Like everything, inertial navigation has its strengths and weaknesses. While inertial navigation systems are undoubted good at measuring position, orientation and dynamics, the one Achilles heel of basic un-aided inertial navigation systems is drift. Un-aided means systems that only use accelerometer and gyro measurements to calculate their position. Drift is the term used to describe the accumulation of small errors in the accelerometer and gyro measurements, which gradually cause the INS position estimate to become more and more inaccurate.
Understanding why drift occurs is quite easy. Imagine measuring a length of wood with a single 5 m-long tape measure. If you can read the divisions on the tape to an accuracy of 1 mm, then you could easily declare that this piece of wood is 4 m long ± 1 mm. If on the other hand the only tape measure you could find was 0.5 metres long, and you can still only read it to an accuracy of 1 mm, then by the time you have measured and moved along with the tape measure eight times, you would only be able to say that the wood is 4 m long ± 8 mm. In fact, you might not make it 4 m at all.
Drift in the INS accumulates in the same way. Each time an accelerometer or gyro is read, there is a minuscule error in the reading. If we were just taking a single reading to work out how fast we were accelerating or turning, this wouldn’t be a problem. But because the navigation computer is adding up each measurement to work out how it has moved on from the previous position estimate, the minuscule error grows with time.
How drift can accumulate in un-aided INS
This diagram gives a very simplified view of how drift can accumulate in an un-aided inertial navigation system. A great deal of work has been put into systems that minimise the accumulation of these errors, but there’s no getting away from the fact they are there. That does not, however, mean that the principle of inertial navigation systems is useless—or that it’s inferior to GPS for example. Far from it.
At the beginning of this page, we said that drift was the Achilles heel of un-aided inertial navigation systems—so what about aided ones? When you combine an INS with GPS to create a GPS-aided INS (also written as GPS+INS), you solve the problem of drift and also solve the problems that affect GPS too. This is why OxTS can provide customers with the ability to measure with confidence – by providing a complete solution to measure position, orientation and dynamics in all environments. We discuss this in detail in our article ‘What is GNSS?’.
This is one of a series of articles in our ‘What is an inertial navigation system?‘ series.