Ephemeris error is caused by which of the following?

• A. The GPS satellite data being corrupted.
• B. The INS experiencing drift.
• C. Satellite not being exactly where it was thought to be.
• D. Jamming causing the signal to fail.

Answer: (C)

Ephemeris error comes from the satellite not being exactly where its predicted orbital parameters say it should be. The satellites broadcast ephemeris data that describe their orbit and position as a function of time. If those parameters are imperfect or the actual orbit is perturbed beyond what the model accounts for, the calculated satellite position used in ranging is off. That mismatch directly translates into errors in the measured range to the satellite and, consequently, the user’s computed position.

Other options don’t fit because data corruption would imply broader data integrity issues or spoofing rather than a systematic mismatch in satellite position; INS drift concerns a separate inertial navigation system, not the GNSS satellite’s actual location; and jamming affects signal reception quality rather than the server’s or satellite’s true position.