How to Build a Navigation System for GNSS-Denied Environments

The core element of an inertial navigation system is the inertial measurement unit (IMU). IMUs come in different grades: industrial, tactical, navigation and strategic.

Cost grows logarithmically. Industrial IMUs can cost as little as a few dollars, while navigation-grade IMUs can exceed $100K and strategic-grade IMUs $1M.

Inertial navigation cannot be jammed, but by its very nature it "jams itself": it drifts. Inertial navigation systems need a known starting position and rely on initialization systems.

Unaided navigation hardly exists in practice. Aiding units improve navigation performance and provide robustness when GNSS is unavailable.

A CRPA (Controlled Radiation Pattern Antenna, also known as Controlled Reception Pattern Antenna) is an anti-jam antenna. Unlike a conventional antenna, it algorithmically estimates the direction of a jammer and shapes its antenna pattern to suppress signals from that direction, while still receiving signals from all other directions.

The number of elements determines how many jammers it can suppress at the same time. The general rule is "number of elements minus one": a 4-element CRPA can suppress jammers from 3 different directions, and an 8-element CRPA from 7.

It can suppress a jamming signal by up to 40–50 dB. However, its effectiveness can drop against very powerful or very numerous jammers.

This method is known as VINS (visual-inertial navigation system). Navigation drift is reduced using IMU measurements and the motion of points tracked in the image. It gives no absolute correction, but drift can be brought down to linear levels. It works well on land and can work in the desert, but not at sea.

Think of it as the maritime equivalent of airspeed: current is to the sea what wind is to the air. A speed log gives speed through water; you can estimate the current and derive your speed over ground from the log measurement. Like any device used at sea, speed logs are significantly affected by seawater. Regular cleaning improves accuracy, and with a tactical-grade IMU you may be able to navigate for hours without GNSS.

Used on land vehicles, this is the same method familiar from cars, where distance travelled is estimated from wheel rotation: the wheel turned this much, so I travelled this far, and that information feeds the navigation filter. Slip, mud and rough terrain directly affect performance.

A compass uses Earth's magnetic field to estimate your heading. It is an inexpensive method: for a few tens of dollars, good sensors give you a rough heading estimate. But where you mount a compass matters, and it needs platform-specific calibration. Quantum compasses have also begun to be used for magnetic anomaly detection.