I have a question about antenna heights for the FACET, specifically so I can submit the data to OPUS.
I’ve had initial success with a pair of FACETS and the Holybro radios in a Base-Rover RTK configuration. Currently I’m letting the base “survey-in” to an approximate location and getting good relative accuracy (within an inch of measurements made by a professional surveyor on my property corners.)
My next step is to gather hours of raw data and submit it to OPUS so I can get an accurate PPP position for my Base. OPUS has a selectable antenna type “SFETOP106” - SFE being SparkFun Electronics! Sounds like that’s the antenna to use…but…
I suspect that the OPUS antenna "SFETOP106 refers to the SparkFun Electronics “GNSS Multi-Band L1/L2 Surveying Antenna (TNC) - TOP106” antenna and NOT the FACET, even though the FACET has the same internal element.
The FACET and the “GNSS Multi-Band L1/L2 Surveying Antenna (TNC) - TOP106” have different offsets from the antenna phase center to the Antenna Reference Points (ARPs) on the bottom of the units.
The L1 phase center for the “GNSS Multi-Band L1/L2 Surveying Antenna (TNC) - TOP106” is 43.60mm above the ARP.
The L1 phase center for the FACET is 61.40mm above the ARP.
SWMaps simply wants the height of the antenna phase center above the survey mark, and the FACET hookup guide gives a very useful example of what to enter for SWMaps: (“So if your monopod is 67 inches long fully extended, the antenna offset is 1701.8mm + 61.4mm = 1763mm. You would want to enter 1.763m into SW Maps to get accurate altitude of the point being measured on the ground.”)
But OPUS is a little smarter than SWMaps, it tries to save you that math, and just wants the height of your monopod/tripod.
If my tripod is 2 meters above my survey marker, then the bottom of the FACET is 2 meters above the survey marker. That is, the ARP is 2 meters above the survey marker. If I tell OPUS my ARP height is 2 meters and I’m using the SFETOP106 antenna, then OPUS will think the L1 antenna phase center is 2 meters + 43.60mm (2.0436 meters) above the survey marker.
But the FACET’s phase center will actually be 2.0614 meters above the survey marker, 0.0178 meters above where OPUS thinks it is.
Do I tell OPUS that my 2 meter tripod (ARP height) is 2.0178 meters above the survey marker to get OPUS to give me good results because I’m using a FACET but telling OPUS I’m using the TOP106?
I know we’re talking about a couple centimeters here, but might as well eliminate all the errors that I can. I’m working with a professional surveyor who uses Trimble GNSS receivers; I am attempting to repeat his results to prove the usefulness of the FACETS for our project. He’s already gathered data from the same survey marker and gotten the OPUS output; I want to do the same and compare my OPUS output to his.
Or can someone get the FACET added to the OPUS antenna selection option?
Thanks!
Tony.
PS I’m working in the southern US. The project and my colleagues use NAVD88. So I don’t believe I can use the Canadian CSRS-PPP service as that uses the Canadian vertical datums. Therefore I want to use OPUS.
PPS I believe the FACET hookup guide uses the term “ARP” (antenna reference point) when it should use “APC” (antenna phase center).
- What is an ARP?
From https://geodesy.noaa.gov/ANTCAL/FAQ.xhtml
"The antenna reference point (ARP) is the physical point on the antenna to which antenna calibration values are referenced. ARP is preferably an easily accessible point on the lowest non-removable horizontal surface of the antenna. Typically, the ARP coincides with the axis of attachment of the antenna to a monument or surveying instrument. "
PPPS Oh when I dig deep into the OPUS antenna calibration files I see that the vertical offset for the TOP106 isn’t modeled at exactly 43.60cm. But let’s go with what the TOP106 datasheet says.
