Re: errors in GPS reported height

Hi all,

Some interesting assertions here and I couldn't quite see how the
mathematical transformation of three SV x, y and z ECI-centric
coordinates to obtain one set of ECEF coordinates would contain any
bias towards the z vector. If positional errors mainly arise from
atmospheric refraction, ephemeris, or orbital errors, these should
apply to latitude, longitude and height equally. As I couldn't work it
out, I checked a reference book (Introduction to The Global Positioning
System, Ahmed El-Rabbany, Artech House).

"The positioning accuracy . . . is 16m for the horizontal component and
23m for the vertical component (95% probability level)." Note that this
refers to basic trilateration with three SVs - the key point is the
HDOP / VDOP ratio.

"Because a GPS user can track only those satellites above the horizon,
VDOP will always be larger than HDOP. As a result, the GPS height
solution is expected to be less precise than the horizontal solution."

This makes sense when you think about it and I should have twigged
myself. Look at the $GPGSV NMEA sentence structure. For each satellite
in view, elevation is reported to a maximum of 90 degrees (ie 180
viewable) whereas azimuth range is 0 to 359 degrees. This allows
latitude and longitude to be corrected, under ideal conditions,
literally from all angles.

Incidentally, the author discusses solar flares extensively so I don't
think their effect on a relatively weak radio signal is an eye opener
to the GPS community.

For those who think that GPS height is "more accurate these days", I
suspect it is more likely a consequence of newer receivers taking the
GPS reported height (ie height above the centre of the sphere of
reference) and calibrating it against the WGS-84 ellipsoid. Within the
standard margin of error, this should align the GPS height with the
satellite sub point. You can check the 'Geoid separation' value in the
$GPGGA sentence to see if this is done, then if you are truly
obsessive, visit an online geoid calculator and triple check. My GPS
reports my height as 613 metres while my location's geoid correction is
19.3 metres, according to
http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/intptW.html.
This concurs pretty closely with the height reported by my altimeter.

Cheers,

Bill

.

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