There are well-known ways to improve GPS accuracy, such as dGPS [1], inertial guidance, and beam-riding [2]. I think it unlikely that the US military has a sec

There are well-known ways to improve GPS accuracy, such as dGPS [1], inertial guidance, and beam-riding [2]. I think it unlikely that the US military has a secret second constellation of GPS-like satellites, operating on an undetectable secret frequency. Firstly, it would be difficult to launch and operate such a constellation in secret, and secondly, because augmenting the GPS data with independent, secondary sources operating with a different approach provides more redundancy and is proven technique for increasing accuracy. The capability to limit accuracy in the satellite signal already existed – the fact that it was shut off is a strong signal that the perceived benefit is no longer important. [1] https://en.wikipedia.org/wiki/Differential_GPS https://en.wikipedia.org/wiki/Differential_GPS [2] https://en.wikipedia.org/wiki/Beam_riding https://en.wikipedia.org/wiki/Beam_riding