What is an Inertial Navigation System? How does it work?

Jay Carr
  • What is an Inertial Navigation System? How does it work? Jay Carr

    In this question, the answer notes that some ADS-B systems are connected to Inertial Navigation Systems (INS), and says they are less accurate than GPS. What exactly is an INS?

    A couple sub questions:

    • Why is it less accurate?
    • The other post mentions they tend to develop biases as they get older, why is that?
    • Are they being phased out? Or are they used as backup?

  • Inertial Navigation Systems, unlike other navigation systems, do not depend on external (radio) measurements. Instead an INS keeps track of its position by accurately measuring acceleration (accelerometers) and rotation (gyroscopes). It therefore works in remote areas where there are no ground based navaids available.

    Initially, the INS gets its position from pilot input at the gate, or in more recent systems from GPS, sometimes even during flight. By measuring all the accelerations and rotations and integrating them into speed and direction the position is tracked. In doing this, the INS has to correct for the rotation of the earth and the related Coriolis force.

    Due to small inaccuracies in the measurements the velocity error and, with that, the position errors grow over time.

    Wikipedia has quite a detailed article about the history and working of the INS.

    While raw INS position has mainly been replaced by GPS measurements on oceanic flights, the system is not being phased out. GPS is a sensitive systems and by crosschecking it with INS errors can be detected in both systems. It is often used as a backup to GPS.

    Integrated GPS / INS systems provide better systems integrity and allow the navigation system to coast through short GPS outages with high accuracies. In addition to that, the (laser) gyroscopes of the INS are often used to provide data to the digital artificial horizon displays in the cockpit and to the auto flight systems. In this case the term Inertial Reference Unit (IRU) is used, which acts as a basis for the INS.

  • Inertial Navigation System it uses a planes motion as detected by on-board accelerometers and gyroscopes to find where it is going using no external reference points.

    A plane has a certain known heading and speed, when the planes makes a turn, hits a headwind,... accelerometers and gyroscopes will detect the change in motion and orientation and update accordingly. This means that initial accuracy depends on the how accurate the initial position speed and heading are when INS is initialized. The accelerometers and gyroscopes have limited granularity/accuracy and extrapolating your position from relative angle changes will compound an earlier error immensely.

    For example if the System is off by 1 degree on a turn then after 500 miles it will be off by (500*crd(1°)=500*2*sin(0.5°)=) 8.7 miles assuming all other information is correct. (this example is not fully correct as the gyros measure rate of turn rather than actual angle turned)

    INS is used primarily as a supplemental source of navigation data and as a fall back when other sources are unavailable.

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