When FIX Is Wrong
The most unsettling failure in RTK is not FLOAT lingering too long, and not an obvious dropout. It is a false fix: a receiver that resolves integer ambiguities with complete internal confidence, reports FIX exactly as it would for a good solution, and is simply wrong.
A false fix occurs when a receiver's ambiguity resolution converges on the wrong whole numbers but still passes its own internal confidence check. The result looks identical to a genuine FIX, often centimeters to a meter or more away from the truth, with no obvious warning sign.
How a Wrong Answer Passes the Test
Resolving integer ambiguities is a statistical search: the receiver evaluates candidate sets of whole numbers and looks for one that fits the observed carrier-phase data dramatically better than any other candidate. Under good conditions, the correct set stands out clearly. Under degraded conditions, heavy multipath or poor satellite geometry, an incorrect set can occasionally fit the data almost as well as the correct one, and the receiver's confidence test can be satisfied by the wrong answer.
What Makes a False Fix More Likely
Several conditions raise the odds of a false fix, and they tend to arrive together rather than one at a time: a cluttered environment with strong multipath, poor satellite geometry with few usable satellites or a bad spatial distribution, and a baseline stretched near the edge of what the correction source can reliably support. None of these conditions announce themselves on the data collector. The screen still just says FIX.
A false fix does not look different from a genuine one on the screen. Both display the same confident status. The difference only becomes visible through independent verification, or through statistics the receiver may report but the field software does not always surface prominently.
A false fix is a receiver confidently reporting FIX on an incorrectly resolved set of integer ambiguities. It looks identical to a genuine FIX on screen, and recognizing the conditions that make it more likely is the first line of defense against it.
Multipath was named as a major contributor to false fix without yet being explained. It deserves its own careful look, since it is one of the most common, least visible sources of RTK trouble in the field.