What is M-Code?

GNSS 

What is the M-Code signal that is used by the US Military?

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Editorial Team - everything RF

Jul 6, 2019

M-code is a new military signal used in the L1 (1575.42 MHz) and L2 (1227.60 MHz) GPS bands. It is designed to improve security and anti-jamming properties of military navigation using GPS.

M-Code offers several operational benefits:

Jamming Resistance: M-code signals can be delivered to specific regions using spot beam transmissions from GPS Block III satellites. The high-gain directional antenna on the satellites aims the M-code signal at a specific region of the earth, with much greater satellite power in that region. The transmitted signals are expected to be around 20-dB more powerful than the conventional full-Earth coverage beam.

Blue Force Electronic Attack Compatibility: Blue Force Tracking provides the US military to with location information about friendly forces. The M-code signal because of its modulation type, allows to selectively jam the commercial GPS L1 C/A signal (C/A code) and continue receiving the M-Code signal from friendly military forces. This practice is called “blue force jamming” or “blue on blue jamming”.

Anti-Spoof: The M-Code signals are encrypted and their receivers are able to detect and reject false signals.

The M-code Signal shares the same bands with existing GPS signals that use the P(Y) code and the C/A code, on both L1 and L2 bands. However, it will not interfere with older codes as it uses binary offset carrier (BOC) modulation which is very different from the binary phase shift key (BPSK) used with the legacy C/A and P(Y) signals. The M-Code signal modulation has a binary offset carrier signal with a sub-carrier frequency of 10.23 MHz and spreading code rate of 5.115 M spreading bits per second, denoted a BOC (10.23,5.115) (abbreviated as BOC (10,5)) modulation.

M-code is designed to be autonomous, i.e. the military receiver can determine its position with the M-code alone while with the P(Y) code, the receiver has to acquire the C/A code first. The M-Code signal is encrypted using a Modernized Navstar Security Algorithm (MNSA), to ensure the system is secure.

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