Electromagnetic wave propagation: diagnosing reflected pseudo-range errors, signal bounce, and antenna ground planes in urban residential yards.
The Physics of Multipath Radio Propagation
Satellite signals travel over 20,000 kilometers from space at the speed of light. In a perfect world, signals travel in a direct, straight line to the antenna. In real-world residential yards, microwave satellite signals bounce off dense physical surfaces: standing-seam metal roofs, brick masonry walls, aluminum siding, and solar panels. These bounced, indirect signals arrive at the antenna nanoseconds later than the direct wave—a phenomenon known as multipath interference.
How Multipath Corrupts Centimeter Positioning
Because GPS measures distance by calculating signal time-of-flight, a signal that bounced off a brick chimney travels a longer physical distance. This delay introduces a pseudo-range error, deceiving the mower internal Kalman filter into calculating its position 2 to 3 meters away from reality. The mower abruptly stops, spins in circles, or triggers boundary violation safety halts.
Mitigation Strategies: Choke Ring Antennas and Metallic Ground Planes
Professional surveying equipment combats multipath using metallic ground plane discs or stepped choke ring antennas placed beneath the antenna element. A simple 6-inch diameter circular aluminum ground plane disc mounted beneath your RTK antenna blocks reflected upward-bouncing ground signals while allowing direct downward-arriving satellite waves to penetrate.
Technical Specifications & Engineering Matrix
| Reflective Surface | RF Signal Reflection Coefficient | Multipath Interference Severity | Safe Distance Buffer |
|---|---|---|---|
| Standing-Seam Metal Roof | High (Reflects > 90% of RF Waves) | Severe (Acts as Massive Mirror) | Mount Mast at Least 4 Ft Above Roof |
| Solid Brick / Stone Masonry Wall | Moderate to High | Moderate (Ghost Signals in Alleys) | Maintain 6 Ft Clearance |
| Large Glass Patio Sliders | Moderate (Reflective Surface) | Moderate | Maintain 5 Ft Clearance |
| Wooden Cedar Fence | Low (Wood Absorbs RF Energy) | Minimal | Safe for Direct Mounting |
💡 Expert Field Recommendation
Mount a 6-inch circular aluminum plate or metal pie tin directly beneath the RTK antenna base. The metal plate acts as an RF shadow shield, blocking reflected signals bouncing up from metal roofs or moist ground soil.
Diagnostic Troubleshooting Checklist
- Mower Performs Erratic Donuts Next to Garage Wall: The garage aluminum siding is creating severe multipath reflections; map an excluded corridor buffer along the wall or enable visual VSLAM positioning.
- RTK Fix Drops Only on Sunny Dry Afternoons: Dry asphalt driveways reflect radio waves differently than wet ground; relocate the base station to an elevated rooftop position above reflection planes.
- Satellite Count Drops from 28 to 9 Satellites Suddenly: A metal vehicle or delivery truck parked near the base station is blocking and reflecting satellite sightlines.
Frequently Asked Questions
What is the difference between “RTK Fix” and “RTK Float”?
“RTK Fix” means integer carrier-phase ambiguity is solved, delivering 1-2cm precision; “RTK Float” means signals are noisy or ambiguous, dropping precision to 20-50cm (unsafe for close edge mowing).
Can trees cause multipath interference?
Dense wet leaves both absorb and scatter microwave signals, causing fluctuating phase errors during spring and summer leaf growth.
Article researched, lab-tested, and verified by Brett Sorenson, dedicated technical specialist and editorial contributor.
