Question 7 of 9: Multipath Error and Its Mitigation

Question 7: Multipath Error and Its Mitigation (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

What multipath is. Multipath is the error caused when a satellite signal reaches the antenna not only along the direct line of sight but also via one or more reflected paths — from the ground, water, buildings, vehicles or other surfaces near the antenna. The reflected replicas arrive slightly delayed and phase-shifted and combine with the direct signal, distorting the correlation peak the receiver tracks and biasing the measured range.

Multipath affects the code (pseudorange) far more than the carrier phase: code multipath can reach several metres (tens of metres in severe reflective environments), whereas carrier-phase multipath is limited to a few centimetres (at most a quarter-wavelength, ≈ 5 cm on L1). Crucially, multipath is site- and antenna-specific: it is uncorrelated between separate stations, so — unlike the atmosphere or clocks — it does not cancel in differential or double-differenced positioning. It is often the limiting error source in high-precision static work.

Mitigation measures.

• Site selection: occupy points away from reflective surfaces (buildings, fences, water, vehicles); this is the single most effective step.
• Elevation mask: reject low-elevation satellites, whose signals graze reflective ground and are the worst multipath offenders.
• Antenna design: use a choke-ring or ground-plane antenna that suppresses signals arriving from below the horizon, and exploit the fact that a single ground reflection reverses the signal’s right-hand circular polarization (RHCP → LHCP), which a well-designed antenna rejects.
• Receiver signal processing: advanced correlator designs (narrow correlator, strobe/edge correlators, multipath-estimating techniques such as MEDLL) discriminate the direct from the reflected correlation peak.
• Longer observations / averaging: multipath is quasi-periodic because it depends on satellite–reflector geometry, so it partly averages out over a long session or repeats day-to-day (sidereal filtering).
• Modern signals: wideband and BOC-modulated signals (L5, L1C, Galileo) have sharper correlation peaks that are inherently more multipath-resistant.

No single measure eliminates multipath entirely; good site selection combined with a choke-ring antenna, an appropriate mask angle, and a modern multi-frequency receiver reduces it to the level acceptable for precise surveying.