GNSS RTK: Escaping FLOAT Purgatory & Achieving True FIX

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A GNSS receiver displaying "FLOAT" is not providing a centimeter-accurate solution. It is estimating, often with meter-level errors. This protocol outlines the minimum required steps to force a transition from FLOAT to FIX and to verify the integrity of the FIX solution.

1. Environmental Constraint Assessment

RTK performance is primarily limited by the sky view. Obstructions and multipath are the leading causes of persistent FLOAT status.

📡 Sky View & Expected Fix Probability

🌤️ Open Sky

>95%

Fix Probability

🏢 Urban Canyon

<40%

Fix Probability

🌲 Tree Canopy

~60%

Fix Probability

⚡ Power Lines

<20%

Fix Probability

2. Base Station Verification

If the base station is not transmitting corrections, or if its position is incorrect, the rover will never achieve FIX.

📡 Base Station Checklist

• Base is set up on stable ground with clear sky view.
• Base coordinates are correctly entered (or base is in "Auto" mode).
• Radio antenna is elevated and has line-of-sight to rover.
• Transmit light is blinking (for internal radio) or NTRIP caster is confirmed active.

3. Rover Configuration & CQ Analysis

Even with a perfect base, the rover must be configured correctly and its internal quality metrics must be monitored.

📋 Rover Checklist

• Rover is set to the same radio channel / NTRIP mount point as the base.
• "Age of Corrections" is < 2 seconds.
• **CQ (Coordinate Quality)** value is monitored. A value < 0.02 is required for high confidence.

📊 CQ Value Confidence Simulator

0.015

🟢 High Confidence (CQ < 0.02)

4. Troubleshooting Persistent FLOAT

If FLOAT persists after the above steps, use this decision tree.

🔧 FLOAT Troubleshooting Flow

📡 FLOAT > 2 mins? ➔ Move to open sky

📶 Age of Corrections > 2s? ➔ Check radio/NTRIP link

🔄 FIX-FLOAT-FIX cycling? ➔ Reduce distance to base

🛰️ Low satellite count? ➔ Wait 15 mins or change location

📐 Reference: Expected CQ Value vs. Baseline Length

CQ (m) ≈ 0.01 + (Baseline km × 0.001)

Rule of thumb: Expect CQ to degrade by 1mm per km from base.

Frequently Asked Questions

Q: Is FLOAT data usable for topographic survey?
A: No. FLOAT solutions can have meter-level errors. Never store points in FLOAT mode.

Q: Can I use a mobile hotspot for NTRIP?
A: Yes, but be aware of high latency. A dedicated UHF radio is more reliable for critical work.

📘 Complete GNSS RTK Mastery Guide

This protocol is a preview. The full guide includes advanced NTRIP configuration, PPK workflows, and Trimble/Leica-specific troubleshooting matrices.

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