GPS M-Code: A Powerful, Encrypted Military Code Satellite Signal

GPS has at least 30 satellites in orbit that provide positioning, navigation, and timing (PNT) data to receivers on earth. Owned by the US government and operated by the US Air Force, the satellite constellation sends signals on earth, some for commercial use and others for military operations. However, these communication frequencies were non-secure, which made it easy for adversaries to jam and manipulate them. So it was necessary to upgrade the system to have an encrypted military signal that became the M code. We will analyze this GPS M code signal to see its capabilities, characteristics, and operational benefits. Read on to learn more!

Table of Contents

What Is M-Code?

One of the United States Air Force’s plans to modernize the Global Positioning System included introducing encrypted M-code signals that enhance anti-jamming and spoofing protection. So the M code is a powerful, highly secure military code signal that is more secure than the GPS signals used today.

Since multiple branches of the US government rely on PNT data, intentional or unintentional jamming or spoofing can cause signal disruption with severe consequences.

The upgrade began as the M-Code Early Use (MCEU) program, which gave command and control of M-Code capability to 12 GPS IIF and eight GPS Block IIR-M satellites. The program will provide the same functions to the new GPS III satellites.

A GPS satellite in orbit

A GPS satellite in orbit

M-Code vs. P(Y)-Code

The satellites transmit the M-Code on the same L1 and L2 frequencies used in the previous P(Y) military code. You might be wondering how they send data on the same frequency. M-Code places most of its energy around the edges, away from the C/A and P(Y) carriers.

And unlike P(Y), M-Code is autonomous. So military receivers can calculate user position using only the M-code signal. P(Y)-Code receivers must first lock onto the C/A code, then transfer the lock to the P(Y)-Code.

Compared to traditional GPS designs, M-Code broadcasts from two sources: a high-gain directional antenna and a full-earth, wide-angle antenna. Satellites can aim the spot beam from the directional antenna signal to a specific area on the planet, increasing the signal strength by about 20 dB. This increase equals 100X the power and 10X the voltage field strength. And the area covered by the spot beam is usually several hundred kilometers in diameter.

Directional antennas on a mast

Directional antennas on a mast

The full-earth, wide-angle antenna M-Code signal is already available on the GPS IIR-M satellites since they are already in space. However, the spot beam antennas are only available in the new GPS III space vehicles, which are still under deployment.

Additionally, the M-Code depends on OCX Block 2, the next-generation Operational Control System (control segment). Scheduled to enter service in October 2016, the OCX Block 2 project has experienced multiple delays. So M-Code is still not fully functional but is under the early use phase, with operational acceptance achieved in November 2020.

M-Code Capabilities

M-Code gives military receivers better defense against jamming and spoofing because it supports a more flexible and secure cryptography architecture that detects and rejects false signals. Additionally, it provides a more powerful power signal with better accuracy and availability than the previous military code. So it enables military users to operate in dense jungles with precise GPS tracking.

A soldier using a tablet with a map and GPS for navigation

A soldier using a tablet with a map and GPS for navigation

On top of that, the code provides military forces with a dedicated GPS signal separate from the civilian one. Therefore, when fully operational, the M-Code upgrade will eliminate the possible interruption risk by the armed forces on the standard GPS signal used for commercial applications.

M-Code Characteristics

  • Requires satellite signals from two antennas: a spot beam and whole-earth coverage antenna
  • It takes 24 MHz of bandwidth.
  • Binary offset carrier (BOC) modulation, abbreviated as BOC (10,5) and denoted as BOC (10.23, 5.115). It implies a 10.23 MHz sub-carrier frequency and a spreading code rate of 5.115 M spreading bits/second.
  • It can include error detection and FEC
  • 20dB more powerful spot beam than the wide-angle, whole-earth coverage beam
  • It uses four effective data channels and can send different data on each frequency and each antenna
  • Transmits as new navigational messages (MNAV) in packets, not frames, allowing flexible data payloads
  • M-Code signal strength on earth’s surface: -138dBW for the spot beam antenna and -158dBW for the whole-earth, wide-angle antenna

M-Code Operational Benefits

This code has the following operational benefits.

Blue Force Electronic Attack Compatibility

Blue Force Tracking gives the United States military location data about their friendly forces for allied operations. The M-Code signal allows selective jamming of the C/A code (commercial L1 C/A GPS signal) while still receiving the M-Code signal from friendly military forces due to its signal modulation technique. This application is known as blue-on-blue or blue-force jamming.

Resistance to Jamming

The spot-beam signal transmissions from the high-gain antennas in GPS Block III satellites give better defense against jamming. These signals should be about 20dB more powerful than the full-earth wide-angle antenna beam transmitted by Block II satellites.

A mobile network jamming device

A mobile network jamming device


Spoofing attacks involve falsifying data to get an illegitimate advantage. But with the encryption applied on M-Codes, advanced M-Code GPS receivers can detect and reject false signals to prevent unauthorized access.

A handheld GPS receiver

A handheld GPS receiver

Encryption creates a closed-loop system between the space and ground segment that adversaries cannot hack. So the code gives secure access to weapons systems (advanced missile systems), ground forces, and aerial systems.

Spoofing can lure these assets off-course, which is something you don’t want during a military battle.

Wrap Up

In conclusion, the GPS M-Code is a game changer for military systems because it introduces a stronger and more secure signal to the GPS positioning, timing, and navigation system. Although not yet fully deployed, the code promises more accurate performance in its signal modulation technique. It won’t be available to the public, though. Only military users will be able to access it. That’s it for this article. Drop a comment or give a thumbs up to the article below if you found it insightful. Cheers!

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