GNSS Multipath Interference: What It Is and How to Fix It

Applies to: All GNSS receivers used in RTK survey, machine control, and layout applications

What Is GNSS Multipath?

Multipath occurs when GNSS satellite signals bounce off nearby surfaces — buildings, metal structures, vehicles, rock faces, or water — before reaching the receiver antenna. The reflected signal arrives slightly delayed compared to the direct signal. The receiver, designed to track one signal per satellite, receives both and produces a blended, corrupted signal. This results in position noise, cyclic position errors, difficulty achieving RTK fixed, and positions that may be systematically off by 5–30cm even with RTK fixed status showing.

Signs of Multipath Interference

• Position noise — repeated observations of the same point show large scatter (>5cm) with RTK fixed status
• RTK float that cannot resolve to fixed despite good satellite count and correction link
• Cyclic position error — position drifts in a repeating pattern as satellites move across the sky
• Elevated RMS or DOP values despite adequate satellite geometry
• Position suddenly improves when moving a few meters away from a structure

Common Sources of Multipath

• Buildings and walls — strongest single-structure multipath source
• Metal roofs, tanks, and equipment — highly reflective to L-band GNSS frequencies
• Vehicles — reflections off steel machine bodies, especially near the base station
• Water surfaces — open water produces strong multipath when satellites are low on the horizon
• Rock cuts and earth berms — can focus reflections from multiple surfaces

How to Reduce GNSS Multipath

1. Move the antenna away from reflective surfaces — even 1–2m of clearance significantly reduces multipath
2. Increase antenna height — raising the antenna above surrounding objects reduces reflection angles
3. Use a ground plane or choke ring antenna — these antenna types attenuate signals arriving from low elevation angles, which is where multipath signals predominantly arrive
4. Apply an elevation mask — set the receiver to ignore satellites below 10–15° elevation to exclude low-angle multipath-prone signals
5. Wait for satellite geometry to improve — multipath intensity varies with satellite positions throughout the day
6. For base stations: set up on high ground, away from vehicles and metal structures; avoid setting up near buildings
7. Use multi-constellation GNSS (GPS + GLONASS + Galileo + BeiDou) — more satellites means more redundancy to dilute multipath effects

Multipath vs. Poor GNSS Signal — How to Tell

Poor GNSS signal (few satellites, weak signal strength) typically shows as low satellite count, high PDOP, and inability to initialize RTK. Multipath often shows as apparently good satellite count and geometry, RTK fixed status, but inconsistent or noisy positions when checking the same point multiple times. If repeatability is poor despite RTK fixed, multipath is the likely cause.

Related Topics

GNSS Integer Ambiguity Errors | Leica E01 RTK Float | GNSS RTK Float Guide