How does multipath affect GNSS measurements, and what are typical mitigation strategies?

• A. It biases and degrades accuracy; mitigated with antenna design, choke rings, processing techniques, and multipath rejection algorithms.
• B. It enhances signal strength; mitigated by reducing bandwidth.
• C. It only affects attitude measurements; mitigated by calibration.
• D. It improves pseudorange accuracy; mitigated by averaging.

Answer: (A)

Multipath changes the path length that GNSS signals travel to the receiver by adding reflected copies of the same signal. This creates biases in the code measurements and adds noise to the measurements, which degrades position accuracy. Reflections can interfere constructively or destructively, so the bias can be systematic and persistent, especially for pseudorange measurements, while carrier-phase measurements can be less affected but still distorted.

Mitigation combines hardware and processing approaches. Hardware solutions include antenna designs with choke rings and proper ground planes that suppress signals coming from reflective directions and reduce low-elevation multipath; choosing a mounting location away from shiny surfaces like buildings, water, or metal structures also helps. On the processing side, receivers implement multipath rejection algorithms and robust estimation techniques to detect and downweight or exclude measurements affected by multipath, along with methods like code-carrier smoothing and satellite selection to minimize the impact. These strategies together reduce the bias and noise introduced by multipath and preserve GNSS accuracy.

Multipath does not improve signal strength or pseudorange accuracy; it is not limited to attitude measurements, and simple averaging does not resolve the biases it introduces.