The Inertial Navigation Renaissance: Why Dead Reckoning Is Defense's Backup Plan for GPS Denial

GPS is jammed over eastern Ukraine on a daily basis. Chinese electronic warfare exercises routinely spoof signals across the Taiwan Strait. Every defense planner knows it: the Global Positioning System is contested infrastructure, and anyone who built weapons, logistics, or command systems assuming clean GPS access built on sand.

Photo by Ylanite Koppens on Pexels.

The response has been predictable in some ways and surprising in others. Predictable: the Pentagon has poured money into quantum sensing, signals diversity, and alternative PNT programs. Surprising: a technology that predates GPS by decades is quietly having its best decade ever. Inertial navigation is back, and it's better than most people outside the IMU world realize.

Dead reckoning sounds like a fallback. Track your velocity, integrate to get position, apply known heading. Simple enough in principle; brutal in practice because every sensor error compounds over time. Classic ring-laser gyroscopes were precise but expensive, fragile, and bulky. MEMS-based inertial measurement units shrank the hardware but introduced higher drift rates. For decades, the honest answer was: use GPS, use inertial as a short-duration bridge.

That answer is being revised.

Two hardware threads are changing the equation. First, MEMS fabrication has matured to the point where navigation-grade MEMS IMUs (think Honeywell's HG4930 or Safran's Sagem line) can maintain position accuracy for minutes rather than seconds before drift accumulates to operationally unacceptable levels. Tactical-grade MEMS units are shipping at volumes and price points that let programs embed them redundantly throughout a platform rather than treating the IMU as a single precious component.

Second, photonic IMUs are moving from laboratory demonstrations into early program-of-record conversations. Optical gyroscopes based on integrated photonics, rather than bulk optics, preserve the drift characteristics of fiber-optic or ring-laser designs while shrinking toward chip-scale form factors. DARPA's CHIPS program (Chip-Scale Combinatorial Atomic Navigator) has been pushing this direction since 2021. The physics are favorable: photon-based rotation sensing doesn't suffer the same vibration sensitivity that plagues MEMS at high dynamics, which matters enormously for missiles, hypersonic glide vehicles, and rotary-wing platforms.

Here's where software enters the picture. Raw inertial data is only part of the story. Modern INS/GPS integrated systems use Kalman filters to fuse IMU outputs with whatever external reference is available: GPS when clean, radar altimetry, terrain-referenced navigation, star trackers, signal-of-opportunity ranging from cellular towers or LEO satellite constellations. GPS denial doesn't mean those other signals disappear. A well-engineered navigation solution treats GPS as one input among many, not the arbiter of truth.

The Kalman filter itself has been modernized. Adaptive variants can detect GPS spoofing by monitoring consistency between inertial-predicted position and received GNSS fixes. When the two diverge faster than the IMU's known error budget allows, the filter down-weights or rejects the GPS input autonomously. No human in the loop, no latency waiting for an operator to recognize an anomaly. The sensor fusion software is doing threat detection.

graph TD
    A[/IMU Raw Data/] --> B(Kalman Filter)
    C[/GPS Signal/] --> D{Spoofing Check}
    D -->|Consistent| B
    D -->|Anomaly Detected| E[GPS Rejected]
    F[/Terrain Ref Nav/] --> B
    G[/Star Tracker/] --> B
    B --> H((Position Estimate))

What makes this moment different from previous inertial revivals is the combination of cheaper hardware, better fusion algorithms, and a procurement community that has finally absorbed the GPS-denial lesson operationally rather than theoretically. Programs like MAPS (Mounted Assured Positioning, Navigation, and Timing System) and the Army's IPPS program aren't hedging against GPS denial. They're designing around it as a baseline assumption.

There are real limits to acknowledge. Drift is physics, not an engineering oversight. Even the best inertial-only system degrades over time without an external reference update. Chip-scale atomic clocks help by maintaining timing precision that lets the Kalman filter constrain some error sources. Terrain-referenced navigation works where you have detailed terrain data and a radar altimeter; it's less useful over open ocean or flat desert. Star trackers work at altitude and when the sky isn't obscured. No single backup is universal.

The honest picture is a layered navigation stack, with inertial as the always-on substrate that ties every other source together. GPS becomes one optional input rather than the foundation. That inversion, subtle as it sounds on paper, represents a significant shift in how navigation systems get designed from the ground up.

Silicon Valley built the GPS-integrated world. The companies rebuilding navigation for contested environments are largely not in Silicon Valley. They're in Tucson, in Bristol, in Tel Aviv, running programs that most tech press has never covered. The work is unglamorous. The physics are hard. And right now, it's some of the most consequential hardware engineering happening anywhere in defense.