Multipath: The Invisible Enemy
A satellite signal is meant to travel one path: a straight line from the satellite to the antenna. Multipath is what happens when it does not, and it is one of the most common, least visible sources of GNSS error a field crew will ever encounter.
Multipath occurs when a satellite signal reaches the antenna by more than one path, typically a direct path and one or more reflected paths off nearby surfaces, and those paths interfere with each other before the receiver ever sees a clean signal.
A Reflection, Arriving Slightly Late
A signal that bounces off a wall, a vehicle, a metal roof, or even wet ground before reaching the antenna travels a longer path than the direct signal, and arrives slightly delayed. The receiver, trying to measure the arrival time of a single clean signal, instead measures a distorted blend of the direct signal and its delayed reflection. That distortion can bias both code and carrier-phase measurements, and it does so differently for every satellite, depending on the geometry between that satellite, the reflecting surface, and the antenna.
Multipath does not add random noise the way atmospheric delay does. It biases individual satellite measurements in ways specific to the local environment, which is why the same receiver in the same spot can behave differently depending on what happens to be standing nearby that day.
Where Multipath Comes From in the Field
Common sources include parked vehicles, chain-link fencing, metal buildings, water, and even the tripod or pole itself if a ground plane or choke ring is not being used. Urban canyons, job sites full of equipment, and areas near large reflective structures are classic multipath environments. None of these sources are exotic. Multipath is less an edge case than an ordinary condition of working near buildings, vehicles, and equipment, which describes a great deal of everyday fieldwork.
Multipath is not primarily a satellite-signal-strength problem, and boosting antenna gain does not fix it. It is a local reflection problem, caused by the antenna's immediate surroundings, and it is addressed by antenna design and site selection, not signal strength.
What Actually Reduces It
A survey-grade antenna with a good ground plane or choke ring design is built specifically to reject signals arriving from low angles and from below, where most reflected paths originate. Elevation masks, ignoring satellites below a certain angle above the horizon, help for the same reason, since low satellites are both weaker and more prone to reflecting off nearby surfaces before reaching the antenna. Simply moving a setup a few feet away from a reflective surface can meaningfully change the multipath environment, sometimes dramatically.
A rover reports FIX every time near a large metal storage building, but the points collected there consistently disagree with nearby control by several centimeters. Is the receiver malfunctioning? Probably not. That pattern, precise but inaccurate, near a strongly reflective surface, is a textbook multipath signature.
Multipath occurs when reflected signals blend with the direct satellite signal, biasing measurements in a way specific to the local environment rather than adding random noise. Antenna design, elevation masks, and simple site awareness are the practical defenses, since multipath is a local reflection problem, not a signal-strength problem.
Multipath is one environmental factor among several. The next chapter looks at a different one: the arrangement of satellites overhead, and why that arrangement alone can make or break a solution's quality.