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[DPRG] Using GPS HDOP for KF purposes
Subject: [DPRG] Using GPS HDOP for KF purposes
From: Dean Hall
dwhall256 at gmail.com
Date: Fri Jan 25 23:32:34 CST 2008
Chris Jang, all,
I suggested back in November that using the NumberOfSatellites from
the NMEA stream of a GPS unit could provide an index into a lookup
table to get the variance value to use in a Kalman Filter (http://
list.dprg.org/archive/2007-November/031055.html).
Well, today I stumbled across a better idea. I was reading through
this article (http://www.cs.unc.edu/~welch/kalman/Levy1997/
Levy1997_KFWorkhorse.pdf) that suggested on page 8 that pseudoranges
are preferred outputs to use in the KF. So I looked again through
the fields that my SiRFstarIII chip provides... nope, no
psuedoranges. However, serendipity befell me and I found the
Dilution of Precision (DOP) field in the GSA sentence and read up on
it at http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)
And there it was, in little black text near the bottom:
The DOP factors are functions of the diagonal elements of the
covariance matrix of the parameters, expressed either in a global or
a local geodetic frame.
I followed the reference wikipedia gave, read the article (http://
www.developerfusion.co.uk/show/4652/2/) and found these two gems:
"As a general rule of thumb, an HDOP value of six or less is
recommended for any application which makes suggestions to the user
based on the current location."
Accuracy of GPS Device * DOP = Maximum Allowable Error
I plan to use the HDOP in my KF. Also, note that the HDOP is usually
much smaller (=better) than the VDOP and mean DOP. Here are real-
world measurements I made: with 10 sats in view, I got a HDOP of 1.8;
with 12 sats (max for my GPS), HDOP = 0.9. Although I cannot confirm
it, it seems (from measuring and comparing to Google Earth) that *DOP
values have a unit of meters. So it looks like the HDOP value will
provide a nice input to calculate my GPS module's measurement error
variance. The downside is that the GSA sentence containing the HDOP
does not arrive as often as the RMC and VTG sentences in the default
configuration.
!!Dean
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