Differentiate GPS time, International Atomic Time (TAI), Coordinated Universal Time (UTC), and Terrestrial Time (TT).

• A. TAI is the continuous atomic time scale; GPS time is tied to TAI by a fixed offset and does not include leap seconds; UTC is civil time with leap seconds; TT is a dynamical time scale used for ephemerides; TT = TAI + 32.184 seconds.
• B. TAI is measured in leap seconds; GPS time equals UTC; TT is meaningless for ephemerides; TT = TAI - 32.184 seconds.
• C. TAI equals GPS time, UTC equals TT; TT = TAI + 10 seconds.
• D. GPS time is the same as UTC and includes leap seconds.

Answer: (A)

Different time scales are designed for different purposes, and they relate to each other with fixed offsets or with leap adjustments. TAI is the continuously running atomic time scale defined by SI seconds, without any leap seconds. It serves as a stable reference for precise timekeeping and for other time scales used in science.

GPS time is anchored to TAI with a fixed offset and does not incorporate leap seconds. This means GPS time ticks forward uniformly, independent of the leap-second adjustments that apply to civil time. As a result, GPS time slowly drifts relative to UTC because UTC periodically inserts leap seconds to stay aligned with Earth's rotation.

UTC is the civil time standard used worldwide and includes leap seconds to keep it close to UT1, which reflects the actual rotation of the Earth. Because leap seconds are added irregularly, UTC is not a continuous, fixed-scale like TAI.

TT, or Terrestrial Time, is the time scale used for precise ephemerides and is defined as a uniform progression that is tied to TAI by a fixed offset: TT = TAI + 32.184 seconds. This makes TT suitable for astronomical calculations, independent of leap seconds.

So the best-fit description says: TAI is the continuous atomic time scale; GPS time is tied to TAI by a fixed offset and does not include leap seconds; UTC is civil time with leap seconds; TT is a dynamical time scale used for ephemerides, with TT = TAI + 32.184 seconds.