Speed readout accuracy?

I use a Tom Tom Rider on my bike and in my car. The speed reedout on the
gps is always about 6 kph slower than that of the speedos. I know vehicle
speedos have an inbuit error margin (usually read slower than actual) but
does anyone know just how accurate gps units are?

Jetrascal wrote:



Horizontal component.

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 or 0.4 mph) 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".

vehicle

At a constant speed they are relatively accurate. When I got my 98 ST1100
people were constantly coming up on me and passing me even though I
normally run +7 to the speed limit. Scary! When I put the GPS on the bike it
showed the speedo was 5 MPH optimistic. So then I timed it with Interstate
mile markers for verification and the GPS was correct. That GPS reads
exactly the same as my car and the GPS in my new IPAQ.

Since then I've discovered that a lot of bike mfg make the speedo optimistic
in case the owner puts a different sized tire on the bike. IOW lawyers are
involved.

The car being off is a little surprising.

On Thu, 28 Feb 2008 11:28:53 +1100, Jetrascal wrote:


Car speedos always show higher speed than actual. GPS is much more accurate

Not these days apparently. My car for instance reads about 3% low compared
to the GPS, and others have made similar comments in previous threads on
this subject. I'm not sure why this is; it makes sense to provide margin in
the other direction, but apparently that's not done these days (although I
think there may be some places that it is required by law).

John