Plain English Meaning

The GPS signal slows down a little as it passes through the lower atmosphere, which throws off the receiver's distance calculation by a small amount. The receiver corrects for most of it, but not all.

Full Definition

A GPS positioning error caused by the slowing of satellite radio signals as they pass through the troposphere, the lowest layer of the atmosphere. Water vapor, temperature, and pressure in this layer bend and slow the signal, making the satellite appear slightly farther away than it actually is. The receiver compensates with a built-in atmospheric model, but residual error remains and contributes to overall GPS position inaccuracy.

Context Anchor

Seen in instrument flying discussions of GPS accuracy, especially when explaining the sources of GPS position error.