Definition
Float Solution GNSS refers to a Real-Time Kinematic (RTK) or post-processed GNSS positioning mode in which the receiver has resolved the carrier phase ambiguities to non-integer values. Unlike a fixed solution, which constrains ambiguities to whole numbers, a float solution maintains ambiguities as decimal values, resulting in decimeter to sub-decimeter level accuracy rather than centimeter-level precision.
The term "float" indicates that the ambiguity parameters are allowed to "float" or vary continuously rather than being locked to discrete integer values. This represents an intermediate positioning confidence between code-based solutions (meter-level accuracy) and fixed solutions (few centimeter accuracy).
Technical Characteristics
Ambiguity Resolution Process
Float solutions occur during the initialization phase of RTK surveying before the receiver achieves integer ambiguity resolution. The GNSS processor performs a least-squares adjustment of the carrier phase double-difference observations while treating ambiguities as real-valued parameters rather than integers. This computational approach requires fewer observations and less convergence time compared to achieving a fixed solution.
The mathematical relationship between rover and base station observations creates double-difference combinations that eliminate atmospheric biases and clock errors, but without integer constraints, the solution remains probabilistic rather than deterministic.
Accuracy Expectations
Float solutions typically provide accuracy ranging from 10 to 50 centimeters horizontally, depending on:
The uncertainty in float solutions is substantially larger than fixed solutions but significantly better than standard positioning without carrier phase information.
Float vs. Fixed Solutions
The distinction between float and fixed solutions is fundamental in RTK surveying. A fixed solution occurs when the integer ambiguity search engine successfully resolves ambiguities to whole numbers with sufficient confidence, typically indicated by an ambiguity ratio test exceeding a threshold (commonly 2.0 to 3.0). Fixed solutions achieve few-centimeter accuracy suitable for cadastral surveying and precise stakeout operations.
Float solutions, conversely, indicate that either:
Applications in Surveying Practice
Long Baseline Operations
When the distance between base station and rover exceeds 10-20 kilometers, ionospheric effects become increasingly difficult to model, and integer ambiguity resolution becomes unreliable. Float solutions provide a practical compromise, delivering decimeter-level positioning sufficient for reconnaissance surveys, preliminary boundary investigations, and large-scale topographic mapping.
Rapid Reconnaissance Surveys
In preliminary field investigations where speed is prioritized over highest precision, float solutions allow surveyors to quickly establish approximate positions and evaluate site conditions. Once survey requirements are confirmed, subsequent observations can target fixed solutions at critical control points.
Network RTK Operations
Network RTK systems using regional base station networks sometimes deliver float solutions when atmospheric conditions are poor or when rover positions are located at network peripheries with geometry constraints. These solutions remain valuable for many surveying applications while waiting for fixed solution convergence.
Kinematic Surveying
When conducting continuous kinematic surveys of features such as road centerlines, utility corridors, or shorelines at moderate speeds, float solutions provide continuous positioning with acceptable accuracy when fixed solutions are intermittently lost due to signal obstruction.
Related GNSS Positioning Modes
Float solutions exist within a spectrum of GNSS positioning methods. Standard code-based positioning provides meter-level accuracy using pseudorange observations alone. Floating-point carrier phase processing represents an intermediate step toward fixed solutions achieved through integer ambiguity resolution. Differential GNSS (DGNSS) can augment any of these modes with real-time or post-processed corrections.
Post-Processed Kinematic (PPK) surveying frequently benefits from float solution analysis, particularly when analyzing solution quality during field data review or when processing baseline segments where fixed solutions were not achieved.
Practical Surveying Recommendations
Surveyors should understand float solution characteristics when:
While float solutions are generally considered interim positioning results, they provide documented positioning confidence levels suitable for many surveying applications and serve as diagnostic indicators of system performance and atmospheric conditions.
Conclusion
Float Solution GNSS represents a practical positioning mode in modern GNSS surveying that bridges the gap between basic code-based positioning and high-precision fixed solutions. Understanding when float solutions occur, their accuracy characteristics, and their appropriate applications enables surveying professionals to optimize field procedures and deliver reliable results appropriate to project specifications.