Trying to Understand GPS Antenna Offests
For the first time, I'm trying to mix two receivers (Ashtech Locus and Topcon Hiper Pro), so I wanted to do some testing and make sure I know what I'm doing.
I did some testing today, by placing the two next to each other on a flat surface, let them collect data for 2 hours, and processed to see if I could get the elevation to agree.
When I process in Topcon Tools, everything is fine, because Tools has both antenna types programmed in it. The elevations come up less than 1 hundredth apart.
I tried it again in Ashtech Solutions, and it didn't work so great. Solutions doesn't have Topcon offsets built in, so I had enter them myself. This is where it gets weird. It appears that both published "up" offsets are significantly wrong (or I don't understand what they mean).
Ashtech published the "UP" offset as 125mm, and topcon has their L1 "UP" offset as 105.9mm. (0.347 feet). Now that topcon offset immediately looks funny to me, because the Hiper itself is less than that height.
When I used "none" for antenna, and used those heights, I'm off about 0.10'.
However, I look up the NGS numbers for the two GPS, and it has Locus as 139.85mm, and Topcon Hiper as 87.85mm.
So, basically, it's 125mm published vs. 139.85mm NGS, for the Locus. (about -0.05')
And, 105.9mm vs. 87.85mm published for the Hiper. (about +0.06')
So there is my tenth.
When I used those numbers, its again within 0.01' ...
So, why the big differences? Am I misunderstanding what the "published" numbers refer to?
Unless you open the antenna files and compare all the offsets you will never know.
Be aware that some older software still uses an antenna file with the old relative offset differences not the newer NS/IGS absolute offsets.
Also Ashtech has the offsets in the file opposite the NGS published 5° increments.
Some TOPCON antennas have NGS offsets that produced different OPUS elevations from that same data used in Solutions or the Topcon software.
Paul in PA
Just to be clear, I'm using the older "Ashtech Solutions", not the newer "GNSS Solutions". "Ashtech Solutions" doesn't have a lot of antennas included, so I have to enter them manually.
If I understand you correctly, you are asking why the values printed on your antenna differ from those in the NGS antenna information files.
The reason is that there is a physical distance between the antenna reference point (in most cases the bottom of the antenna) and the point at which the signal is captured by the antenna. You will note that the NGS and IGS antenna calibration files include not only a physical offset in terms of N,E and U but also azimuth and elevation dependent values. I am sure that the folks performing the calibrations do not open the antenna to make th physical measurement between the antenna base and each strip antenna. These values are solved from signal tracking data.
Recollect that when you send a file to OPUS it asks for the height to the Antenna Reference Point and the exact antenna model. If you carefully examine the OPUS extended output you will see the offset values used in processing. In addition to the UP offset, there are horizontal offsets though usually not enough to worry surveyors.
Bad news was that Ashtech's original locus antenna file was based on physical antenna measurements and not on a NGS calibration. Unfortunately NGS did't do a calibration on it until near the EOL for the units.
Good news was that under the Sokkia brand NGS did do a calibration and it can be found on the NGS site under Sokkia Locus.
Bad news is that most people then using locus with opus could tell that it was not right for some reason and based on messages on the then still active ashtech forum users came to other numbers based on actual experiences. Ashtech forum messages may still be available. I think I have some emails on the topic archived.
So caution is advised. Also as I recall it was revealed the AS didn't use any of the non vertical data in the file. i.e. only straight up offsets.
SOK_LOCUS NONE Sokkia Locus (L1, 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