2.27870@news.xtra.co.nz...
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph?
Its just that I was driving my car and noticed the car was showing about
7% faster than my Garmin 60cs.
Regards,
John.
|
GPS displays show one second intervals. Since SA was turned off, I've never
seen more than 2 mph differential at steady cruising. If you can find an
interstate with measured mile markers, you can calibrate your speedo with a
stopwatch. 10 miles is 10 times more accurate than 1 mile. Or find a
measured mile and use it 10 times and average the times. |
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Mxsmanic
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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John Dekker writes:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
|
The car is wrong. Lots of cars have their speedometers set to show
inaccurately high speeds, presumably based on the idea that people will
stay below the speed limit more if their speedometer is inaccurate.
--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly. |
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Tumbleweed
Guest
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| Posted:
Wed Dec 22, 2004 8:16 am Post subject:
Re: speed accuracy compared to car |
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"Mxsmanic" <mxsmanic@hotmail.com> wrote in message
news:hj9is0dq2ae61dv0o9to55ielqeu1nqge9@4ax.com...
| Quote: | John Dekker writes:
Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph?
Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
The car is wrong. Lots of cars have their speedometers set to show
inaccurately high speeds, presumably based on the idea that people will
stay below the speed limit more if their speedometer is inaccurate.
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Not quite, its based on the idea the manufacturer wont get sued!
--
Tumbleweed
Remove my socks for email address |
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Mxsmanic
Guest
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| Posted:
Wed Dec 22, 2004 3:14 pm Post subject:
Re: speed accuracy compared to car |
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Tumbleweed writes:
| Quote: | Not quite, its based on the idea the manufacturer wont get sued!
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I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly. |
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Colin Hughes
Guest
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| Posted:
Wed Dec 22, 2004 4:05 pm Post subject:
Re: speed accuracy compared to car |
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From www.speed-trap.co.uk
"..vehicle construction and use regulations [UK] require a vehicle
speedometer accuracy to be in the range of -0->+10%. The implications are
that it must never under-read - for obvious reasons - but may over-read. As
the cost of manufacturing a speedometer with -0% error would be very costly
they all over-read by a few percent without exception. Even if speed is
measured correctly the display may not be accurate, so a speedometer error
is allowed." |
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GSV Three Minds in a Can
Guest
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| Posted:
Wed Dec 22, 2004 4:24 pm Post subject:
Re: speed accuracy compared to car |
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Bitstring <25iis0p2lboqn9uh038map8g7q7hremlc4@4ax.com>, from the
wonderful person Mxsmanic <mxsmanic@hotmail.com> said
| Quote: | Tumbleweed writes:
Not quite, its based on the idea the manufacturer wont get sued!
I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
|
In the Uk it is, iirc, legally required that they be anywhere between
accurate and 10% 'fast'. I.e. if you are travelling at 60mph, they can
indicate anywhere between 60 an 66 .. 59 is illegal.
And they have to do that whatever the state of tyre wear and inflation
happens to be (most of them still measure propshaft revs and calculate
MPH from that). So with well worn, under-inflated, tyres you can expect
the reading to be 10% 'fast' compared to actual road speed. And 'miles
travelled' and MPG (if you have a trip computer) are similarly affected.
I can look at my MPG over time graph (which I keep to detect when the
engine is getting sadly sick) and spot the points at which i swapped
worn tyres for new ones. 8>.
--
GSV Three Minds in a Can
Outgoing Msgs are Turing Tested,and indistinguishable from human typing. |
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Hans-Georg Michna
Guest
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| Posted:
Wed Dec 22, 2004 4:45 pm Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 11:14:03 +0100, Mxsmanic
<mxsmanic@hotmail.com> wrote:
| Quote: | Tumbleweed writes:
Not quite, its based on the idea the manufacturer wont get sued!
I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
|
It is like that in Germany and probably in many more countries.
The rule is actually that the speedometer must never show a
lower speed than the actual speed.
Since tires differ in their diameter from type to type and from
new to worn, most speedometers necessarily have to show a speed
that is considerably higher than the true speed.
Hans-Georg
--
No mail, please. |
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Thomas Schäfer
Guest
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| Posted:
Wed Dec 22, 2004 5:39 pm Post subject:
Re: speed accuracy compared to car |
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"John Dekker" wrote
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph?
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My Garmins manuals for SP3 and 76S state, that an error
of <0.05m/s (about 0.1 mph) is typical for unaccelerated driving
(no brake, accelerator or turn, good satellite reception).
I trust the manuals, because both GPS units show the same speed
indepently and my bord computer (with speed from ABS
sensors and engine rev counter) agrees usually to the last digit.
