NASHUA, N.H. — Guided missiles use navigation, guidance and control systems to reach a target. Each handles a different part of the flight. Navigation systems track the missile’s position and movement. Guidance systems use that information to calculate a flight path, while flight controls steer the missile along that path.

The equipment varies with the weapon and its mission. Inertial navigation systems (INS) can track movement throughout the flight, while GPS or external platforms can provide position updates. Missiles designed to engage moving targets may also use onboard seekers during the later stages of flight.

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How does a missile determine its position?

Many guided missiles use an INS to track their movement without continuously relying on signals from outside the vehicle. An inertial measurement unit (IMU) provides the measurements needed for that process. Accelerometers measure changes in motion along different axes, while gyroscopes measure rotation. A navigation computer processes those measurements to estimate the missile's position, velocity and orientation as it moves.

Inertial navigation has an important advantage for military systems because it operates independently after initialization. It does not need to transmit a signal or maintain a continuous radio link with an external source.

Its weakness is drift. Small errors in inertial measurements accumulate as the navigation computer repeatedly calculates changes in position and velocity. The longer the system operates without another source of position information, the larger those errors can become.

Some missiles combine inertial navigation with GPS or another global navigation satellite system. Satellite navigation can provide position information that helps correct accumulated inertial errors during flight.

What does the guidance system do?

The guidance system compares the missile's current state with the trajectory needed to reach its intended destination. It can then calculate corrections when the missile moves away from that path or when the target's position changes.

For a missile flying toward a fixed location, the guidance computer may use stored coordinates along with navigation data to keep the weapon on its planned route. A system intended to engage a moving aircraft, ship or vehicle faces a different problem because simply flying toward the target's previous position may not result in an intercept.

Guidance calculations can account for the relative motion of the missile and target. The system continually updates the required flight path as new information becomes available.

That information does not always come from the missile itself. Depending on the system, targeting or position updates can also come from aircraft, ships, ground systems or other sensors over a data link.

How does a missile find a moving target?

Some guided missiles use a seeker to detect or track the target during flight. Seekers can operate using several types of sensing technology. The type of seeker depends on the target and operating environment. Radar and infrared seekers can provide information directly to the missile, while laser-guided weapons rely on reflected laser energy from an illuminated target.

The INS estimates the missile's own movement and position, while the seeker tracks the target relative to the missile. Inertial and satellite navigation can bring the missile toward the expected target area, while a seeker can provide updated target information during the final portion of the flight.

Not every guided missile follows that arrangement, though. The sensors and guidance methods vary with the weapon and its mission.

How does a missile change direction?

Once the guidance system determines that the missile needs to change its trajectory, the control system has to make that correction happen. Flight-control computers turn guidance commands into instructions for the missile's control hardware. Depending on the design, that hardware can include movable fins or other aerodynamic control surfaces. Some missiles can also change the direction of engine thrust to alter their flight path.

Actuators physically move the control surfaces to the commanded positions. Sensors then continue measuring the missile's motion, allowing the flight-control system to determine how the vehicle responded.

The process forms a continuous feedback loop. Navigation provides information about the missile's movement, guidance determines the required trajectory and control systems make the physical adjustments needed to follow it. New measurements then feed back into the system for the next calculation.

What happens when GPS is unavailable?

If GPS is lost, a missile equipped with an inertial navigation system can still continue estimating its movement without satellite signals. The challenge is maintaining accuracy as inertial errors accumulate. Systems can use other sources of information to limit those errors, including terrain or scene matching, external updates or onboard sensors that compare observed features with stored information.

A missile designed to operate in an environment with heavy electronic warfare cannot assume that every external signal will remain available throughout the flight.