# [time-nuts] Impact of GPS antenna height measurements

Mark Barettella n2md at aol.com
Mon Jun 27 18:05:42 EDT 2016

```Appreciate all the input.  I now realize that I know nothing about this subject!

For fun ran my gpsdo with the antenna height calculated from an available Topo chart it really made the timing intervals and EFC show much larger swings.  So answering one of my own questions, YES there does seem to be an effect of antenna height on the gpsdo performance.
I did change my elevation mask to 15 degrees as Michael suggested to see what difference this will make.  My autosurvey mode is set at max on 10000 and runs about 3 hours so I don’t believe I can lengthen that.

Thanks to Bob and Chris I now know that “elevation” or “altitude” have a number of definitions!  So the answer to my question was not as simple as I thought.  After reading some of the responses I researched a bit more and now see that the GPS may be using the Ellipsoid height not the Topo (Orthometric) height I was measuring.  I was amazed at how far apart all these measurements can be and vary significantly by location on the earth.

Here’s the Geoid height of my antenna location.  I entered my coordinates and GPS height into an online calculator:

Your Input Coordinates and GPS Height:

Latitude = 29.1953319444444° N = 29° 11' 43.19" N
Longitude = 80.9982327777778° E = 80° 59' 53.64" E
GPS ellipsoidal height = -18.39 (meters)
Geoid height = -43.609 (meters)
Orthometric height (height above EGM96 geoid which approximates mean sea level) = 25.219 (meters)

(note: orthometric height = GPS ellipsoidal height - geoid height)

Cheers!

Mark
n2md
```