What is multipath, and how does it affect pseudorange and carrier-phase measurements?

• A. Multipath artifacts do not affect GNSS measurements.
• B. Multipath occurs when signals reflect before reaching the receiver; it biases pseudorange and carrier-phase measurements, degrading accuracy; mitigated by antenna design, shielding, and signal processing.
• C. Multipath only affects Doppler measurements.
• D. Multipath improves accuracy by providing extra paths.

Answer: (B)

Multipath happens when a GNSS signal takes more than one path to reach the receiver—usually because it reflects off nearby surfaces like buildings, water, or ground before arriving. The receiver then sees a mixture of the direct signal and reflected copies, and those multiple paths interfere with each other.

For the pseudorange measurement, which is based on the time it takes the code to travel from satellite to receiver, this extra reflected path makes the measured travel time appear longer than it actually is. The receiver interprets that longer time as a longer distance, so the pseudorange value is biased. This bias degrades position accuracy, especially in environments with many reflective surfaces.

For the carrier-phase measurement, the situation is similar but more subtle. The receiver tracks the phase of the incoming carrier, and the reflected signal changes the phase seen by the tracking loop. The result is a biased or unstable phase measurement and possible changes in the inferred integer ambiguities. Because carrier-phase is much more precise than code, multipath can cause noticeable, highly correlated errors and can lead to cycle slips if severe.

Mitigation focuses on reducing the amount of multipath reaching the receiver and filtering its effects. Using antennas designed to minimize multipath reception (such as choke rings and proper ground planes), shielding the antenna from nearby reflectors, and applying an elevation mask to ignore low-angle signals help. On the processing side, advanced correlation and tracking techniques, multipath-resistant code designs, and careful calibration further lessen the impact.