How to Perform RTK Initialization and Verify Fixed Solution

Applies to: Trimble R10/R12i, Topcon HiPer HR/VR, Leica GS18 T, Spectra Geospatial SP80, South Galaxy G1 Plus

RTK initialization is the process by which the GNSS receiver resolves integer carrier phase ambiguities — the mathematical step that converts meter-level GPS accuracy into centimeter-level RTK accuracy. Every RTK session starts with an initialization period, and every initialization ends either in Fixed status (reliable centimeter accuracy) or Float status (sub-decimeter but not centimeter). Knowing how to initiate, verify, and recover initialization is a foundational skill for anyone doing RTK survey or stakeout.

Step 1: Confirm the Correction Link is Active

RTK initialization cannot occur without a continuous stream of correction data from a base station or NTRIP network. Before powering up the rover, confirm the base station is operating and broadcasting. On a UHF radio base, the transmit light should flash at the correction broadcast interval. For an NTRIP network, confirm cellular data is active on the rover or field controller and the NTRIP caster credentials are entered correctly.

On the field controller or rover display, look for a corrections received indicator. Most systems show the age of corrections — the number of seconds since the last correction was received. Age of corrections above 5 seconds indicates a link problem; above 30 seconds means the receiver is operating without corrections and cannot achieve or maintain Fixed.

Step 2: Achieve Open-Sky Conditions for Initialization

Initialization requires tracking at least 5 satellites on two frequencies with a geometric spread (low PDOP) and a continuous correction data stream. Move to an open area with clear sky view before attempting initialization — avoid trees, buildings, and overhead power lines. On the receiver display, monitor satellite count and PDOP. Target 8 or more satellites with PDOP below 3.0 for reliable, fast initialization.

Stand still during initialization. Moving the rover during the ambiguity resolution process forces the receiver to restart the computation. On a high-quality dual-frequency receiver with good sky, initialization typically completes in 15-60 seconds. If initialization is taking longer than two minutes, the sky view or correction link is compromised.

Step 3: Confirm Fixed Solution Status

When initialization completes, the solution status indicator changes from "Searching" or "Float" to "Fixed." On Trimble systems this is displayed as "RTK Fixed" with a green indicator. On Topcon systems, solution quality is displayed as "Q1" (Fixed). On Leica GS18, the display shows "RTK fixed" with a signal quality bar. The Fixed solution means integer ambiguities are resolved and positions are at centimeter-level accuracy.

Do not rely on the visual Fixed indicator alone. Check the estimated horizontal and vertical precision values displayed alongside the solution. For construction stakeout, acceptable precision is below 0.010m horizontal and below 0.020m vertical. Values larger than 0.050m horizontal indicate a marginal solution — initialization may have resolved to an incorrect set of integers (a "wrong fix"), which produces positions that are consistently off by a small amount.

Step 4: Verify on a Known Control Point

Occupy a second known control point — one not used as the base station. Compare the rover's reported coordinates against the published values. Navigate to the check point in the data collector and read the residuals (difference between observed and known coordinates). Acceptable check shot residuals for construction stakeout: below 0.025m horizontal and 0.050m vertical. For structural or high-precision layout: below 0.010m horizontal and 0.020m vertical.

If the check shot residuals exceed tolerance, do not begin work. Common causes: wrong-fix initialization (reset and re-initialize), base station moved or coordinates entered incorrectly, coordinate system mismatch, or antenna height error on the base or rover. Resolve the discrepancy before staking any points.

Step 5: Re-initialize After Sky Obstruction

When the rover passes under a bridge, through a tree canopy, or near a large structure, it may lose Fixed status and revert to Float. After returning to open sky, the receiver will attempt automatic re-initialization. On most modern dual-frequency receivers, re-initialization in open sky takes 15-30 seconds. Monitor the solution status indicator during re-initialization and do not take production measurements in Float status.

If the receiver fails to re-initialize after two minutes in open sky, force a manual reset: on Trimble data collectors, disconnect and reconnect the Bluetooth link or select "Reset RTK" in the survey configuration. On Topcon, restart the survey job. After forced reset, repeat the check shot before resuming production work.

Frequently Asked Questions

What is the difference between RTK Fixed and RTK Float?

Fixed means the receiver has resolved integer carrier phase ambiguities and is computing positions at centimeter accuracy (typically plus or minus 10-20mm). Float means ambiguities are not fully resolved and accuracy is sub-decimeter (50-200mm) — not suitable for construction stakeout or precision layout. Always wait for Fixed before beginning work.

How long does RTK initialization take?

On a modern dual-frequency receiver with good sky view and a valid correction link, initialization takes 15-60 seconds. Older single-frequency receivers or poor sky conditions can take several minutes. If initialization exceeds two minutes, troubleshoot the sky view or correction link before continuing.

What is a wrong fix in RTK GNSS?

A wrong fix occurs when the receiver resolves carrier phase ambiguities to an incorrect integer set. The solution shows "Fixed" but coordinates are systematically offset by one to several centimeters. Wrong fixes are detected by check shots — they produce consistent residuals that are outside normal Fixed accuracy but too small to be obvious from the Fixed indicator alone. This is why check shots are mandatory, not optional.

Can RTK work inside buildings or under bridges?

RTK requires clear sky view and cannot maintain Fixed under solid obstructions. Near bridges, canopies, or in urban canyons with limited sky, Fixed status is unreliable or impossible. Use a total station for layout work in obstructed environments where RTK cannot maintain Fixed.