Facet antenna height questions?

Hello,
I’m very new to the GNSS world, I started by connecting a drone to an Ntrip server, which peaked my interest in positioning accuracy, that led me to a single Facet casting position corrections. Hey a second Facet used in a base/ rover configuration could send corrections to a drone and the other Facet, wow! I’m am creeping closer and closer to higher precision / accuracy. This lead me to taking a GIS course at a local college.

I started taking measurements with items recommended in the SparkFun tutorial, an old tripod and an old monopod for data collection, these worked well for testing but using them for work presented issues, mostly accuracy / precision. I recently purchased the SparkFun Survey Pole, making setup much easier.

Now my question. The Facet has 2 antennas L1 and L2, L1 at 61.4 mm offset and L2 at 57.4 mm offset, which would I use for the antenna height in the collection software, the upper L1 or lower L2 or possibly the difference between the two 59.4 mm?
Here I’m thinking about precision / accuracy 2 - 4 mm is not a lot of distance in the real world. Up until now I had been using the higher L1 as I wasn’t sure which to use.
Thanks for any insight you may provide.
Mike

The product page has the following

  • Antenna Phase Center:
    • L1: 67.3mm
    • L2: 71.8mm
    • Average: 69.55mm

Which leads me to believe you should use the average as a best-fit type of thing…try that and then you can run a verification

Hey TS-Russell
Thanks for replying but I don’t think those numbers are correct for the Facet, both of mine show 61.4 and 57.4 on the label attached to the underside of the units.
I’ve taken a photo but can’t seem to upload it.

Gotcha; I would use the numbers on the device …and I’ll see if we need to update the product page

If you’re using two Facets in a base-rover setup, use the same antenna offset (antenna phase center to antenna reference point distance) for both the base and the rover when you configure them. A centimeter or two difference in the offset you specify won’t matter really if you are consistent at the base and rover.

I generally use the L1 antenna offset. The important thing is to be consistent at the base and rover.

Why does SparFun have two different sets of offsets in their documentation for the Facets? The Facet offsets were initially specified with the offsets calculated from the specs of the internal antenna part and the measurements of the SparkFun housing (that’s my understanding at least.) A couple years later the US NGS calibrated the Facet and that gave us slightly different and more accurate offsets.

Again, being consistent at both ends (base and rover) is more important than a few mm. Using absolutely precise numbers for the offsets matters more when mixing and matching different models of antennas.

If you use highly accurate post processing software (eg OPUS, NRC CSRS-PPP, Trimble BC, Carlson Survey GNSS, and I believe RTKLIB), you’ll generally give the software the height of the ARP (the bottom of the Facet) above your survey mark. You also tell the software your antenna/receiver model. The software will then lookup both the L1 and L2 offsets (and other data) for that model from the NGS calibration data. These s/w packages solve the mixing and matching antenna/receiver models issue (and a number of other issues) by using the NGS calibration data, which is one reason many surveys prefer to use NGS calibrated equipment.

Fun fact: the effective offsets can vary by azimuth and the NGS calibration determines this. Quality GNSS antennas have a north orientation. Post processing software can take this all into account.

Figure out the north orientation for your antenna/receiver and set your equipment up accordingly. Most GNSS receivers have the HMI (human machine interface akathe control panel) facing north. The Facet’s HMI faces south, and there’s a north arrow moulded into the bottom
of the Facet.

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Thank you toeknee, I sent in for an Opus solution adding the offset to the antenna height, I will redo using just the elevation to the bottom of the Facet. I assumed that the arrow on the bottom of the Facet was for northward positioning, thanks for that too.

I am also assuming that the offset needs to be added to the pole height in software such as SWmaps?

So consistently is the key to accuracy.