A GPS positioning error caused when satellite signals reflect off nearby surfaces (glass, metal, water) before reaching the receiver. The reflected signal arrives later than the direct signal, causing the RTK algorithm to miscalculate the mower's position by centimeters to meters.
How RTK Positioning Works (And Where It Breaks)
Real-Time Kinematic (RTK) positioning achieves centimeter-level accuracy by comparing satellite signals received at two points: a fixed base station at a known coordinate, and the moving rover (your mower). The base station calculates the error in the satellite signal and transmits correction data to the mower in real time.
This system assumes both receivers see the same satellite signals traveling the same direct path. When a signal bounces off a reflective surface before reaching the mower, it travels a longer distance. The RTK algorithm cannot distinguish the reflected signal from the direct one, and the correction data becomes invalid for that satellite.
The positioning accuracy degrades from the expected ±1cm to anywhere between ±10cm and ±2 meters — enough to send the mower across a boundary line, into a flower bed, or into a "stuck" loop.
The Worst Offenders: Surfaces That Cause Multipath
| Surface Type | Reflection Severity | Typical Error Range | Affected Zone |
|---|---|---|---|
| Large glass windows / sliding doors | High | ±30–200cm | 0–3m from surface |
| Aluminum or metal siding | High | ±20–150cm | 0–4m from surface |
| Stucco / painted concrete | Medium | ±10–50cm | 0–2m from surface |
| Parked vehicles (metal/glass) | Medium–High | ±15–100cm | 0–3m radius |
| Swimming pools / water features | Medium | ±10–60cm | 0–2m from edge |
| Wood fencing | Low | ±2–10cm | Minimal impact |
Diagnosing Multipath Problems on Your Property
Before attempting fixes, confirm that multipath is actually the issue. The symptoms are distinct from other positioning failures:
- Location-specific drift: The mower operates perfectly in the center of the yard but consistently drifts or crosses boundaries within 2–3 meters of the house, garage, or fence.
- Time-of-day variation: The problem worsens at certain hours because satellite geometry changes throughout the day. A satellite that is directly behind a reflective surface at 2 PM may be in clear view at 10 AM.
- "RTK Float" status: Check the mower app for positioning mode. If it drops from "RTK Fix" (centimeter accuracy) to "RTK Float" (decimeter accuracy) near structures, multipath is the likely cause.
- Satellite count drops: If the satellite count decreases by 3 or more near a building, the RTK algorithm is rejecting corrupted signals — a direct indicator of multipath.
Five Proven Solutions for Multipath Interference
1. Relocate and Elevate the Base Station
Mount the RTK base station as high as possible — ideally on the roof peak or a pole mount at 3+ meters. The higher the antenna, the weaker the reflected signals become relative to the direct signals. Avoid placing the base station near metal chimneys, satellite dishes, or HVAC units.
2. Use a Ground Plane Antenna
A ground plane is a flat metal disc mounted beneath the GPS antenna. It blocks reflected signals coming from below the antenna's horizon. Many professional-grade RTK systems include ground planes, but aftermarket options are available for consumer mower base stations. A 10cm aluminum disc can reduce multipath by 40–60%.
3. Set a Boundary Offset Near Reflective Surfaces
If multipath cannot be eliminated, work around it. Set your mowing boundary 50–100cm away from the house, glass walls, or metal structures. This keeps the mower in the "clean signal" zone. Use a manual string trimmer for the narrow strip between the boundary and the structure.
4. Schedule Mowing for Optimal Satellite Geometry
Satellite positions change throughout the day. Use a GNSS planning tool (many are free online) to identify the hours with the best Position Dilution of Precision (PDOP) for your location. A lower PDOP means satellites are well-distributed across the sky, reducing the impact of any single reflected signal. Morning hours (8–11 AM) often provide the best geometry in North America.
5. Upgrade to Tri-Fusion Navigation
The most robust long-term solution is a mower with multi-sensor navigation. Models like the Mammotion LUBA 3 AWD combine RTK, LiDAR, and Vision AI. When satellite signals degrade near a building, the mower seamlessly switches to LiDAR-based positioning, maintaining centimeter accuracy without any GPS dependency. This eliminates multipath as a failure mode entirely.
Real-World Case Study: The "Glass Wall" Problem
A common scenario reported on Reddit involves homes with large floor-to-ceiling windows or glass sunrooms. The mower operates correctly across most of the yard but consistently crosses the boundary by 30–50cm when mowing parallel to the glass wall. The owner verified "RTK Float" status in the app during these passes.
The solution involved two changes: (1) elevating the base station from a fence post (1.5m) to a roof-mounted pole (3.5m), and (2) setting the boundary 80cm away from the glass wall. The combined effect eliminated boundary crossings entirely, though the grass strip near the glass still requires manual trimming.
Frequently Asked Questions
Multipath interference occurs when GPS satellite signals bounce off reflective surfaces — glass windows, metal siding, stucco walls, or cars — before reaching the mower's antenna. The reflected signal travels a longer path than the direct signal, causing the mower to miscalculate its position by 10–50cm or more.
Houses create "urban canyon" effects on a small scale. Glass windows, aluminum siding, and even painted stucco reflect GPS signals. The mower receives both the direct satellite signal and the reflected copy, confusing the RTK correction algorithm. This is most severe within 2–3 meters of the structure.
Yes, in many cases. Moving the base station away from reflective surfaces and elevating it above the roofline reduces the base station's own multipath errors. However, the mower will still experience interference when it physically drives near reflective surfaces.
Mowers with Tri-Fusion navigation (RTK + LiDAR + Vision AI) like the Mammotion LUBA 3 handle multipath best because they can fall back to non-satellite navigation near buildings. Husqvarna EPOS and Kress RTK networks also include multipath mitigation algorithms.
Yes. If you mapped your lawn boundary while standing near a reflective surface, the boundary coordinates themselves may contain multipath errors. Re-mapping the boundary on a clear day, walking slowly, and pausing at corners can improve accuracy.