Differentiate carrier-phase and pseudorange measurements in GNSS; why is carrier-phase essential for high-precision positioning?

• A. Pseudorange measures approximate distance to a satellite (code-based); carrier-phase measures the phase of the carrier wave (more precise but requires ambiguity resolution); carrier-phase enables centimeter-level positioning when ambiguities are resolved.
• B. Pseudorange measures phase; carrier-phase measures distance.
• C. They are the same; there is no difference.
• D. Carrier-phase is less precise than pseudorange.

Answer: (A)

Carrier-phase versus pseudorange measurements differ in what part of the signal they lock onto and how precisely they can define distance. Pseudorange uses the code phase of the pseudorange code to estimate the distance to a satellite. This gives a usable range, but it’s limited by code noise, clock errors, and environmental factors, so the accuracy is typically at the meter level.

Carrier-phase measures the phase of the carrier wave itself, which means each carrier cycle corresponds to about a small physical distance (for L1, roughly 0.19 meters). The phase measurement can be resolved with millimeter to centimeter precision, but only if you know exactly how many whole carrier cycles have occurred between the satellite and the receiver. That unknown integer number of cycles—the ambiguity—must be solved in a process called ambiguity resolution. Once the ambiguities are fixed, carrier-phase provides very high-precision positioning, especially in RTK or PPP-AR setups that use long baselines and precise clock modeling.

So the reason carrier-phase is essential for high-precision positioning is that its phase measurement carries far finer inherent resolution than code-based pseudorange, provided you can reliably resolve the integer ambiguities and manage cycle slips and atmospheric and clock errors.