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Trying to Understand GPS Antenna Offests  

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Shawn Billings
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Jerry nailed it. The Locus values were physical, not the more precise electronic phase center offset. That 125mm offset (from way back memory) would produce 0.06' error when mixed with other receivers. As a system, ie Locus to Locus, the errors canceled and great elevations could be determined. Mixed with other receivers, though, users had to determine a custom offset.

Also, as Jerry mentioned, I think most consumer post processing software uses the vertical offsets and not the individual elevation offsets. I could be wrong about that.

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The Pseudo Ranger
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Thanks to everyone so far for the help. Some addition notes.

NGS also has an Ashtech Locus calibration, which looks identical:

ASH_LOCUS NONE Integrated L1 only Display to South NGS ( 1) 04/08/24
2.5 0.9 139.9
0.0 -0.1 -0.5 -1.0 -1.7 -2.4 -3.0 -3.5 -3.8 -3.7
-3.6 -3.3 -3.1 -3.0 -3.0 -3.2 -3.7 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 END OF FREQUENCY
END OF ANTENNA

I looked at GNSS Solutions, and they have the "UP" as 159mm.

So, we have 0.125 published in the original manual, 0.1399 by the NGS, and 0.159 by GNSS Solutions.... Weird that there would be so much variation. Did Ashtech really blow it by 0.11' on their first try? I'm going to download some CORS data, and run the pair in Ashtech Solutions, GNSS Solutions, and Topcon Tools, and see what I come up with. I'll post the results.

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The Pseudo Ranger
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Yeah, that's always how I have used them in the past ... Locus to Locus, and whatever height error was present, it cancelled out. Now, I have a project that I could use few extra GPS on, and still have 3 working locus laying around, so I'm going to give it a shot.

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The Pseudo Ranger
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So, here are the results for the two "side by side" points when I process against a CORS about 3 miles away:

Tools - Using the built in antenna models ... also should note, the vector from the CORS to the Hiper was L1/L2, were as all the other solutions were L1 only.

Locus
N 42.443
E 09.852
Z 19.020

Hiper
N 41.443
E 10.412
Z 19.042

GNSS Solutions - Using built in antenna models (Note:, had to use Topcon Hiper Plus G/G because the Hiper Pro Model is blank). L1 Only for both points.

Locus
N 42.421
E 09.826
Z 18.996

Hiper
N 41.427
E 10.387
Z 19.016

Ashtech Solutions, using the NGS heights manually added to the 2 meter poles, and the antenna type set to "none". L1 Only for both points.

Locus
N 42.444
E 09.870
Z 18.996

Hiper
N 41.449
E 10.419
Z 19.021

So, between all 3, I have a range of about 0.025' vertically, between the topcon and Ashtech results. Could be something having to do with Tools using L1/L2 for a vector, where as I don't have that activated for the Ashtech processing. The two Ashtech products agreeing exactly for the LOCUS is interesting. The key thing I was looking for was to make sure I could integrate the two units without a systematic error, and I think that's about as close as I'm going to get to weeding out the odd vertical offset component.

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Paul in PA
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Solutions Locus Antenna Offsets

From Ashtech Solutions antenna.inf file.

[800372]
Description=Locus
Radius=100.0
SHMPOffset=125.0
L1Offset=0.0 0.0 125.0
L1Phase=0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
L2Offset=0.0 0.0 0.0
L2Phase=0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

From GNSS Solutions antenna.ini file.

[800372]
Description=Locus
Radius=100.0
SHMPOffset=125.0
L1Offset=0.0 0.0 125.0
L1Phase=0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
L2Offset=0.0 0.0 0.0
L2Phase=0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Paul in PA

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