GNSS measurements: pseudorange and carrier phase
The positioning technique described in Chapter 2 is referred to as a pseudorange-based technique because the receiver uses the ranges derived from the correlation of the satellite PRN codes. This results in positioning accuracies of a few metres or yards. For some applications, such as surveying, higher accuracies are required. Carrier-based techniques such as Real-Time Kinematic (RTK) and Precise Point Positioning (PPP) have been developed to provide positions that are orders of magnitude more accurate than pseudorange-based GNSS.
- Phase modulation of the carrier wave using the PRN code is used to differentiate satellite signals and to provide signal timing information for range measurements. See item A in Figure 40.
- Measurements based on the PRN modulation are unambiguous, but precision is limited to sub-metre or foot level. See item B in Figure 40.
- The carrier wave for the GNSS signal is a sine wave with a period of less than one metre (one yard) (19 cm [7.5 inches] for L1), allowing for more precise measurements. See item C in Figure 40.
- Measurements of the carrier wave’s phase can be made to millimetre (subinch) precision, but the measurement is ambiguous because the total number of cycles between the satellite and receiver is unknown. See item D in Figure 40.
Resolving or estimating the carrier phase ambiguities is key to achieving precise positioning with RTK or PPP. The two methods use different techniques to achieve this, but both make use of:
- Pseudorange (code-based) position estimates
- Mitigation of positioning errors, either by using relative positioning or correction data
- Multiple satellite signal observations to find the ambiguity terms that fit best with the measurement data
Therefore, the method employed by the receiver, code-only or code-and-carrier-based measurements, impacts the positioning performance.