What is GNSS-denied navigation?

GNSS-denied navigation is keeping a vehicle on course when satellite positioning, GPS, Galileo, GLONASS and BeiDou collectively, is jammed, spoofed, or simply unreachable. The workhorse answer is inertial navigation, and the grade of the inertial sensor decides how long you can stay accurate.

The 30-second answer

Satellite signals are astonishingly weak, roughly the power of a light bulb transmitted from 20,000 km away. A small jammer can blank them over kilometres. A spoofer can do worse and quietly hand your vehicle a false position.

When that happens, navigation falls back on sensors that need nothing from outside the vehicle: an inertial measurement unit measuring rotation and acceleration, combined over time into heading and position with help from aiding sensors such as speed. That is dead reckoning, and everything about GNSS-denied performance follows from how slowly its errors grow.

Where GNSS actually fails

Deliberate jamming and spoofing. Now routine in and around conflict zones, with commercial aviation logging large numbers of interference events every month in affected regions.

Contested airspace. For defense platforms, assuming GNSS will be available on the day is no longer a plan.

Physics and geography. Tunnels, urban canyons, indoors, underground and underwater, where the signal cannot reach at all. A subsea vehicle lives its whole mission GNSS-denied.

Ordinary degradation. Multipath and short outages that corrupt position exactly when precision work like grading, surveying or docking needs it most.

How inertial navigation fills the gap

An IMU senses six or more axes of motion: three of rotation from gyroscopes, three of acceleration from accelerometers, and sometimes three of magnetic field from magnetometers and two of inclination from inclinometers. Integrate rotation and you know which way you are pointing. Combine that heading with your speed and you know where you have gone. No signal, no infrastructure, nothing to jam.

The catch is drift. Every small gyro error is integrated into a growing heading error, which turns distance travelled into position error. Two numbers on the datasheet govern it: gyro bias stability, in degrees per hour, the slow wander, and angular random walk, in degrees per root hour, the noise.

This is why sensor grade is the whole game. A consumer-grade part is lost in seconds. A tactical-grade unit is useful for minutes to hours with aiding. Navigation-grade instruments can free-navigate far longer.

For the full grade ladder, see [What is an IMU?]

What the difference looks like on a real road

We drove a 21 minute route through Montreal with GNSS positioning denied, running a photonic IMU, the OSCP MK2E2, and a tactical MEMS IMU, the OSCP MK2M2, side by side. Heading came from each unit’s gyro alone, with a 1 Hz speed input for distance and no EKF.

The photonic unit finished 5.7 m from ground truth. The MEMS unit finished 20.5 m away. Both ran identical aiding, so the 3.6x gap is set almost entirely by heading drift, and it grows with every minute of denial.

The full data is published in [our case study].

Aiding: stretching inertial accuracy

Real systems rarely run pure dead reckoning. They blend the IMU with whatever the platform already has: wheel odometry on vehicles, air data on aircraft, Doppler velocity logs subsea, vision or terrain matching where the scenery allows, magnetometers where the local field is clean.

The better the IMU, the less often those aids have to correct it, and the more gracefully the system survives when one of them drops out too.

Choosing a sensor for GNSS-denied work

Define the denial window. Seconds through a tunnel, or hours of contested operation?

Set a position-error budget at the end of that window, and work back to the gyro grade it demands.

Count your aiding sources. Odometry alone materially extends any IMU.

Check the environment. Vibration, shock, EMI and temperature will find weak sensors. Light-based gyros are far less sensitive to the first three.

Check exportability. The best sensor is one you are allowed to ship. ITAR-free removes the longest pole in many programs.

Frequently asked questions

Is GNSS-denied the same as GPS-denied?

Effectively yes. GPS is the American satellite constellation. GNSS is the collective term including Galileo, GLONASS and BeiDou. Jamming rarely respects the distinction, so engineers say GNSS-denied.

Can you navigate with no GNSS at all?

Yes. Inertial navigation needs nothing external. The question is never whether, but for how long within your error budget, and that is set by sensor grade and aiding.

What accuracy can an IMU hold without GNSS?

It depends on grade and aiding. As one real data point: over a 21 minute drive with GNSS positioning denied and a 1 Hz speed input, a photonic tactical IMU held position to 5.7 m where a MEMS tactical unit drifted to 20.5 m.