Re: Deternining when GPS data is good

Anne wrote:

Hi!

I am new to GPS's and I am writing software application for the
PocketPC to get the location using I-Blue737 Bluetooth GPS. I can get
and parse the NMEA string okay and get the latitude and longitude
values, but I'm not sure when the location is within the 2D-RMS spec
of less than 3 meters. Can I assume after the cold start time, the
data is accurate? What do GPS applications typically do to determine
when the data is good? Any help would be appreciated!


Hi,

from the data in the standard NMEA messages, it is not possible
to find a definitive answer to that question. As Sam said, only
comparison with a known surveyed position will do that.

DOP (Dilution Of Precision, check the GSA sentence), does not
give an accuracy, just a dilution. It's a measure of the geometry
of the satellites used in the solution. For instance, if all
visible satellites are within a small spot of the sky, you will
get high DOP values. Any measurement error gets effectively
multiplied with the DOP.

Inside the receiver, it is possible to calculate an upper bound
of the position/velocity error. Most receivers will not output
that information. The ones that do, output it it in the GBS (errors),
GRS (residuals) and GST (noise stats). This feature is known as
RAIM and only works with 6 or more satellites in view.

If your receiver doesn't do RAIM, you have two options.

1) extract the pseudo-ranges and ephemeris data from the receiver
and calculate your own solution. The residuals from the overdetermined
solution give you an upper bound for the error. This is what a RAIM
receiver does internally. Unfortunately, the necessary information
is not available in NMEA mode - you'll have to switch into the
manufacturer's 'binary' mode.

2) Guess. Look at how many satellites are used in the solution
(GGA or GSA) and look at their signal strengths (GSV). A large
number of satellites, good signal strength (absolute and relative)
and a low DOP number means good precision.

You can also compare the positions with integrated velocities.
Typically, atmospheric errors affect the position more than they
affect the velocity. However, GPS manufacturers know this as well
and filter the output accordingly. You may or may not be able to
outguess the manufacturer's firmware.

Kind regards,

Iwo

.

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