How to interpret the HDOP value
Bendik Engebretsen
Does anyone know how to interpret the HDOP (horizontal dilution of
precision) value ?
I've written a software that decodes various NMEA sentences, among these
$GPGGA where you get (from some GPS receivers) a HDOP value. I know this
can tell me something about the precision of the fix, but how should I
interpret it? If it's possible I'd like to "convert" it to give an accuracy
measurment in meters. Is this possible ? (I don't suffer from math-o-fobia,
so technical descriptions are welcome...)
(PS: I'm new in this group, so please forgive me if I'm out of context
here...)
Bendik Engebretsen
bendik.en...@mmisys.no
Robert S. White
In article <01bc1900$8d759900$651a13c2@bendik>, bendik.en...@mmisys.no
says...
>
>Does anyone know how to interpret the HDOP (horizontal dilution of
>precision) value ?
HDOP * User to Satellite Range error, 1 sigma = Radial error in user
position, 1 sigma, in the horizontal plane
Typical rms value is designed to be about 1.45. Note that the MIT real
time observations show about 50 meters accurate. That would mean that what
the Standard Position Service (SPS), with SA, is currently seeing about 34.5
meters User to Satellite Range error.
_______________________________________________________________________
Robert S. White -- an embedded sys software engineer
Whi...@CRPL.Cedar-Rapids.lib.IA.US --long/cheap alternate I-net address
Adrian Jansen
My understanding is that the HDOP measures the degree of possible
( horizontal ) error in the position solution due to the satellite
geometry. ie if all sats are grouped in a small area of the
sky, the triangulation gets worse, and the HDOP goes up.
The best possible HDOP is 1, ie the geometry contributes no error
to the solution.
So if the receiver estimates the error as, say 20m, based on its
internal conversion routines, and the HDOP is 2, then the overall
estimate is 40m. This still takes no account of SA and other
external errors.
Unfortunately the NMEA sentences dont output the receiver error
estimate either, so as far as I can see, the HDOP is fairly
useless, other than as a rough guide to how bad the geometry is
getting.
You may have noticed that you can also get the VDOP and PDOP from
the GSA sentence, as well. These give you the vertical and
'overall' position dilutions in a similar way.
In article <01bc1900$8d759900$651a13c2@bendik>, "Bendik
Engebretsen" (bendik.en...@mmisys.no) writes: >Does anyone know
how to interpret the HDOP (horizontal dilution of >precision) value ?
>
>I've written a software that decodes various NMEA sentences, among these
>$GPGGA where you get (from some GPS receivers) a HDOP value. I know this
>can tell me something about the precision of the fix, but how should I
>interpret it? If it's possible I'd like to "convert" it to give an accuracy
>measurment in meters. Is this possible ? (I don't suffer from math-o-fobia,
>so technical descriptions are welcome...)
>
>(PS: I'm new in this group, so please forgive me if I'm out of context
>here...)
>
>Bendik Engebretsen
>bendik.en...@mmisys.no
>
>
--
Regards,
Adrian Jansen aja...@brisbane.DIALix.oz.au
Design Engineer J & K Micro Systems
Ph (076) 394 676
Microcomputer solutions for industrial control
Sam Wormley
Ref:
1. Hoffmann-Wellenhof B, Lichtenegger H, and Collins J, "GPS Theory and Practice",
2nd Ed, Springer-Verlag, New York, 1993.
2. Clarke B, "Aviator's Guide to GPS", TAB Books, McGraw-Hill, New York, 1994
The User Range Accuracy (URA), also known as User Equivalent Range Error
(UERE) is a 1-sigma statistic. This UERE is transmitted via the navigation
message. In combination with a DOP factor, UERE allows for an estimation of
the achievable point position precision. When SA was implemented UEREs were
set to 32 meters.
Assuming you want to esimate the horizontal position error based in HDOP of 1.6
then Estimated Position Error (EPE) = UERE * HDOP
EPE = 51.2 m = 32 m * 1.6 (1-sigma)
EPE = 102.4 m = 32 m * 1.6 * 2 (2-sigma)
Since the Standard Positioning Service (SPS) specifys user 2-D (horizontal)
position accuracy shall be 100 meters (328.1 feet) 2d rms or better based
on HDOP 1.6, VDOP 2.2, PDOP 2.7, and TDOP 1.3, the above formulation has
the ring of truth about it.
__________________________________________________
Sam Wormley - http://www.cnde.iastate.edu/gps.html
Sam Wormley
When SA was implemented UEREs were set to 32 meters.
Assuming you want to esimate the horizontal position error based on HDOP of 1.6
then Estimated Position Error (EPE) = UERE * HDOP
EPE = 51.2 m = 32 m * 1.6 (1-sigma)
EPE = 102.4 m = 32 m * 1.6 * 2 (2-sigma)
Since the Standard Positioning Service (SPS) specifies user 2-D (horizontal)
position accuracy shall be 100 meters (328.1 feet) 2d rms or better based
on HDOP 1.6, VDOP 2.2, PDOP 2.7, and TDOP 1.3, the above formulation has
the ring of truth about it.
DOPs can be calculated for your position on the Earth, at a given date
and time.... These DOPs are based on NO obstructions of any kind....
Realistically there ARE obstructions from the terrain, buildings, vehicles,
possibly your body... that make the DOPs higher than minimum. This is
the primary reason your GPS position can be off by more that 100 meters...
because of a higher effective DOP.
Inge Nesbö
aja...@jkmicro.DIALix.oz.au (Adrian Jansen) wrote:
|
|My understanding is that the HDOP measures the degree of possible
|( horizontal ) error in the position solution due to the satellite
|geometry. ie if all sats are grouped in a small area of the
|sky, the triangulation gets worse, and the HDOP goes up.
|
|The best possible HDOP is 1, ie the geometry contributes no error
|to the solution.
|
HDOP is a measure of the geometrical strength of a fix as you say, but
then it can be less than 1. The number of satellites used in the fix
and the direction to the satellites will have an effect upon HDOP. The
real error in the observations depends upon the quality of your
receiver, but the HDOP value is only dependent upon where in the world
you are, how many satellites you see and where they are. If now your
GPS receiver showed HDOP=1.0 we can expect to get a HDOP smaller than
that if we have a receiver that can observe all GPS satellites and all
GLONASS satellites (maybe you can see 24 satellites if you are lucky).
|So if the receiver estimates the error as, say 20m, based on its
|internal conversion routines, and the HDOP is 2, then the overall
|estimate is 40m. This still takes no account of SA and other
|external errors.
|
SA is a bias, not a random error and that is why SA cannot be included
into a total random error. Unlike people, computers are happy with
lots of numbers. If we allowed them to have all available satellites
(GPS and GLONASS) the computer would be able to compute the random
errors for the satellites and to compute and remove the effect of SA
from GPS. The computer could then use the estimate for random error of
an observation to scale the HDOP and VDOP to get best estimate of
horisontal and vertical position error.
--
_inge