INS: Frames of reference

Industry Articles October 5, 2020

In our page on how does an INS actually work? we mentioned that an inertial navigation system comprises two-distinct parts; the first is the IMU (inertial measurement unit)—sometimes called the IRU (inertial reference unit). Here we’ll explain what terms like IMU frame mean.

In the above-mentioned page, we talked about a positive measurement on the z-axis accelerometer when the product accelerates down, but what happens if we flip the INS so it’s oriented, as shown in this image? Now when the INS accelerates down, the z-axis would register a negative value. This is why frames of reference are important.

To both you and I, up means ‘above us’ and down means ‘below us’. Equally, we both know which way is right and left, and if I said I moved forwards one metre, you can picture exactly what I’ve done—because you’ve put yourself into my frame of reference. As humans, we’re very good at doing that. In fact, we find it so easy to ‘see things from another point of view’ that it’s easy to forget each object has its own frame of reference, and how we describe movement depends on the frame of reference being used.

For example, picture yourself stood on a train platform. Imagine you can see someone stood inside the train carriage looking for a seat. As the train pulls out of the station the person starts to walk towards the rear of the train. From the person’s point of view, they are walking forwards at a constant speed. But to you, looking into the train from the platform’s reference frame, the person initially appears not to be moving—because they’re walking forwards (in their frame) at the same speed as the train is moving forward in its frame. As the train gets faster, however, the person will appear to move in the direction of the train’s travel. To you, they are moving backwards, but to the person on the train, they are still moving forward. Both points of view are correct, they’re just using different frames of reference.

Luckily most inertial navigation systems are smart enough to be able to convert movement from one frame of reference to another – as long as they’re given a little bit of information to begin with. So imagine holding an INS upside down, so the z-axis points up. As long as we tell the INS it’s being held upside down before we start, then when you move your hand upwards the inertial measurement unit will register a positive value on the z-axis (which points down as far as it’s concerned) – but the inertial navigation system (the computer) knows it’s actually being held upside down in your hand. So it spins all the measurements around and puts them into a different reference frame that makes sense to us; one that says the INS is moving upwards.

This is one of a series of articles in our ‘What is an inertial navigation system?‘ series.  To fully understand how an inertial navigation system works, you also need to know about the sensor types used in most inertial navigation systems –  accelerometers and gyros.