>[!abstract] >Multipath interference happens when a radio signal takes multiple paths to reach a receiver from the transmitter. These paths may include: >- **Direct** [[Line of sight|line-of-sight]] (LOS) path. >- **Reflected** paths (off buildings, terrain, water). >- **Diffracted** paths (around obstacles). >- **Scattered** paths (through trees, foliage, rough surfaces). > >Because radio signals are waves, when they arrive at the receiver from multiple paths, they can interfere with each other. Depending on how the waves line up, they can interfere **constructively** (resulting in a stronger signal) or **destructively** (resulting in a weaker or even canceled signal). ## Applications to UAV operations UAVs are mobile, fly at low altitudes, and often operate in noisy RF environments, which makes them particularly vulnerable to multipath effects, such as: - Urban interference: buildings reflect signals, creating many reflected paths to the receiver and causing erratic signal even in excellent LOS conditions. - **Low altitude**: flying in close proximity to the ground or over water also causes reflected paths to bounce off the surface. Separately from transmitter-receiver signal, the GPS/GNSS signal from satellites may also reflect off surfaces. The signal delay may cause positional errors and unwanted drift while hovering near buildings or cliffs. ## Related - [[Fresnel zone]] - [[Line of sight]]