Re: Roughtly converting DOP to metric error

On Mar 5, 2:28 am, Nick <nhop...@xxxxxxxxx> wrote:
On 4 Mar, 20:49, speleo...@xxxxxxxxx wrote:

Garmin units usually send a proprietary PGRME sentence that reports
horizontal, vertical and overall position error [...]

Forgive me, but I don't think GPS receivers can report errors in
position, to do this they would need to know their true position.

Regards, N.

There is a rough indication of signal error sent down from each GPS
satellite - the URA or user range error. It is defined in IS-GPS-200D
available at http://www.navcen.uscg.gov/gps/geninfo/IS-GPS-200D.pdf.
Here's the text describing it:

6.2.1 User Range Accuracy. User range accuracy (URA) is a
statistical indicator of the ranging accuracies
obtainable with a specific SV. URA is a one-sigma estimate of the
user range errors in the navigation data for the
transmitting satellite. It includes all errors for which the Space
and Control Segments are responsible. It does not
include any errors introduced in the user set or the transmission
media. While the URA may vary over a given
subframe fit interval, the URA index (N) reported in the NAV message
corresponds to the maximum value of URA
anticipated over the fit interval.

Currently the URA's smallest value is zero, meaning there is a one-
sigma range error estimate for that fit interval between 0 and 2.4
meters. The URA values increase from there.
When CNAV data begins transmission, the URA will have a different
meaning, and can take on negative values. This gives a much better
precision to the URA estimates. CNAV data will be available on the
new L2C signal.
Ted
.