How do GPS time and UTC relate, and what role do leap seconds play?

• A. GPS time runs continuously since 1980 with no leap seconds; UTC includes leap seconds; GPS time is offset from UTC by a fixed difference that changes when leap seconds occur.
• B. GPS time and UTC are identical; there is no offset.
• C. GPS time has leap seconds; UTC includes leap seconds.
• D. GPS time started in 1970 and is unrelated to UTC.

Answer: (A)

GPS time advances continuously starting from its 1980 epoch and does not include leap seconds. UTC, the civil time standard, regularly adds leap seconds to stay in sync with the Earth's rotation, so it can jump by one second at those moments. Because only UTC incorporates leap seconds, GPS time and UTC diverge by a fixed offset that equals the total number of leap seconds inserted since GPS time began. That offset remains constant between leap seconds and then increases by one whenever a leap second is added. In other words, GPS time is ahead of UTC by a changing amount: you subtract the current offset to get UTC from GPS time, and add the offset to UTC to get GPS time. The role of leap seconds is to modify UTC to keep it aligned with Earth's rotation, while GPS time keeps running without any such adjustments, creating the stepwise relationship between the two.