Garmin GPS - Speed Display 76CS

Author

Speed Display 76CS - Error??

MacArthur

2006-01-31, 2:48 am

I took my new GPS MAP76cs on the highway for a round trip of 140 Miles.

The speed/distance unit is set to Metric.

Throughout the trip the displayed speed was off by 20-30 kph.

I was driving at ~110 kph and the speed displayed on the map page was 75-95
kph, generally on the low end.

The value I have selected for display "is" Speed; not min, not max, not
average, etc.

The track log shows that I was travelling at 62-69 MPH for most of the trip,
which makes sense.

Any suggestions as to what the problem might be?

--
Live strong and have a nice day, - "Nil carborundum illegitimi"!

M

Over 1200 Links at Http://MacArthur.Funknstyle.Com - NL Link of The Week:
"Newfoundland and Labrador Heritage"

Laszlo Nemeth

2006-01-31, 5:49 pm

MacArthur

2006-01-31, 11:48 pm

"Laszlo Nemeth" <laz.junk2@cybox.com> wrote in message
news:43DFC2E4.7080207@cybox.com...
> MacArthur wrote:
>
> your cars speedometer and tires. on my motorcycle i have seen
> over 15% error when running out a pair of tires. replace tires and
> back to 5% error. most cars i have checked are around 2-7% off.
>
>
>
> laz

Thanks Laz,

As I pointed out in the response to your email:

Indeed! I would expect to see a speed ~95% of what my speedometer is
reading.
However, My speedometer indicated I was driving at 110% (ie 110 kph) of the
posted speed limit : )

The traffic stream around me, and the track log downloaded from the GPS
verify that the speedometer was ~correct.

68 28/01/2006 12:52:35 507 ft 0.224 mi 00:00:12 67 mph 199° true N47 31.901
W52 50.635

69 28/01/2006 12:52:47 513 ft 0.213 mi 00:00:12 64 mph 184° true N47 31.717
W52 50.726

70 28/01/2006 12:52:59 540 ft 0.217 mi 00:00:12 65 mph 185° true N47 31.532
W52 50.746

71 28/01/2006 12:53:11 549 ft 0.212 mi 00:00:12 63 mph 206° true N47 31.344
W52 50.768

72 28/01/2006 12:53:23 581 ft 0.214 mi 00:00:12 64 mph 207° true N47 31.180
W52 50.889

Any other ideas would be welcome!

--
Live strong and have a nice day, - "Nil carborundum illegitimi"!

M

Over 1200 Links at Http://MacArthur.Funknstyle.Com - NL Link of The Week:
"Newfoundland and Labrador Heritage"

Randy Given

2006-01-31, 11:48 pm

Mine is within 3%. Still, even with tire variations and such, you should be
within 15% I would think. Try doing timed miles (go 60mph for 60 seconds and
see if it lines up with the mile markers).

Sam Wormley

2006-01-31, 11:48 pm

MacArthur wrote:

> I took my new GPS MAP76cs on the highway for a round trip of 140 Miles.
>
> The speed/distance unit is set to Metric.
>
> Throughout the trip the displayed speed was off by 20-30 kph.
>
> I was driving at ~110 kph and the speed displayed on the map page was
> 75-95 kph, generally on the low end.
>
> The value I have selected for display "is" Speed; not min, not max,
> not average, etc.
>
> The track log shows that I was travelling at 62-69 MPH for most of the
> trip, which makes sense.
>
> Any suggestions as to what the problem might be?
>

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
rewritten...

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".

MacArthur

2006-01-31, 11:48 pm

Thanks Sam,

However, I would expect the track log to reflect what the display was
indicating.

The display never achieved more than 92 kph and was generally much lower, in
the low to mid 80's.

The track log shows a constant speed above 60 "MPH" ie GT 100 kph.

I had the log set to record at 5 obs per minute, so the data stream was
fairly rich..

--
Live strong and have a nice day, - "Nil carborundum illegitimi"!

M

Over 1200 Links at Http://MacArthur.Funknstyle.Com - NL Link of The Week:
"Newfoundland and Labrador Heritage"

"Sam Wormley" <swormley1@mchsi.com> wrote in message
news:1zVDf.746166$_o.87885@attbi_s71...
> MacArthur wrote:
>
>
> 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
> rewritten...
>
> 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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