Heading, Course, and Crab Angle

What's the Difference?

The terms "heading" and "course" are often used interchangeably to describe the direction something is moving. In common aerospace terms, however, they actually mean different things. In this article, we'll briefly discuss the differences.

Heading (yaw) is used to describe the direction an object is pointing. In contrast, the course angle refers to the direction an object is actually moving. Suppose, for example, that we have a sensor mounted to a helicopter. Because a helicopter can move in any direction - not just the direction it is pointing - heading and course can be very different. A helicopter pointing east (heading = 90 degrees) and banking left is moving north (course = 0 degrees).

Heading Vs Course1

 

The difference between course and heading is called the crab angle, or side-slip angle. On an airplane, wind often pushes the airplane sideways, so that the direction it is pointed is different from the direction it is actually moving. An airplane is "crabbing" if this is happening.

Because course and heading are different, measuring heading can be difficult even if you have GPS, because GPS measures the direction that you are moving, not where you are pointed. Magnetometers are often used to measure heading directly by observing the Earth's magnetic field - this is the approach used by the UM7, the UM7-LT, and (optionally) the GP9.

The downside of using a magnetometer is that unpredictable distortions in the Earth's magnetic field can cause arbitrarily large heading measurement errors. Calibrating a magnetometer to compensate for large distortions near the sensor can also be difficult. For this reason, it is best to avoid relying on a magnetometer whenever possible. The GP9 is a GPS-Aided AHRS that uses GPS and a novel filter to figure out its heading without relying on the magnetometer. The GP9 also uses its onboard GPS to make its attitude and heading estimates robust even on fast-moving, dynamic platforms like airplanes and helicopters.