Why INS Is Needed Even When GPS Is Available

INS stands for inertial navigation system, and the reason militaries keep investing in it despite the ubiquity of GPS is simple: INS works when GPS does not. An inertial navigation system determines position, velocity, and orientation entirely from its own internal sensors, accelerometers and gyroscopes that measure motion, without receiving any signal from outside, so it cannot be jammed, spoofed, or blocked the way GPS can. On a battlefield where GPS is increasingly contested, that independence is not a nice-to-have but a necessity, which is why nearly every serious military platform, from aircraft and missiles to ships and vehicles, carries inertial navigation alongside GPS rather than relying on GPS alone.

What INS does and how it works

An inertial navigation system tracks where it is by measuring its own motion and calculating its position from a known starting point, using accelerometers that measure changes in velocity and gyroscopes that measure rotation. Because it senses its own movement directly, it needs no external reference, so it keeps working in the places and situations where GPS fails, in tunnels, underground, underwater, and, crucially, when an adversary jams or spoofs the GPS signal. This self-contained operation is the whole point of INS, and it is why inertial navigation was the standard for military navigation before GPS existed and remains essential now, since a system that depends on no external signal cannot be denied by an enemy attacking that signal. The technology has advanced enormously, from the bulky mechanical systems of early aircraft and missiles to compact modern units, including ring-laser and fiber-optic gyroscopes that measure motion with great precision and, increasingly, small MEMS sensors that put inertial navigation into ever-smaller and cheaper packages.

INS has one fundamental weakness, which is precisely what makes it complementary to GPS rather than a replacement for it: it drifts. Because an inertial system calculates position by continuously accumulating measurements of its own motion from a starting point, small errors in those measurements accumulate over time, so an INS operating alone gradually drifts away from the true position, its error growing the longer it runs without correction. GPS, by contrast, provides an absolute position fix from the satellites that does not drift, being fixed to the actual geography rather than accumulated from motion, but GPS can be jammed, spoofed, blocked, and denied, which INS cannot. So the two systems have exactly opposite strengths and weaknesses: INS is independent and un-jammable but drifts over time, while GPS is accurate and drift-free but depends on a vulnerable external signal, which is why they are combined, each correcting the other's weakness.

Why the two are combined

Modern navigation systems fuse INS and GPS so that each covers the other's blind spot, using GPS to correct the drift of the INS while the INS carries the navigation through any period when GPS is unavailable. When GPS is working, its drift-free position fixes continuously correct the accumulating drift of the inertial system, keeping the combined system accurate, and when GPS drops out or is jammed, the inertial system carries on providing navigation from its own sensors, holding the position through the outage until GPS returns and corrects any drift that accumulated. The mathematics that combines them, typically a Kalman filter that weighs the reliability of each input, produces a navigation solution more accurate and far more robust than either system alone, and this fused INS/GPS navigation has become the standard for military and many civilian applications precisely because it delivers both the accuracy of GPS and the resilience of INS. The combination also delivers other benefits: the inertial system provides orientation, which way the platform is pointing, that GPS alone does not, and it updates position far faster than GPS, providing the rapid, continuous motion data that fast-moving platforms and control systems need, so the fused system is better than GPS alone in several ways beyond resilience.

The military value of INS has grown sharply as GPS has become contested, since the resilience that inertial navigation provides is exactly what is needed on a battlefield where adversaries jam and spoof GPS. The war in Ukraine made this vivid, with intense GPS jamming and spoofing on both sides degrading GPS-dependent systems, and the platforms and weapons that carried good inertial navigation kept functioning through the GPS denial while those that depended on GPS alone suffered. This has driven renewed emphasis on inertial navigation and on the assured positioning, navigation, and timing that combines resilient inertial navigation with anti-jam GPS and other means, precisely because a military that can navigate and guide its weapons only when GPS is available has handed an adversary a way to cripple it by denying GPS. The answer to why INS is needed even when GPS is available, then, is that GPS is not reliably available when an enemy is trying to deny it, and inertial navigation, needing no external signal, keeps working when GPS is jammed, spoofed, or blocked, providing the resilient, independent navigation that ensures a force can still find its way and guide its weapons in the contested environment where GPS cannot be counted on.

If GPS is so accurate, why not just harden it against jamming instead of using INS?

Because hardening GPS against jamming, while worthwhile, cannot make it fully reliable against a determined adversary, so inertial navigation provides an independent backup that does not depend on the GPS signal surviving at all. GPS signals are inherently faint, coming from distant satellites, and while anti-jam antennas and other protections make GPS more resistant to jamming, they cannot guarantee that GPS will work against a capable enemy who jams and spoofs it heavily, since a sufficiently determined and capable adversary can overpower or deceive even hardened GPS in a contested area. Inertial navigation, by contrast, needs no external signal at all, so it cannot be jammed or spoofed by any means, providing navigation that is entirely independent of the GPS signal and thus reliable regardless of how badly the enemy denies GPS, a fundamentally different kind of resilience than hardening GPS provides. This is why militaries use both, hardening GPS to make it as reliable as possible while also carrying inertial navigation as an independent backup that works even when GPS is completely denied, since the combination of a more resilient GPS and an independent INS provides far more assurance than either alone. The inertial system covers the case that GPS hardening cannot fully address, the possibility that GPS is denied entirely despite the hardening, so that even if the enemy defeats the hardened GPS, the inertial navigation keeps the platform navigating and its weapons guiding, which is exactly the resilience that a contested environment requires and that GPS hardening alone cannot guarantee, making inertial navigation an essential complement to even the most hardened GPS.