PXI-6682 Reports Altitude incorrectly

[BruceBarnes]

I am using a PXI-6682 to track position and it is reporting my altitude (3rd floor of building at sea level) as being 15 to 30 meters below sea level.  The altitude measurements vary quite a bit with time while the latitude and longitude are stable.  Why am I getting these results and is there any thing I can do to improve it?

Thanks,

Bruce

[EBL]

Bruce,

The altitude accuracy of GPS is notoriously worse than the horizontal accuracy. This is due to fundamental geometric limitations, but it can be exacerbated by poor satellite reception, so make sure your antenna has a clear view of most of the sky.

As an example, I have a nice little handheld Garmin receiver, which does very well for horizontal position but is quite often off by several hundred feet of altitude. Fortunately, it also has an aneroid barometer for measuring altitude. Interestingly, since it has no way of knowing the actual sea-level pressure, it gently calibrates the barometer over the long term by averaging the GPS-derived altitude data over time. This seems to work fairly well, and as a result it's always right to within a few dozen feet.

If you're sure your antenna is well-positioned, you might try averaging the altitude data over time.

Hope this helps,
Ed

[Sheldon_Stokes]

Are you sure, it's not just the geoid separation?  I'm assuming that this card is set for the WGS84 datum, which for the virginia area for example has a geoid separation of about 36 meters if memory serves me correctly.  So depending on where you are, that could be very reasonable. 

Sheldon