Fixed RTK (Real-Time Kinematic) represents the highest-accuracy positioning mode achievable with carrier-phase differential GNSS techniques, delivering centimeter-level precision suitable for the most demanding surveying, construction, and autonomous navigation applications. The term ‘fixed’ refers to the successful resolution of carrier phase ambiguities to their correct integer values, a critical mathematical achievement that distinguishes centimeter-level positioning from meter-level float solutions.
The carrier phase measurement is extraordinarily precise, providing range information to millimeter levels, but it presents an inherent ambiguity problem. A receiver can measure the fractional phase of the incoming signal precisely, but cannot directly determine how many complete wavelengths (approximately 19 centimeters for L1 GPS signals) exist between the satellite and antenna. This unknown integer number of wavelengths, the carrier phase ambiguity, must be resolved through mathematical techniques using observations from multiple satellites over time.
Achieving a fixed RTK solution requires favorable conditions: adequate satellite visibility with good geometric distribution (low DOP), strong signal quality (high carrier-to-noise ratio), reliable correction data from a base station or network service, and reasonable baseline length. When these conditions are met, the receiver’s ambiguity resolution algorithm can confidently determine the correct integer values, typically within seconds to tens of seconds. The position solution then transitions from ‘float’ (ambiguities estimated as real numbers) to ‘fixed’ (ambiguities constrained to integers), dramatically improving accuracy.
Modern RTK systems indicate solution status through quality indicators, allowing users to verify when fixed solutions are achieved. Loss of fixed status, reverting to float, can occur due to signal obstructions, multipath interference, communication dropouts, or excessive baseline distances. For applications requiring centimeter accuracy, such as autonomous vehicle lane positioning, machine guidance, or cadastral surveying, confirming fixed RTK status before recording measurements or relying on position data is essential for quality assurance.
