GPS Modernization With Improved Civilian Signals

In addition to the new signals to be broadcast under the GPS modernization project, there are to be two significant changes to the existing civilian signals, both designed to improve the performance of GPS receivers. The first is an additional data-free pilot signal and the second is the addition of forward error correction (FEC) encoding to the navigation message.

The new data-free signal will be broadcast alongside the normal data signal, acting as an easy-to-acquire pilot signal. Once acquired by a GPS receiver, the pilot signal will be used to acquire the data signal. This new scheme will improve the acquisition of the GPS signal and boosts power levels at the correlator.

The second improvement, the addition of forward error correction encoding to the navigation message, is designed to make the overall GPS signal more robust. This is because the navigation message is broadcast at such a low data-rate (around 50bit/s) that even a small interruption to the signal can have a significant effect on the reception of the message.

The forward error correction works by adding systematically generated redundant data (or error-correcting code) to the message. The carefully designed redundancy allows the receiver to detect and correct a limited number of errors occurring anywhere in the message without the need to receive additional data.

Designers of GNSS equipment can prepare for GPS modernization by simulating all the new signals today using Spirent's range of GNSS simulators. Not only do these instruments already cover all the new and enhanced signals that will be provided by the modernized GPS constellation, they also cover all signals for the GLONASS and Galileo GNSSs as well as the regional and local augmentation systems.

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