Explain the term 'cycle slip' in carrier-phase GNSS signals.

• A. A sudden discontinuity in carrier-phase measurements caused by loss of lock or heavy multipath; interrupts ambiguity resolution.
• B. A gradual drift in satellite clock biases.
• C. A change in the receiver's position.
• D. A temporary loss of GNSS signal that does not affect phase.

Answer: (A)

Cycle slip is a sudden discontinuity in the carrier-phase measurements, where the observed phase jumps by an integer number of carrier wavelengths and the tracking of the phase becomes broken. This usually happens when the receiver loses lock or experiences strong multipath or scintillation, causing the phase to reset or wrap unexpectedly. Because carrier-phase processing relies on a continuous phase quantity and fixed integer ambiguities, a slip disrupts the ambiguity resolution and can degrade or invalidate precise positioning until the slip is detected and the ambiguities are re-estimated. Other scenarios like a gradual drift in satellite clock biases, a change in the receiver’s position, or a temporary loss of signal that doesn’t affect phase do not produce this abrupt phase jump.