What is the tropospheric delay, and how does elevation affect it?

• A. A dispersive delay caused by free electrons in the ionosphere; mitigated by dual-frequency measurements.
• B. A delay due to satellite clock errors, mitigated by precise clock products.
• C. A non-dispersive atmospheric delay caused by water vapor and air; more pronounced at low elevation angles.
• D. A delay caused by multipath reflections, mitigated by antenna shielding.

Answer: (C)

Tropospheric delay is the portion of the signal’s travel-time delay caused by the Earth’s lower atmosphere, mainly the air and water vapor. It is non-dispersive, meaning the delay is essentially the same across microwave frequencies, unlike ionospheric delay which depends on frequency. The atmosphere can be thought of as adding a small extra path length to the signal, with the wet (water vapor) part being more variable than the dry part.

Elevation affects it because the signal’s path through the troposphere is longer when the satellite is lower in the sky. As elevation decreases, the slant path through the atmosphere increases, so the delay grows. In practice, this relationship is described by a mapping function that scales the zenith delay by a factor roughly like the secant of the zenith angle, making the delay much larger at low elevations and smaller at high elevations.

So, the delay is a non-dispersive atmospheric effect due to air and water vapor, and it becomes more pronounced as elevation angles drop.