GPS multipath explained

Direct and reflected signals

A reflection makes the radio journey longer.

GNSS ranging depends on the apparent travel time from satellite to receiver. The cleanest measurement comes from a direct line of sight. Near a reflecting surface, another copy of the signal can bounce before reaching the antenna. That copy arrives later because it has travelled farther.

The direct and reflected copies can interfere at the antenna and distort the receiver's code or carrier measurement. In a more severe non-line-of-sight case, the direct signal is blocked and the receiver tracks only a reflected path. The resulting range appears too long, which can displace the calculated position.

The error changes as the satellite, receiver and reflecting surface move relative to one another. Multipath is therefore not a fixed offset that an app can subtract once.

Outdoor reflectors

Street canyons and rock walls create similar radio geometry.

Tall buildings are the familiar example because their broad faces both block and reflect satellite signals. Outdoor routes can reproduce the same problem beside cliffs, quarry walls, steep-sided gullies, large metal structures or tightly packed vehicles. Water and other surfaces can contribute reflections under suitable geometry.

Low-elevation satellite signals are especially exposed because their direct path travels close to surrounding surfaces. Receiver antenna design and signal processing influence how well those unwanted arrivals are rejected. Two devices side by side can therefore respond differently.

Canopy is more often discussed as attenuation and scattering than as one clean mirror. A poor woodland position should not automatically be labelled multipath. Blockage, weak signals and reduced satellite geometry may be involved at the same time.

Symptoms, not proof

The line can wander while the person does not.

• Stationary cloudSuccessive fixes spread around a stopped receiver and add false distance.
• Parallel offsetA track follows the broad direction of travel but sits away from the mapped path.
• Sudden excursionOne or more points jump towards open space, through a wall or across terrain.
• Recovery at an openingThe position improves after leaving the reflecting or obstructed corridor.

Each pattern has other possible causes. A bad map, stale fix, poor geometry, radio interference or aggressive filtering can create similar evidence. A route screenshot alone cannot prove that a particular satellite signal reflected from a particular surface.

Practical limits

Improve the direct view before trying to repair the line.

When the current position matters, move away from a large wall or cliff if the route and conditions allow it. Give the receiver a broader sky view and time to acquire a fresh set of measurements. Keep the device reasonably exposed rather than deep inside dense kit.

During recording, preserve the original even if a later tool removes obvious spikes. Cleaning can improve presentation, but it cannot recreate the exact path through a period of corrupted measurements. Strong smoothing may also hide the symptom while shifting genuine corners.

Compare the track with timestamps, reported accuracy, pauses and known landmarks. A repeatable deterioration beside the same kind of reflector strengthens the diagnosis. If the problem follows one device everywhere in open sky, investigate permissions, power settings, antenna obstruction and hardware before blaming the landscape.