FIX Is Not the Same as Correct
Few indicators carry as much unearned confidence in the field as the word FIX. A crew sees it on the data collector and treats the job as done: the position is precise, so the position is right. FIX is a real and meaningful achievement, but it is a narrower claim than most people give it credit for.
FIX means the receiver judges that it has correctly resolved the whole-cycle ambiguities, together as one connected set, for whichever satellites it included in that solution. That judgment can be wrong, and even when it's right, it says nothing about the reference frame, the base coordinate, or anything else in the chain beyond that one relative solution.
What FIX Is Actually Vouching For
The last chapter explained the integer ambiguity, the missing whole-cycle count between receiver and satellite that has to be resolved before a carrier-phase measurement means anything. Resolving that count correctly is what a FIX solution represents.
That resolution is usually right, but it isn't guaranteed. A false fix happens when the receiver's own confidence check is satisfied by the wrong set of whole numbers, and the receiver reports FIX with just as much apparent confidence as a correct one. Companion 3 covers this failure mode in depth; for now, the point is simpler: FIX is the receiver's own best judgment that the ambiguities were resolved correctly, not an independent guarantee that they were.
FIX is a statement about the quality of one relative geometric solution. It is not a statement about the reference frame, the base coordinate, the antenna height, or anything that happens to the coordinate after this stage.
Everything FIX Cannot See
A receiver in FIX has no way to know whether the base station's own coordinate was surveyed correctly. It has no way to know whether the base and rover are even using the same reference frame and epoch. It has no way to know whether the antenna height was entered correctly, whether the correct geoid model was applied, or whether a map projection downstream is about to distort the displayed number. FIX describes the last few centimeters of a calculation. It says nothing about everything that calculation was built on.
A rover in FIX is not automatically correct. Even a genuine, correctly judged FIX says nothing about the base coordinate, the reference frame, or the antenna setup underneath it, any of which can still be wrong.
A Habit Worth Building Early
From this point in the companion forward, treat FIX as the beginning of a checklist, not the end of one. It answers exactly one question well: did the receiver resolve a precise relative position? It leaves every other question, about frames, datums, projections, and site adjustments, completely open. Those are the questions the rest of this companion is built to answer.
A rover reports FIX and a coordinate that looks perfectly reasonable. Has anything in this chapter told you the coordinate is correct? No. At best, it has told you that one particular calculation, deep inside the chain, was judged to be resolved correctly.
FIX means the receiver judged its own resolution of carrier-phase ambiguities to be correct. It does not certify the base coordinate, the reference frame, the antenna setup, or any transformation applied after the position is solved. Precision and correctness are answered by different parts of the chain.
So far this companion has stayed close to the ground: satellites, a base, a rover, a FIX. The next chapters go somewhere less intuitive. Before a receiver ever produces a latitude and longitude, it solves its position in a coordinate system built around the one point every satellite orbit shares: the center of mass of the Earth itself.