Short answer
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.
Interactive: circular polarisation
Right-hand circular polarisation: viewed from the source toward the receiver, the electric field rotates clockwise. Every civilian GNSS signal is broadcast RHCP — the universal convention that lets a single antenna design pick up GPS, GLONASS, Galileo, BeiDou, QZSS, NavIC, and SBAS without polarisation matching loss.
Reflection flips the handedness
When an RHCP wave reflects off a smooth surface — a wet road, a glass facade, a vehicle roof — the handedness inverts and the reflected ray comes back as LHCP. A well-designed GNSS antenna with high RHCP polarisation purity rejects that reflection by 10–20 dB, suppressing the strongest multipath component for free.
Detailed explanation
Circular polarisation means the electric field vector traces a circle as the wave propagates. Right-hand (RHCP) means it rotates clockwise looking from the antenna toward the source; left-hand (LHCP) rotates counter-clockwise. All civilian GNSS signals are broadcast RHCP from the satellite — this is a deliberate choice to avoid the Faraday rotation effect that would otherwise scramble linear polarisations as signals pass through the ionosphere.
When an RHCP signal reflects off a smooth surface (a building wall, a wet road, a metal vehicle roof), the handedness inverts — the reflected signal is LHCP after one bounce. This is a free multipath-rejection tool: an antenna designed to receive only RHCP and reject LHCP automatically suppresses the strongest reflected paths by 10–20 dB.
Antenna polarisation purity is quantified by the axial ratio (in dB) — the ratio of the major to minor axis of the polarisation ellipse the antenna actually radiates / receives. A perfect RHCP antenna has 0 dB axial ratio everywhere; real antennas have <3 dB at boresight (zenith) and 3–6 dB at low elevations. Choke-ring antennas hold AR <3 dB across the full upper hemisphere, which is part of why they're the standard for high-precision work.
Two-way satellite systems (BDS RDSS, satellite telephony) sometimes use LHCP transmit + RHCP receive (or vice versa) to isolate the two paths in the same antenna. Combo antennas for these services are dual-polarised by design.
Related terms
Multipath
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.
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).
RDSS
BeiDou's unique two-way satellite messaging service. Users transmit a short message + position request on L-band uplink (1610–1626.5 MHz), and receive a response + correction on S-band downlink (2483.5–2500 MHz). The only GNSS with built-in global SMS-grade communication.