Anyone who has run RTK near trees knows the status bar can flip from FIX to FLOAT for no obvious reason. If you are choosing a receiver for an orchard robot, a lawn mower, a forestry survey or an outdoor AMR, you need a test that tells you how it will behave under canopy, not a brochure number measured in an open field.
We recently reviewed a multi-receiver field test logged under roadside trees in rain. The results were interesting: three of four record groups held a 100% fixed solution for 39–61 minutes, while one dropped to 83–89%. The review also showed how easy it is to draw the wrong conclusion from data like this. This checklist is what we would do differently next time.
1. Decide what "good" means before you start
Fix ratio alone is not enough. Write down the metrics you will report:
- Fix ratio: share of epochs with a fixed RTK solution (Q=1 in RTKLIB)
- Accuracy: horizontal and vertical RMS and 95th percentile against a known truth
- Time to first fix after a cold start and after losing the fix
- Availability: share of epochs with any usable solution
A receiver that reports 100% fixed but sits 10 cm off the truth is worse than one that reports 95% fixed and is right.
2. Get a truth
Without a truth, you are only comparing status flags. Options:
- A surveyed control point with known coordinates in a stated frame
- A long static session post-processed against a nearby CORS station
Put the antenna on the control point, or measure the offset precisely.
3. Control the variables
The review we looked at changed canopy, baseline, site, correction source and weather between sessions, so no single factor could be blamed for a difference. Avoid that:
| Keep the same | Vary on purpose |
|---|---|
| Site and antenna position | Open sky vs under canopy |
| Baseline and correction source | Dry vs wet |
| Receiver settings and firmware | (one factor at a time) |
Repeat every combination at least three times, ideally on different days, so you see the effect of satellite geometry.
4. Log raw data, not just screenshots
Log raw observations (RINEX, or the receiver's native format such as SBF) and the solution file. With raw data you can:
- Reprocess every receiver with the same engine and settings in RTKLIB (
rnx2rtkpor RTKPOST) - Check whether fix decisions were correct
- Reproduce plots later instead of relying on screenshots
Also log: antenna height, canopy photos, rain intensity, firmware version, elevation mask, ambiguity resolution mode, satellites used, PDOP and C/N0.
5. Plot with RTKPLOT
RTKPLOT (part of RTKLIB) is the quickest way to see what happened:
- Ground track view: fixed points (Q=1) should form a tight cluster or a clean path; float (Q=2) and DGPS (Q=4) show up as scattered jumps.
- Position view: E, N and U against time, which shows when the fix was lost and how big the jumps were.
- Nsat view: satellites used over time, which helps explain dropouts under canopy.
Its status bar gives the epoch count N and the Q=1/Q=2/Q=4 split. Note those numbers for each file; they are your fix ratio.
6. Use enough units
One unit per model cannot separate model performance from unit-to-unit variation. Use at least two units per model, and on dual-antenna receivers log heading at the same time, since heading is often what fails first under canopy.
7. Handle the statistics honestly
At 10 Hz, neighbouring epochs are strongly correlated, so 36,000 epochs is not 36,000 independent samples. Decimate to 1 Hz before computing statistics, state the effective sample size, and don't build confidence intervals from raw N.
8. Report what you can and cannot conclude
A good test report says clearly:
- What was controlled and what was not
- Which device produced which file (by serial number)
- Whether the conclusion is about status (fix ratio) or accuracy (error vs truth)
The field review behind this checklist
The review that prompted this checklist covers 36 screenshots from three sessions (half sky + rain, under trees + rain, and under trees with network RTK), with Septentrio mosaic-X5 and mosaic-H receivers as used in the Eview HB56 and HB56H. It includes the full fix-ratio tables, the figures and a retest design:
RTK Fix Ratio Under Tree Canopy and Rain: Field Review
Disclosure: I work with Eview GNSS. Test data was supplied by the test team and is used with permission. RTKLIB is open source: rtklib.com.
Top comments (0)