List the main GNSS error sources that degrade positioning accuracy.

• A. Solar activity and weather conditions only.
• B. Multipath, ionospheric delay, tropospheric delay, satellite clock errors.
• C. Receiver noise and antenna phase center variations.
• D. Satellite clock errors, orbit (ephemeris) errors, ionospheric delay, tropospheric delay, multipath, receiver noise, and antenna phase center variations.

Answer: (D)

The main concept is that GNSS positioning errors come from a combination of satellite-related, atmospheric, and receiver-related factors that bias the measured range to the satellites. The biggest contributors are satellite clock errors, which occur when the satellite’s onboard clock isn’t perfectly synchronized; orbit (ephemeris) errors, which are inaccuracies in the reported satellite position; ionospheric delay, where the signal slows as it travels through the charged layer of the ionosphere; tropospheric delay, from the neutral atmosphere affecting the signal; multipath, where the signal reflects off surfaces before reaching the receiver and creates biased range measurements; receiver noise, the inherent measurement noise in the receiver electronics; and antenna phase center variations, where the effective point from which the signal is considered to originate shifts with angle and frequency, causing range biases.

This collection covers the full range of common, practical error sources that degrade positioning accuracy. Other options omit one or more of these essential elements—for example, focusing only on environmental factors like solar activity or weather, or listing only atmospheric delays or only receiver-related issues—so they don’t capture all the major contributors.