Inertial Navigation Systems

What is an INS?

An inertial navigation system (INS) consists of at least three accelerometers, three gyroscopes, and a computer. The accelerometers measure the acceleration of the helicopter longitudinally, laterally and vertically. The gyros measure the angular rotation of the helicopter about three perpendicular axes. From these values, and an initial known state, the INS computer estimates position, velocity, and attitude (pitch, roll, and yaw) of a helicopter.

Since the INS only measures acceleration (change in velocity) and rotation (change in orientation), it cannot estimate the actual position, speed, or orientation of a helicopter without an initial known state. INS units are typically combined with GPS and other sensors like magnetometers to form an EGI—Embedded GPS/INS system. This system combines the position measurement of the GPS with the INS tracking between GPS updates.

What's INS used for in a Helicopter?

The INS system provides values to (1) the flight control system, (2) the pilot displays, (3) navigation system / maps, and (4) weapon aiming solutions (military helicopters). We’ll discuss what INS does for these systems below.

The Flight Control System

Most modern helicopters have a flight control system (FCS) or at least a SCAS. These systems can completely or partially control the helicopter to e.g. hold speed, hold hover position, or hold roll angle. While a GPS can partially help with such tasks, the lower update rate, poor resolution, and lack of acceleration cues cause the helicopter to drift around the target (e.g. hover position). With INS, these systems are much more precise.

Pilot Displays

Screens in front of the pilot(s) indicate the current aircraft position, orientation (pitch, roll, yaw), ground speed, climb rate, and turn rate, among other things. These values are critical for the pilot to fly the helicopter, and they are derived partially or completely from the INS.

Sensor fusion is used for many of these values. This means a computer combines INS outputs with values from other systems to create the value shown to the pilot. For example, climb rate often uses a fusion of both the vertical speed estimated by the INS and the change in air pressure measured by an air data system.

Navigation and Maps

The navigation system and maps displayed to the pilot primarily rely on GPS. However, the INS is used to smooth/correct the GPS position and to completely provide position when GPS inputs are temporarily lost.

Weapon Systems

Military helicopters often fire both guided and unguided munitions. All of these munitions start with the velocity of the helicopter when fired. If the helicopter is not flying straight toward the target, then the munition should not be aimed directly at the target. For example, if an AH-1Z Cobra is climbing at 1,000 ft/min when firing its cannon, the bullets will also be climbing 1,000 ft/min initially. If it was aimed at a target straight in front of it, the bullets would miss and fly above that target. In this case, outputs from the INS may be used to point the cannon slightly downward, offset from the direction to the target.

For an unguided rocket, the helicopter can only fire it straight out of the pod from which it’s launched. It can’t aim it in a different direction as we just described with the cannon. In this case, the aiming cue in the pilot’s helmet mounted display already includes the offset. In the prior example, the cue would already indicate that the rocket will travel upward and will be centered on targets above the orientation of the rocket itself.

For guided munitions, like a Hellfire missile, the INS or EGI often provide the initial velocity/orientation to the missile. The missile’s navigation system uses this information for steering, until other mechanisms take over, like laser seeking.

Why not GPS?

There are many reasons why an INS is useful, even when a GPS receiver is present. For one, the INS can provide better real-time estimates of speed and acceleration than GPS. A GPS provides granular, infrequent measurements of position that can easily have 10+ feet of error. Deriving speed and acceleration from this will have substantial error, in addition to being delayed.

For military helicopters, GPS cannot be relied on. GPS receivers work by receiving radio signals from GPS satellites. Enemies can “jam” the GPS by broadcasting similar signals toward the helicopter’s receiver. While there are ways to overcome some jamming attempts, there’s an active “arms race” around this. Attackers iteratively improve jamming/spoofing while defenders add anti‑jam/anti‑spoof and alternative navigation options (including INS).

INS Technology

Modern Mechanisms

Modern helicopters use three perpendicularly-mounted accelerometers to measure linear acceleration in the longitudinal, lateral, and vertical directions. There are two main types of accelerometers used.

One type of accelerometer, called a force-rebalance accelerometer, uses a motor to hold a mass in a fixed position. The force required to hold the mass is proportional to the acceleration in the given direction (plus gravitational acceleration vertically). The accelerometer measures the force and outputs the proportional acceleration.

Another type, which is often cheaper but suffers from higher bias, is a micro-electro-mechanical system (MEMS). These systems are often used in smartphones, cars, drones, … as they are cheap, small, and robust. A semiconductor (chip) houses a very small mechanical structure (often a little mass suspended by microscopic springs or beams). When the helicopter accelerates, this mass moves slightly relative to the chip (inertia). This movement changes the electrical properties in a way that can be measured. The electronics on the chip convert those tiny mechanical shifts into an electrical signal that corresponds to acceleration along a particular axis.

Quantum Mechanisms

DARPA is investigating new INS technologies within the robust quantum sensors program (RoQS). Using some highly quantum effects, it may be possible to make an INS with so little error that GPS jamming/spoofing are no longer a problem.

A couple of the companies reportedly working on this program are Q-CTRL and Safran Federal System. Let’s discuss a couple of the approaches used in this program.

Cold-atom interferometry is one of the approaches. Lasers shine on a cloud of atoms from multiple directions to slow their otherwise chaotic, thermal motion. Once the atoms are cooled to near absolute zero, subsequent laser pulses split them into a quantum superposition of two paths, and later recombines those paths.

After the atoms recombine, they create an interference pattern that's extremely sensitive to the motion of the helicopter when they were in superposition. By measuring this interference pattern, the device extracts fine details of the helicopter’s acceleration. Like all INS technology, this may be difficult to implement on a helicopter due to the combination of high vibration and extreme limits on space, weight and power consumption. If they overcome this, researchers believe this could drive an INS with orders of magnitude lower error than existing technology.

Another approach uses diamonds with "nitrogen vacancies." Although originally aimed at detecting rotation, this approach may also be a good measure of linear acceleration. An ideal diamond is a lattice of carbon atoms. One type of defect in a diamond occurs when two adjacent “slots” (where carbon atoms would normally reside), are replaced by a nitrogen atom and a vacant slot—a so called nitrogen-vacancy center (NV center). Diamonds can be engineered to have trillions of these NV centers.

Electrons "orbiting" this defect have well-defined states that can be set and measured with lasers. Furthermore, the electrons transition between these states predictably with strain/stress/rotation of the diamond. Hence measuring these states can reveal fine details about the acceleration and rotation of the helicopter.

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