Base Station Antenna Height: Slant Height, ARP, and the 5/8" Thread

Antenna height errors are the most common vertical bust in GNSS work. How to measure slant height correctly, what ARP and phase centre mean, when tripods and tribrachs matter, and why ground-placed bases skip the problem.

Why a tape measure causes more vertical busts than satellites do

Ask any processing office: the classic vertical error in GNSS work is not the ionosphere — it is a wrong antenna height. The base believes its antenna sits 1.532 m above the mark when it sits 1.687 m, and every rover elevation, drone geotag, and surface on the job inherits a clean 15.5 cm bust that no amount of FIX can cure. Understanding three terms — mark, ARP, and phase centre — plus one measuring technique removes the whole failure class.

The three heights that get confused

The mark is the physical point on the ground your coordinates should describe. The ARP (Antenna Reference Point) is a defined physical feature on the receiver — usually the bottom of the mount — that tape measures can actually reach. The phase centre is where the antenna electrically receives signals: an invisible point above the ARP whose offset the manufacturer publishes and firmware applies automatically via the antenna model (RTCM even carries the descriptor — message 1033, the message tour). Your only field job is the mark-to-ARP distance; software does phase-centre arithmetic. Busts happen when people measure to the wrong feature or enter a slant as a vertical.

Measuring slant height correctly

On a tripod you cannot tape straight up through the instrument, so you measure slant height: from the mark to a defined slant-measurement point on the receiver’s rim, then let software convert using the unit’s known geometry (vertical = √(slant² − radius²), applied automatically when you tell the field app the measurement type). The discipline: measure to the manufacturer’s marked point, not “about the edge”; take two measurements on opposite sides and confirm they agree within a couple of millimetres; and record slant-vs-vertical explicitly in the notes. Emlid Flow, Trimble Access, and every serious field app ask the measurement type for exactly this reason — answer it honestly.

Tripods, tribrachs, and the 5/8"-11 thread

The mechanical stack matters because it defines the ARP’s position over the mark. The industry standard is the 5/8"-11 thread: receivers, poles, tribrach adapters, and tripod heads all speak it, which is why a survey receiver drops onto any survey tripod on Earth. The tribrach adds precise centring (optical or laser plummet) and levelling over the mark — mandatory when occupying a known point, because a 5 mm centring error is a 5 mm coordinate error forever. Rover poles simplify life with a fixed height (typically 1.8 or 2.0 m — enter it once) and tilt compensation now forgives the levelling (how modern tilt works).

When height measurement matters — and when it disappears

The measurement ritual exists to relate the antenna to a specific ground mark. Occupying a monument: full ritual — tribrach, plummet, slant tape, double-check. Rover on a pole: fixed height, entered once. Ground-placed self-converging base: the ritual disappears — the receiver sits directly on the ground, its antenna-to-ground geometry is a fixed constant the firmware already knows, and its coordinates describe its own position rather than a monument’s, so there is nothing to tape and nothing to mistype. This is an underrated share of the “place, power, work” appeal: one deleted procedure is one deleted error class (the coordinates side of the same story).

Drone-workflow specifics

Two height details bite mapping crews. Base height in the corrections: the base position broadcast in RTCM 1006 includes antenna height — if the base setup got it wrong, every rover and geotag inherits the bust, which is why checkpoint verification always includes a vertical (the two-checkpoint habit). Geoid versus ellipsoid: GNSS heights are ellipsoidal; deliverables usually want orthometric — apply the geoid model once, in processing, and never “fix” a constant offset by nudging antenna height, which launders a datum question into a hardware lie.

A checklist that ends height busts

Before logging: confirm mount type (tripod/pole/ground), measure slant twice to the marked point, enter value plus measurement type, photograph the setup with the tape in frame. During: never change the height entry mid-session. After: checkpoint with a vertical comparison; a constant vertical residual across all checkpoints is a height or geoid suspect before it is anything else. Total cost: ninety seconds and one photo — against the most common preventable error in the trade.

How the pros catch height busts after the fact

Even disciplined crews audit. Three detection patterns worth knowing: the constant-vertical-residual signature — every checkpoint high or low by the same amount points at height entry or geoid handling before anything else; the session-boundary step — a surface that jumps a decimetre exactly where one day's data meets the next means the two sessions carried different height entries; and the slant-as-vertical fingerprint — an error suspiciously equal to (slant − vertical) for your setup geometry, usually 10–20 cm on a 2 m tripod. Each signature names its own correction, and all three are recoverable in reprocessing if the raw logs and setup photos exist — one more return on the RINEX habit.

One-line takeaway

Measure slant to the marked point, tell the software what you measured, photograph the setup, and prefer ground-placed self-positioning bases where the whole ritual — and its error class — simply doesn't exist.

Rover-side heights: poles, bipods, and tilt

The rover's version of the problem is gentler but real. Fixed poles at 1.8 or 2.0 m: enter once, verify the collar hasn't been re-pinned by a helpful colleague. Adjustable poles: the collar scale is the truth — read it at eye level, not from above. Bipods for static occupations follow tripod rules. And tilt compensation changes none of the arithmetic — the entered height is still the lever arm the IMU projects through, so a wrong pole height produces tilted-shot errors that grow with the lean angle. One habit covers the fleet: pole heights on a strip of tape on each pole, checked against the app at the first point of the day.

Further reading

The base-coordinates side of setup discipline — where the position itself comes from — is covered method-by-method in setting up without a known point.

One-line takeaway

Three heights, one tape, one honest software field: measure slant to the marked point, declare what you measured, photograph the rig — or run a ground-placed base and retire the entire ritual along with its favourite error.

Frequently asked questions

What is the difference between slant height and vertical height?

Vertical is mark-to-ARP straight up; slant is mark to a rim measurement point, converted by software using the receiver’s radius. Enter the type you actually measured.

What is the 5/8"-11 thread?

The universal survey mount standard connecting receivers, poles, tribrach adapters, and tripods — any survey receiver mounts on any survey tripod through it.

Do I measure antenna height for a ground-placed base?

No — units designed for ground placement carry their antenna geometry as a firmware constant, and their coordinates describe the receiver itself, so there is nothing to tape.

Why do my elevations all sit ~30 m off?

Almost certainly ellipsoidal vs orthometric heights — apply the geoid model in processing. A constant sub-metre offset instead suggests an antenna-height entry error.

Does tilt compensation change height measurement?

No — pole height is still entered once. Tilt corrects the pole’s lean geometrically; it does not alter the height value.