Compass Terminology 101

There is so much terminology when talking about navigation products that it can be confusing. What is an Attitude Heading Reference System (AHRS) and how does it differ from an Inertial Navigation Systems (INS)? Some products have GPS integrated and others don’t.  Welcome to Compass Terminology 101. Let’s go over the basics of these terms and see what they all mean.

  • Accelerometer

Accelerometers measure proper acceleration – the rate at which the velocity of an object is changing. They measure the static (gravity) or dynamic (motion or vibration) acceleration forces of a given object.

  • Attitude Heading Reference System (AHRS)

An Attitude Heading Reference System, better known as an AHRS, is a 3-axis sensor system that provides real-time 3D attitude position – pitch, roll, and heading. The primary function then of an AHRS is to provide orientation data. AHRS are designed to replace traditional gyro-based instruments and to provide superior reliability and accuracy.

  • Digital Compass

This is the most basic compass product.  It usually contains tri-axial magnetometers and accelerometers and provides a measurement of orientation (or attitude). Output is usually heading, pitch, and roll. These devices are intended for stationary or low-dynamic application.

  • Inertial Navigation System (INS)

These devices combine accurate orientation (like an AHRS module) with a navigation algorithm.  The navigation algorithm integrates the tri-axial accelerometers to estimate position and velocity in 3-dimensions. Keep in mind that measurement noise will always cause errors to be integrated into the position and velocity estimates. These errors will grow unbounded over time and will need to be corrected periodically with a known position or set to zero for relative navigation.

  • GPS-Aided Inertial Navigation System (GPS/INS)

GPS refers to the satellite-based Global Positioning System that has become so frequently used today. GPS gives a direct measurement of position on Earth. By using this information in an inertial navigation system (INS), the module can periodically correct its location and minimize the integration errors in the position and velocity estimates. In some situations, the GPS signal can become blocked or interrupted (GPS deprived environment). In these situations, the GPS/INS module performs dead reckoning until the GPS signal is reacquired.

  • Gyro-Enhanced (or Gyro-Stabilized) Compass

This is similar to the digital compass but also contains tri-axial gyroscopic sensors (also known as angular rate sensors). These devices also provide a measurement of orientation/attitude and output heading, pitch, and roll. They are intended for stationary to high-dynamic applications. Accuracy over temperature is typically less than what is available in an AHRS device.

  • Kalman Filter

The Kalman filter was developed by Rudolph Kalman in the 1960s. The Kalman filter is an algorithm that uses a series of measurements over time, even those containing “noise” and other errors, and produces estimates that tend be more accurate than those based on a single measurement.

  • Magnetometer

A magnetometer measures magnetism. Specifically in navigation, a magnetometer measures the direction of an ambient magnetic field.

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