Use an antenna that is NGS calibrated to obtain accurate ARPs. The SparkFun TOP106 antenna has been calibrated and we are in the process of calibrating the RTK Facet and RTK Facet L-Band.
So TOP106 is “NGS calibrated”. But what does that mean?
E.g.:
- Does it mean that every single device is tested and adjusted, so it's calibrated to be within some standard specs?
- Does it mean that NGS took some sample set of these antennas and measured their characteristics, and then published a report/datafile on that?
- What is the result/outcome from such a calibration? Do they adjust something within the antenna? Do they publish a report? Do they publish a data file that can be used by software or receivers to compensate/adjust incoming raw data from that antenna?
Reason I am asking: A customer asked if our equipment (Sparkfun Surveyor with the TOP106 antenna) was “calibrated”. He actually does not know anything about this, and he cant be more specific. I decided to research if, and in which way, the equipment is calibrated, and this is my first question in that research
Well NGS doesn’t certify each device. You submit samples and design materials.
The design and build process is supposed to be robust so that each antenna will be consistent in it’s construction and build quality.
Things like consistency of parts, the placement and orientation of the antenna element on the PCB, and then within the housing of the product, and threaded collar, with it’s bottom face.
The phase-centers of the antenna at various frequencies are mapped in 3D, with respect to the reference point, and this can be found in an ANTEX file, or similar, so this can be factored in vs the direction/angle of attack during post-processing. Basically an XYZ compensation of the incident signals, to where the phase centers lay, to the bottom face of the antenna mount, and the top face of the tripod, as it sits over the survey marker. Also you’d factor in this height above the marker.
There might be some higher end devices that can take ANTEX files, or have a database of antennas, that can apply this on-the-fly, but the u-Blox receivers don’t facilitate this. That’s kind of the trade-off you make on a $200 receiver vs a $20000 one.
The receiver is basically tasked with computing the time and place of the notional phase-center of the antenna. For survey work you’re more interest in the bottom surface on the mounting point. Most survey antenna will have a little diagram showing the offset heights of the L1 and L2 phase centers vs the reference surface/point of the antenna. You can apply this to the reported LLH and your own tripod/antenna height adjustments as you read the position.
