Which of the following signals is primarily used by a GPS receiver operating in differential mode?

In differential GPS (DGPS), a technique used to enhance the accuracy of GPS positioning, the carrier phase signal is fundamental. This signal allows for the measurement of the phase of the carrier wave transmitted by the satellites, which provides higher precision than using code-based positioning methods.

The carrier phase signal is very sensitive to small changes in position, making it ideal for applications requiring high accuracy, such as land surveying or geodesy. By comparing the carrier phase between a stationary reference receiver and a mobile GPS receiver, the system can determine precise locations.

While other signals like radio signals and navigation signals play roles in GPS systems, they do not provide the same level of precision for differential measurements as the carrier phase signal does. The code phase signal is also less accurate than the carrier phase signal due to its reliance on less intricate measurements, making it unsuitable for the high accuracy that differential GPS aims for. Thus, the carrier phase signal is essential for achieving the enhanced accuracy relied upon in differential GPS applications.