An important part of the current GPS modernization program, M-code is the name given to a new signal that is designed to improve both the security and anti-jamming properties of military navigation using GPS. Importantly, the M-code is designed to be autonomous, and so users will be able to calculate their positions using only the M-code signal (unlike the existing military P(Y) code, which also requires use of the C/A code).
Radically, in addition to the normal wide-angle broadcast, the M-code will also be transmitted from a high-gain directional antenna, in a so-called “spot beam” (albeit focused on an area several hundred kilometers in diameter). This will increase the local signal strength by 20dB. And, as a side effect of the two antennas, inside the spot beam the satellite will appear to be two separate GPS satellites occupying the same position.
The M-code signal will carry a new MNAV navigational message, which is packetised instead of framed, allowing for greater flexibility of data content. There will be four effective data channels, and different data can be sent on each frequency and on each antenna. It can also include FEC and error detection.
The full twin-antenna realisation of the M-code signals will not be available until the launch of the GPS Block III satellites, which is tentatively scheduled for 2013. In the meantime, developers of military navigation equipment can prepare their new designs using Spirent's constellation simulators, which can support all the new signals scheduled for use under the GPS modernization program.