Global Positioning System (GPS)

Carrier phase is another processing technique that gathers data via a carrier phase receiver, which uses the radio signal (carrier signal) to calculate positions.

In this technique, PRC has been used too. It is now called the P code or P(Y) code.

This carrier signal has a much higher frequency, so its pulses are much closer together. Thus, carrier phase measurements are highly precise and more accurate than code phase pseudo range measurements and are used on both short and very long baselines.

The phase observable is the difference in phase between the transmitted carrier wave (from the satellite) and the receiver oscillator signal at specified epoch, t. Thus, the phase of a wave only has meaning when it is specified relative to another wave of the same frequency. Once signal acquisition has started the whole number of cycles are counted.

The phase measurements are ambiguous and unless the absolute range difference at the initial epoch is determined, the phase measurement only provides the changes in range over the observed period. The initial integer ambiguity depends on the receiver-satellite combination at the initial epoch and remains the same over a particular observing period. This allows the initial and unknown integer ambiguity to be represented by a single bias term.

A cycle slip can occur when tracking is interrupted due to blockage of the signals, weak signals or incorrect signal processing due to receiver software failure. This cycle slip will alter the integer number of cycles, although the fractional phase measurement after reacquisition of the signal will be the same as if the tracking had not been interrupted.

Several techniques have been developed to fix cycle slips, such as search techniques, discrete Kalman filtering, optimized Cholesky decomposition and in the case of dual-frequency data in code and carrier, widelaning ambiguity fixing. Fast techniques are very important for real time applications and much research is currently in progress to find better and faster ambiguity resolution methods.

Post processed static carrier-phase surveying can provide 1-5 cm relative positioning within 30 km of the reference receiver with measurement time of 15 minutes for short baselines (10 km) and one hour for long baselines (30 km). Rapid static or fast static surveying can provide 4-10 cm accuracies with 1 kilometer baselines and 15 minutes of recording time. Real-Time-Kinematic (RTK) surveying techniques can provide centimeter measurements in real time over 10 km baselines tracking five or more satellites and real-time radio links between the reference and remote receivers.