On 1/23/10 2:56 PM, eric gisse wrote:
Sam Wormley wrote:
[...]
However the live introduction of the
new functions is causing problems wherein some of these receivers are
intermittently not tracking Y-code, and non-compliant civilian receivers
are also reporting continuing problems.

Sounds like some manufacturers didn't pay proper attention to the spec and
relied on something they should not have. Perhaps parts of the spec that
were unused until now were not implemented, or perhaps as Mike drew by
analogy the receiver relied on a part of the signal that was omnipresent but
not part of the spec.


Corrective action could encompass either the Air Force rolling back the
update or revising its software, or the manufacturers modifying GPS
software within the receivers to be totally compliant with the ICD.

In November and December 2009, the new software uploaded operational GPS
IIA and IIR space vehicles with navigation data and completed normal
operational functions. The software is part of the GPS modernization
effort, with its principal features being telemetry, tracking, and
commanding for the new GPS IIF space vehicle ? as yet unlaunched -- and
more robust security for the military M-code signal.

GPS only gives you position. Kinda narrows what the satellites can do unless
they go multi-purpose. I wonder what kind of position accuracy was expected
with the uprades.



Actually one get PVT (Position, Velocity and Time) from the
receivers. Dual frequency P(Y) Code receivers, with out augmentation,
such as WAAS, or other DGPS have positional accuracies of about
one meter, velocities better than 0.1 m/s and time accuracies of
a few nanoseconds.

For nonmilitary single frequency receivers:

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".