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GPS IIF, GPS IIR–M, and GPS III provide the legacy and new modernized signals. These may include L2 civil (L2C) signal and the L5 signal (at 1176.45 MHz) modulated by a new code structure, as well as, the M and L1C codes. These modernized GPS signals improve the ionospheric delay calculation, ranging performance, ambiguity resolution, and overall PVT accuracy.

The GPS Ground Control Segment monitors the GPS signals in space, interfaces with the US Naval Observatory for timing information, and has remote monitor/uplink transmitter sites throughout the globe. Over the years, the GPS GCS has been upgraded and the Next‐Generation Operational Control System (OCX) will monitor all legacy and modernized GPS signals to provide for enhanced PVT solutions for the user segment. See Sections 4.2.8 and 10.5.5.5 and Ref. 18, Chapter 4.