Error sources for GPS usage are a. Satellite clock b. Ionosphere c. Receiver clock d. Satellite orbit e. Lower atmosphere f. Multipath. Which of the following is NOT listed as an error source?

• A. Satellite clock
• B. Ionosphere
• C. Receiver clock
• D. Atmospheric refraction

Answer: (D)

GPS error sources come from biases in the satellite and receiver clocks, errors in the satellite’s orbit (ephemeris), and propagation effects as the signal travels through the atmosphere, plus additional errors from multipath reflections. Among the items listed, the term atmospheric refraction isn’t treated as a separate error category in standard GPS error budgets. The lower atmosphere’s impact is usually referred to as tropospheric delay, which accounts for the signal path length increase caused by the air. So atmospheric refraction isn’t listed as its own error source in this set, making it the one that doesn’t fit with the others. The remaining items—satellite clock, ionosphere, receiver clock, satellite orbit, lower atmosphere (troposphere), and multipath—are all recognized sources of GPS errors.