Thomas |
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Sam Wormley
Guest
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| Posted:
Wed Dec 22, 2004 7:17 pm Post subject:
Re: speed accuracy compared to car |
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John Dekker wrote:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
|
Keep in mind that most GPS receivers employ "smoothing filters" and so
instantaneous velocity reading during acceleration is not necessarily
accurate. However at constant velocity (and assuming no obstruction of
signals), the GPS receiver will likely measure velocity to an accuracy
of 0.2 m/s (0.7 kph) 2drms.
Ref: Misra & Enge "GPS: Signals, Measurements, and Performance" (2001)
Sec. 5.2.1 (pgs 196-197) Velocity Estimation
"The relative motion of a satellite and the user results in changes in
the observed frequency of the satellite signal. This Doppler shift is
measured routinely in the carrier tracking loop of a GPS receiver
[Section 9.6]. Given the satellite velocity, the Doppler shift can be
used to estimate the user velocity. The Doppler shift, or equivalently,
the range rate [Section 1.3.3], can be written as a projection of the
relative velocity vector on the satellite line-of-sight vector. The
measurement, however, is biased by the receiver clock bias rate (i.e.,
frequency offset), and what's actually measured is the pseudorange
rate.
"The delta pseudoranges obtained from carrier phase measurements are
proportional to the average pseudorange rates or the line-of-sight
velocity of the user relative to the satellite over the time interval.
The model for pseudorange rates can be obtained by differentiating
(5.1). It is left as an exercise to show that
[equation 5.28 is true]
where v_sup(k) [a vector quantity] is the satellite velocity vector,
known from the navigational message broadcast by the satellite; v is
the user velocity vector, to be estimated. Both v_sup(k) and v are
expressed in the ECEF coordinate frame. The user-to-satellite unit
vector 1_sup(k) is determined from an estimate of the user position;
b_dot is the bias of the receiver clock (m/s), and the
epsilon_sub_phi_sup(k) denotes the combined error doe to changes during
the measurement interval in the satellite clock, ionosphere and
troposphere. Note that the velocity of an object attached to the earth
is zero in the ECEF coordinate frame.
"The principal source of error in (5.28) throughout the 1990s was the
satellite clock frequency dithering due to SA. Now with SA gone, the
remaining errors arise from changes in the ionospheric and tropospheric
delays and in multipath, and are generally small. Problems, however,
can arise if the user dynamics are excessive. The delta ranges give
only average velocity over a time interval. High accelerations and
jerks would clearly be problematic. The PPS performance specifications
for velocity estimation (0.1 m/s rms in any direction; 0.2 m/s 2drms)
are based on a constant-velocity scenario [JPO(1991)].
"Equation (5.28) is linear in user velocity components, and can be
rewitten...
the combined set of measurements from K satellites can be written as a
set of equations compactly in matrix notation as
[equation 5.29]
where matrix G characterizes the user-satellite geometry, as defined
previously (5.10). It is interesting that the problem of estimation of
user velocity based on pseudorange rates is identical in structure to
that of estimation of user position from pseudoranges (5.9). A
least-squares solution and the DOP parameters can be defined, as
before, and related to the rms error in these estimates". |
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David Phillips
Guest
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| Posted:
Wed Dec 22, 2004 7:43 pm Post subject:
Re: speed accuracy compared to car |
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On 22 Dec 2004 05:27:58 GMT, robertwgross@cs.com (Robertwgross) wrote:
| Quote: | John wrote:
Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
If you are traveling in a relatively straight line with good satellite view,
then a GPS receiver will be good to roughly 0.1 mph. Car speedometers are
notoriously bad, especially as the accuracy changes with tire tread wear.
|
I'll agree that car speedometers are notoriously bad, but argue with
the accuracy change along with tire wear. There is some, but it's
only about 2%.
<tiregeek>
For example, my car uses 225/50-17 tires. That's a 17inch diameter
wheel, with a 225mm treadwidth, and a sidewall height of 225/2mm (and
since you add both sidewalls to get diameter, that's a total of 225mm.
Get the units the same (I'll use inches) and you have a nominal
diameter of 17 + 8.86 = 25.86 inches. Nominal circumference = 81.2
inches.
That's with a tread depth of 10/32 inches. Wear it down to minimum
legal (2/32) and you've lost 1/2 inch of diameter. 25.36 inches,
giving a nominal circumferance of 79.63 inches.
79.63/81.2 = .98
So if your speedometer is accurate with new tires, and reads 60 mph
with old tires, you're going 58.8 mph.
</tiregeek> |
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David Phillips
Guest
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| Posted:
Wed Dec 22, 2004 7:46 pm Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 11:14:03 +0100, Mxsmanic <mxsmanic@hotmail.com>
wrote:
| Quote: | Tumbleweed writes:
Not quite, its based on the idea the manufacturer wont get sued!
I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
|
I would be surprised to find that is actually correct. While I
obviously haven't looked at the motor vehicle code in every
jurisdiction, I've never seen it in my state's laws. It would be a
logistical nightmare for the manufacturer to have an accurate speedo
in one state or country, and an 'optimistic' speedo in another.
My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS) |
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