Distinguish GNSS time from UTC and explain why time synchronization is critical in navigation.

• A. UTC is a continuous time scale used by receivers and satellites; GNSS time is civil time with leap seconds.
• B. GNSS time is a continuous time scale used by receivers and satellites; UTC is civil time with leap seconds; accurate conversion and synchronization are essential for proper positioning and sensor fusion.
• C. GNSS time equals local time; UTC is the time zone.
• D. GNSS time is always ahead of UTC by a constant offset.

Answer: (B)

GNSS time is a continuous, uniform time scale used by satellites and receivers that advances steadily without leap seconds. It starts from a defined epoch and ticks the same every second, independent of Earth's rotation. UTC, by contrast, is civil time used for everyday purposes and includes leap seconds to keep in sync with the irregularities of the planet’s rotation.

Because these two time scales drift relative to each other (especially when leap seconds are added to UTC), converting between GNSS time and UTC requires knowing the current offset. This accurate conversion matters a lot in navigation, where precise time stamps drive all range and clock-bias calculations. If clocks aren’t aligned to GNSS time, the measured ranges become biased, which in turn skews the computed position.

Time synchronization is also crucial when fusing GNSS data with other sensors (like inertial measurement units). A common time base ensures that measurements from different sources correspond to the same moment, enabling reliable sensor fusion and stable navigation estimates.