When do satellite clock errors occur?

• A. The receiver loses lock
• B. The satellite transmits too fast
• C. The time on the satellite clock does not match the master GPS time
• D. The aircraft flies outside coverage

Answer: (C)

Satellite clock errors show up when the onboard satellite clock time does not match master GPS time. Each GPS satellite carries an extremely stable atomic clock that is tied to GPS time, and the navigation message includes a clock bias correction that tells the receiver how far off the satellite’s time is. If that satellite time is off, the pseudorange measurement becomes biased by that offset, which is why this concept is central to understanding why clock errors occur. Ground control continuously updates and uploads clock corrections to keep satellites aligned with GPS time. Losing lock or flying out of coverage doesn’t describe a clock error, and a notion of the satellite transmitting too fast would be describing a rate mismatch rather than a simple offset.