How many atomic oscillators does each Block II GPS satellite contain?

How many atomic oscillators does each Block II GPS satellite contain?

• A. Two
• B. Three
• C. Four
• D. Five

Answer: (C)

The main idea is redundancy in the spacecraft’s time reference. Block II GPS satellites carry four atomic oscillators on board so two can act as the primary time standards and the other two serve as backups. This setup keeps GPS time continuous even if one oscillator drifts or fails, which is crucial because the on-board clock drives the timing for the PRN code and navigation data used to compute positions. Having four oscillators provides solid fault tolerance—enough to maintain accuracy without adding unnecessary complexity or mass—unlike having only two (higher risk of outage) or five (extra hardware with diminishing returns).

The main idea is redundancy in the spacecraft’s time reference. Block II GPS satellites carry four atomic oscillators on board so two can act as the primary time standards and the other two serve as backups. This setup keeps GPS time continuous even if one oscillator drifts or fails, which is crucial because the on-board clock drives the timing for the PRN code and navigation data used to compute positions. Having four oscillators provides solid fault tolerance—enough to maintain accuracy without adding unnecessary complexity or mass—unlike having only two (higher risk of outage) or five (extra hardware with diminishing returns).