The Phase Center and the One Offset RTK Actually Needs
A GNSS receiver does not compute a position relative to the ARP. It computes a position relative to the antenna phase center, APC, the electrical point where the antenna's internal geometry actually resolves an incoming signal. For everyday RTK work, understanding that distinction comes down to one practical question: how does the software in front of you actually want that relationship handled?
The number that matters for RTK is the ARP height plus the antenna's own APC offset. Different field software handles that addition differently, some expecting the user to calculate and enter one combined total, others adding the offset automatically once an antenna model is selected, and assuming the wrong one produces a real, silent height error.
Why the Phase Center Sits Somewhere Else Entirely
The point at which an antenna's internal elements combine an incoming signal into a single measurable phase is a function of its design, not its outer shape, and does not sit at the mechanical ARP a crew can measure to. Manufacturers publish the vertical distance between the two, the APC offset, as a fixed number for a given antenna model.
For everyday RTK work, the antenna's phase center behaves, for all practical purposes, as a single fixed point above the ARP. The more elaborate reality, that this point shifts slightly with signal direction, matters elsewhere, covered later in this companion, but not here.
Two Genuinely Different Ways Software Handles This
Some common field data collection apps expect the user to do the arithmetic directly: measure the pole height to the ARP, add the antenna's published APC offset, and enter that single combined total as the antenna height. Other software, and some receivers with a built-in antenna database, instead ask only for the height to the ARP and add the correct offset automatically once the specific antenna model is selected. Both approaches are genuinely common. Neither is a universal default.
There is no single, universal answer for whether field software wants a combined total or the ARP height alone. Assuming the wrong one produces a real error in either direction, missing the offset entirely if a combined total was expected, or double-counting it if the software was already going to add it.
The One Habit That Actually Matters Here
Before relying on any new combination of receiver and software, confirm directly, from the manufacturer's or software provider's own documentation, which of the two patterns applies. This is a small check with a real payoff, and it is worth repeating any time either the receiver or the software changes, rather than assumed to work the same way it did with previous equipment.
A crew switches to a new field app after years of using one that expected a combined total height. Is it safe to keep entering that same combined number? Not automatically. The new app may expect only the ARP height and add the offset itself, in which case the old habit would double-count it. Confirming which pattern the new software actually uses is worth doing before the first session, not after a discrepancy shows up.
RTK positioning ultimately needs the ARP height plus the antenna's own APC offset, but which party, the crew or the software, actually performs that addition varies by app and receiver. Confirming this directly for a specific setup, rather than assuming it works the way a previous one did, is the one habit that prevents a real, silent height error.
With that practical question settled, it is worth naming the specific mistakes this uncertainty most often produces in the field.