Short answer
Distortion of a GNSS signal caused by reflections off nearby surfaces (buildings, ground, vehicles) arriving at the antenna nanoseconds after the direct path. Adds pseudorange noise of several metres in unmitigated urban environments — the dominant error source for many high-precision applications.
Interactive: direct + reflected interference
Phase delay at L1
947°
Summed amplitude vs direct
0.74× (destructive)
Direct (—) + reflected (—) = summed (—)
Drag the slider to change the extra path length of the reflected ray. At L1 (1575.42 MHz, wavelength 19 cm) every 19 cm of extra path adds a full 360° of phase delay — so the summed amplitude oscillates between constructive (up to 1.7× direct) and destructive (down to 0.3× direct) every half wavelength. This is the multipath fade that an RTK receiver sees as range noise.
Detailed explanation
Multipath occurs when a GNSS signal reaches the antenna both directly from the satellite and via one or more reflections off nearby surfaces. The reflected path is slightly longer, so it arrives a few nanoseconds (and therefore a few metres of equivalent range) later than the direct signal. The receiver sees a sum of the two arriving with a small phase shift, which biases the pseudorange measurement and adds noise to carrier-phase tracking.
In a clean rooftop environment multipath is a few cm of range noise — manageable. In urban canyons it can swing pseudorange by 10+ metres on individual satellites, knocking precision RTK and PPP out of fix. Mitigation strategies live at three layers: antenna (choke-ring design, RHCP rejection of LHCP reflections, ground planes), receiver (narrow correlators, multipath estimators, signal authentication) and processing (Kalman filter weighting by multipath indicators).
Antenna-side mitigation is where GNSource designs spend the most engineering effort. A 3D choke-ring antenna (the TDXL-CA341) uses concentric corrugated grooves λ/4-deep to absorb low-elevation reflections from below. Patch antennas with extended ground planes do a lighter version of the same. Compact helical / patch antennas have less multipath rejection — acceptable for moving platforms where geometry changes every epoch but limiting for static reference work.
Multipath is the dominant error budget item for static geodetic work in built environments. A choke-ring antenna with a stable phase centre (±1 mm or better) is what separates a reliable CORS station from a survey-grade single-frequency rover that occasionally drifts on long observations.
Where you'll see this
High-Precision GNSS Measurement
Browse product lineRelated terms
Phase center
The apparent electrical centre of an antenna — the point from which signal range is effectively measured by a GNSS receiver. Phase-centre stability (the variation in this point as the signal arrival angle changes) is the single most important parameter for survey-grade and geodetic antennas.
Choke ring antenna
A geodetic GNSS antenna with concentric corrugated grooves around the radiating element that absorb low-elevation signal reflections. Provides the best multipath rejection and most stable phase centre of any commercial GNSS antenna, at the cost of bulk and weight (typical 380 mm diameter, 5–10 kg).
Polarization (RHCP / LHCP)
The rotation direction of the electric-field vector in a GNSS signal. All GNSS satellites broadcast right-hand circularly polarised (RHCP) signals. Ground reflections flip handedness to LHCP, so an antenna that rejects LHCP automatically rejects single-bounce multipath.
RTK
A differential GNSS technique using carrier-phase measurements from a known base station to give a moving rover centimetre-level (typically 1–3 cm) horizontal accuracy in real time. The dominant high-precision GNSS technique for cadastral surveying, machine control, and precision agriculture.