Why Carrier Phase Changes Everything
RTK's precision has to come from somewhere. The last chapter explained that RTK works by canceling shared error between a rover and a reference, but that alone doesn't explain why the result can reach centimeters when an ordinary receiver, working the same problem alone, typically lands somewhere in the range of a few meters, not centimeters. The difference comes down to what the receiver is actually measuring against.
Ordinary GNSS ranging times a repeating code signal. RTK instead measures carrier phase directly, a signal dramatically shorter than that code pattern. Measuring a small fraction of a much shorter wave, rather than timing a much longer repeating pattern, is what makes centimeter precision possible, at the cost of one significant complication.
Two Different Signals to Measure Against
Every GNSS satellite broadcasts a repeating code pattern on top of a carrier wave. Ordinary GNSS positioning times the arrival of that code, which is reliable and easy to lock onto, but limits accuracy to a few meters no matter how good the receiver's clock is. RTK instead tracks carrier phase directly, underneath the code, a signal far shorter than the code pattern it carries. Measuring a small fraction of that much shorter wave is what allows RTK to reach centimeter precision in the first place.
The Catch: Which Cycle Is It?
Measuring a fraction of a wave that short is only half the problem. The receiver can tell exactly where it sits within one cycle of the carrier wave, but the distance to a satellite spans an enormous number of complete cycles, and the receiver has no direct way to count how many whole cycles came before the one it's currently measuring. That missing count is the integer ambiguity, and an RTK FIX means the receiver judges that it has resolved that count correctly.
Tracking carrier phase precisely is not the same as having a FIX. A receiver can measure its position within a wave cycle perfectly and still not know which whole cycle it's in. FIX specifically means the receiver judges that missing count has been resolved correctly.
Why More Frequencies and More Satellites Help
That same ambiguity gets easier to resolve as more information becomes available to the receiver. Modern RTK receivers commonly track more than one carrier frequency, and more than one satellite constellation, at the same time. Each additional frequency and each additional satellite gives the receiver another independent piece of data pointing at the same unknown ambiguity. Combining several of them narrows the search considerably, which is why modern multi-frequency, multi-constellation receivers typically resolve FIX faster and more reliably than older equipment tracking a single signal from a single system ever could.
RTK reaches centimeter precision by combining two things: canceling shared error through differencing, and measuring carrier phase directly, a far shorter signal than the code pattern ordinary GNSS ranging relies on. The cost of that added precision is the integer ambiguity, an unknown whole-cycle count that has to be resolved before the measurement means anything. More frequencies and more satellites both help resolve that same ambiguity faster; neither changes what FIX actually certifies.
That whole-cycle count either gets resolved correctly, or it doesn't. The next chapter looks closely at what a FIX solution does and does not actually guarantee.