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.
Plain English
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.
Context Anchor
Seen in instrument flying discussions of GPS accuracy, especially when explaining the sources of GPS position error.
Derivation
Troposphere comes from the Greek tropos, meaning 'turn' or 'change,' referring to the layer where weather and atmospheric mixing occur. The delays happen specifically in this lowest, weather-active layer.
Why Pilots Care
Uncorrected tropospheric delays introduce position errors of several meters that can affect the accuracy of RNAV approaches and enroute navigation.
Analogy
It is like a person walking through shallow water instead of across dry pavement. The path may be the same, but the person takes a little longer to get there; GPS signals can also take a little longer when passing through the lower atmosphere.
Grounding Statement
A GPS signal from a satellite reaches the airplane after passing through miles of lower atmosphere, and that air can slow the signal just enough to matter in position calculations.
Intuition Check
Do not think of tropospheric delays as the GPS receiver being slow. The delay happens because the satellite signal itself is slowed slightly as it passes through the lower atmosphere.
Example Sentence 1
Tropospheric delays are one of the GPS error sources the receiver attempts to model and correct internally.
Example Sentence 2
During high humidity conditions the pilot cross-checked GPS position against DME because tropospheric delays were expected to increase.