Which statement correctly describes GPS time relative to UTC?

• A. GPS time includes leap seconds.
• B. GPS time is identical to UTC.
• C. GPS time is tied to TAI by a fixed offset and does not include leap seconds.
• D. GPS time is an arbitrary civilian time scale.

Answer: (C)

GPS time uses a continuous atomic time scale that is aligned with TAI through a fixed offset and does not include leap seconds. This means it runs in perfectly uniform SI seconds, just like TAI, without any occasional one-second adjustments. When GPS was defined in 1980, a fixed difference of 19 seconds was established between GPS time and TAI, so GPS time is always TAI time minus that constant 19 seconds.

UTC, on the other hand, adds leap seconds from time to time to stay in sync with Earth's irregular rotation. Those leap seconds are inserted into UTC but are not reflected in GPS time at all. As leap seconds accumulate, the difference between GPS time and UTC changes, because UTC is periodically stepped while GPS time continues uninterrupted.

So GPS time is tied to TAI by a fixed offset and does not include leap seconds, which is why this statement is the best description.