Life before GPS
Because even kitchen appliances seem to have GPS receivers fitted these days, it’s hard to imagine life before GPS existed. How did people cope? Is it even possible to drive a car without some disembodied voice telling you to turn left, right and make a U-turn when possible? For a generation raised on sat navs and smartphones, this is a serious question—how did people actually work out where they were?
The truth is, before GPS, if the average person wanted to know where they were, they had to resort to using maps, observations and wildly inaccurate assumptions. And, if the situation got really bad, they asked for directions. However, for individuals and industries with lots of money, plenty of space and a genuine need to know where they were, their salvation frequently came in the form of an inertial navigation system (INS).
What is an inertial navigation system?
Inertial navigation systems come in all shapes and sizes. One thing they have in common though is their use of multiple inertial sensors, and some form of central processing unit to keep track of the measurements coming from those sensors. The sensors an INS uses are typically gyros and accelerometers—and there are normally several of each inside. We’ll look at how an INS actually works in a moment, for now, the most important thing to realise is how they differ from GPS—which you’re probably more familiar with.
Switch on a GPS receiver and, assuming everything works correctly, after a short time it will generate a position measurement. Ignoring the inaccuracies GPS has, the position measurement the receiver generates is quite specific. It says ‘you are at this latitude and this longitude‘—in other words, it gives us an absolute position using a known co-ordinate system. Inertial navigation systems don’t work like that. In their case, the measurement they generate is relative to their last known position. So even after an inertial navigation system has been turned on for several minutes, it can’t say ‘you are at this latitude and this longitude‘, but what it can say is, ‘you haven’t moved from where you started‘.
So why do people use inertial navigation systems at all? If they can’t tell you where you are, how were they able to navigate man to the moon, why don’t submarines crash all the time and how do aeroplanes and missiles find their way? Thankfully the answer to these question is simple. An inertial navigation system works out where it is in relation to where it started—so if you tell the INS where it started, it can easily work out where it is now, based on its own measurements. That is how spaceships, submarines, aircraft and missiles all successfully navigate using an INS—because they know where they started from.
To fully understand inertial navigation systems, its useful to know about INS frames of reference, so that you can accurately interpret the values registered on your x-, y- and z-axis. You’ll also want to know more about the sensor types used in most inertial navigation systems – accelerometers and gyros. To understand how an inertial navigation system keeps track of where it is in three-dimensional space, you also need to know about dead reckoning. And then to appreciate the strengths and weaknesses of INS, you also need to know about drift.
INS: Frames of reference
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.