Work continues in rolling out the resilient M-Code capability across the US Global Positioning Satellite constellation.
The United States Space Systems Command (SSC) Military Communications and Positioning, Navigation and Timing Programme Executive Office (MILCOM PNT PEO) has briefed Armada on the status of the introduction of M-Code. M-Code is a Position, Navigation and Timing (PNT) signal waveform equipping the US Global Positioning System (GPS) Global Navigation Satellite System (GNSS) constellation. A comprehensive discussion of the workings of M-Code can be found here. In a nutshell, M-Code is a secure GPS PNT signal waveform designed to be highly resilient to jamming and spoofing.
The MILCOM PNT PEO told Armada that “M-Code … was designed for military operations in electronic warfare environments.” It is separated from civilian GPS PNT signals in the same frequency bands, allowing the power of M-Code to be significantly increased without interfering with the former. M-Code also features more robust modernised cryptography to prevent spoofing. The main GPS PNT signals, known as L1 and L2, use frequencies of 1.57542 gigahertz/GHz (L1) and 1.22760GHz (L2). Other transmitted signals include L5 (1.17645GHz) and L1C (1.57542GHz), L2C is a new version of the L2 signal transmitted on the same frequency.
M-Code
The encrypted M-Code signal is reserved for use by US and allied militaries. M-Code is transmitted across the L1 and L2 frequencies. Militaries originally used the encrypted P(Y) Code signal, although employing this depended on the GPS receiver first acquiring the civilian C(A) Code transmitted using the L1 channel. Failure to acquire the C(A) Code, for example if the GPS receiver is being jammed, can prevent the acquisition of the P(Y) Code. Although P(Y) Code is encrypted, “M-Code features more robust modernised cryptography to prevent spoofing.”
M-Code is not completely invulnerable to electronic warfare: “The effectiveness of a jammer depends on its overall generating power, the direction the interference signal energy is focused and its proximity to the targeted GPS receiver,” the statement continued. “The combination of these factors means that any receiver, including an M-Code receiver, can be jammed in a specific situation. However, the goal of GPS modernisation is to maximise the availability of secure and accurate position, velocity, and timing information in challenging electronic warfare environments.”
Satellites
The first of the M-Code compatible satellites, known as Block IIRM which were built by Lockheed Martin, was launched in September 2005. The MILCOM PNT PEO says that 24 M-Code compatible satellites are now in orbit. The subsequent Block III satellites, the first of which was launched in December 2018, transmit the M-Code signal with even more power than the original M-Code spacecraft to help counter jamming. The Block III “satellites currently being launched can transmit M-Code approximately seven times stronger than the legacy military signal P(Y) Code.” The forthcoming Block IIIF constellation, which are expected to be launched in the coming years, will see a further increase in M-Code signal power: “The Regional Military Protection (feature) on future Block IIIF satellites will increase M-Code power over a region of interest by an additional factor of 20.” At the same time, military GPS receiver technology is improving through “greatly increased resistance to spoofing, improved encryption of the M-Code signal and integrity checks built into M-Code receiver technology.”
As matters stand at present, seven Block IIRM and twelve Block IIF satellites capable of transmitting M-Code are in orbit, according to the MILCOM PNT PEO. Six of the ten M-Code capable Block III satellites have been launched “with the remainder planned for launch in 2025 and 2026.” The PEO continued that the first Block IIIF satellite is expected to be launched in 2027.
by Dr. Thomas Withington
