What does M-Code mean for GPS simulation?


GPS military codes ensure that America’s armed forces receive the most accurate and precise positioning information anywhere in the world at any time. Delivering that performance consistently and reliably requires extensive testing during receiver development, integration into weapons platforms, and operational maintenance.

GPS simulators let you conduct that testing in the lab rather than in the field. But when military and defense users need to get the job done right the first time, they need simulators that support the latest GPS military code capabilities.

In this article, we will review the evolution of military codes from the original GPS satellites to today’s GPS modernization program.

What are military codes?

GPS is a dual-use service with both civilian and military applications. Civilians use the Standard Positioning System which can deliver positioning accuracy to for smartphones and better for more advanced devices. Civilian GPS receivers, however, only use one of the signals GPS satellites transmit.

The authorized users can acquire two signals to achieve better positioning accuracy. The PPS is also more resistant to adversaries’ attempts at jamming and spoofing.

What is P(Y)-code?

In its original design, GPS satellites transmitted two signals to GPS receivers. The dual-use L1 signal is modulated by both the publicly available C/A code as well as the restricted P(Y) code. To acquire the P(Y) code, the original PPS receivers had to go through a two-step process. They first acquired the L1 signal’s C/A code and used that to transition the acquisition of the signal’s P(Y) code.

Each GPS satellite’s L2 signal is modulated by the P(Y) code only. That keeps it restricted to authorized users. PPS receivers use the two signals to produce more accurate and precise positioning information.

What is SAASM?

Another benefit of the P(Y) code was that it made PPS receivers more resistant to spoofing. If an adversary could mimic the signals from one or more GPS satellites, they could feed false positioning information to GPS receivers in the area. This spoofing could steer warfighters off course and disrupt operations.

The P(Y) code’s encryption should make spoofing impossible. But the original design of PPS relied on the C/A signal for the acquisition of the P(Y) signal. If the C/A signal was spoofed, then a PPS receiver could be affected.

A solution arrived in the early 2000s when military GPS receivers began using the Selective Availability Anti-Spoofing Module (SAASM). Among other benefits for the warfighter, SAASM allows these receivers to acquire the P(Y) signal directly.

What is M-code?

Technologies developed over the decades since GPS was first deployed have created opportunities for new civilian applications as well as improved military performance. At the same time, advancing technologies have increased the threat from jamming and spoofing.

The ongoing GPS modernization program includes the introduction of a new military code, called M-code. A more secure encryption technology, the Modernized Navstar Security Algorithm, will make military GPS receivers more resilient against spoofing.

The first GPS Block II satellite with M-code support was launched in 2005. With the operational acceptance of the fourth GPS Block III satellite in 2020, the US Space Force is one satellite shy of providing full-time, global M-code coverage. Once the full complement of Block III satellites is in orbit, the constellation will improve positioning accuracy and jamming resilience.

Simulating military codes with CAST Navigation

Over the past four decades, the military and defense GPS communities have relied on CAST Navigation’s simulation tools for their GPS testing and integration projects. launched our first simulator in 1989 with support for P(Y) simulations. By the mid-2000s, our development of Field Programmable Gate Array technologies let us support both P(Y) and SAASM development and integration testing.

In January 2020, the Department of Defense granted us MNSA security approval. Authorized military and defense customers can accelerate their M-code development and integration projects by adding MNSA capabilities to their CAST Navigation simulators:

  • GNSS – Our simulations of GPS and other GNSS constellations produce precise and repeatable RF signals with real-time 6 DOF trajectories.
  • CRPA – Precise, coherent, and repeatable wavefronts let you test phased array antenna systems.
  • Inertial – Generate dynamic INS and GNSS signals to test integrated EGI navigation systems.
  • Jamming – Simulate natural, industrial, and hostile interference with control over multiple interference parameters.

Our modular technology lets you configure a scalable solution for your simulation needs. Contact us to learn more about CAST Navigation’s support for GPS modernization and M-code simulation.











November 18, 2021