The Antenna Reference Point (ARP)
Every survey-grade antenna has one specific, defined, physical point that a crew can actually put a tape or a rod tip against. That point has a name, an antenna reference point, and everything else in this companion is ultimately described relative to it.
The antenna reference point, ARP, is a specific, physical, manufacturer-defined point on the antenna, typically the bottom of the mounting threads, that serves as the fixed, touchable point every height measurement is actually made to.
A Point Chosen for Being Measurable
Manufacturers do not pick an antenna's reference point arbitrarily. They pick a point that is physically well defined, easy to identify consistently, and accessible enough that a crew can reliably measure to it in the field, most commonly the bottom of the antenna's mounting thread, where it screws onto a pole or a tribrach adapter. That choice has nothing to do with electronics. It is entirely a practical, mechanical decision.
The ARP is defined for measurability, not for electrical significance. It is the point a crew can find with a tape, not the point the receiver actually uses to compute a position, and those two facts are precisely why the rest of this companion exists.
Where This Point Is Actually Documented
An antenna's specific ARP location is published by its manufacturer, and independently confirmed and distributed by calibration authorities such as the National Geodetic Survey, which maintains diagrams, photographs, and written descriptions of the ARP for a large number of commercially available antennas. Confirming an antenna's documented ARP before relying on it in the field is a small, worthwhile check, particularly for less common models.
The bottom of an antenna's mounting thread is a common, but not universal, choice of ARP. Some antenna designs define it slightly differently, which is exactly why checking the specific, published ARP for a given model matters more than assuming every antenna defines it the same way.
Why This Point Alone Is Not Enough
Measuring height to the ARP with perfect care produces a perfectly accurate description of one thing: where the ARP itself sits. It says nothing yet about where the antenna is actually receiving a signal from, which is an entirely different point, addressed directly in the next chapter, and which is precisely why a height measurement to the ARP is only the first half of getting an antenna's true position right.
A crew measures antenna height to the bottom of the mounting thread with excellent precision. Does that guarantee the position software computes is accurate? Not by itself. The ARP is a defined, measurable mechanical point, but the receiver's actual position solution is referenced to a different point entirely, one this companion covers next.
The antenna reference point, ARP, is a specific, manufacturer-defined, physical point chosen for being easy to measure to consistently, most often the bottom of the mounting thread. It is the foundation every other measurement in this companion builds from, but it is not, by itself, the point a GNSS position is actually computed relative to.
The point that actually matters to the receiver's own math is not physical at all, and understanding it, along with the one number an RTK user actually needs from it, is the subject of the next chapter.