Measuring Motion

Today we take it completely for granted that we know where we are at all possible times and that's thanks to GPS technology that's embedded into all sorts of devices; it's in my phone, it's in navigation systems in cars and so on, but it wasn’t always like this. GPS is actually a fairly recent phenomenon.

GPS works by a large constellation of satellites, 24 satellites in space, sending down radio signals which are received by the little GPS receiver chips embedded into all these devices.

Now, these satellites are very, very complex they contain atomic clocks and they are absolutely massive. Now, this technology was originally developed by the US military to help them work out where all their assets and where all their soldiers were at any particular point in time, really key ability in the business of war fighting to know where everything is. It’s also particularly useful for civilians and driving and so on, but it was not the first application of GPS; it was strictly military.

Now, in the year 2000, President Clinton signed an order which made the high accuracy GPS capability available to everybody on the planet. Previously, the high accuracy was only available to military users and for everybody else the service was deliberately degraded, so it told you roughly where you were, but with an accuracy of 100 meters or something like that.

So, this was a revolution and the fact that he made this order, turned off this deliberate dilution of GPS accuracy is what’s enabled this great revolution in location based services that today we take for granted. So, it is a fairly recent phenomenon, important to keep that in mind.

Now, other countries who don’t want to be reliant on the US-based GPS system, create their own satellite based navigation systems and Europe, Russia, and China are all putting in place or have in place alternatives to the GPS system. So, there are lots of satellites up there providing navigation signals that allow receivers on the ground to know exactly where they are.

Now, a long time ago before there was GPS was the problem with how did you know where you were and this was a particularly important problem during the Cold War. How do you send your nuclear bombers to another country to deliver pay loads if you can’t work out where they are, particularly accurately, and it’s also a real problem under water because these GPS signals don’t penetrate water.

So, if you’re a submarine under the water surface, working out where you are is quite complex; you certainly can’t rely on GPS navigation. If you want to go to the moon and we did go to the moon, sent men to the moon a long time ago, how do you navigate around the moon where there is no GPS system? And back then, there was no GPS system around planet Earth and certainly there was no GPS navigation system around the moon.

So, the way you do that is what’s called inertial navigation, and this is a technique that was pioneered by this gentleman, Charles Stark Draper, back in the 1950s and the 1960s, and he was in fact declared Time Man of the Year in 1961 for his contributions to engineering knowledge, particularly this inertial navigation technology.

So, inertial navigation technology relies on measuring continuously the acceleration of the body, so if I had the sensors on me that measured my acceleration and other sensors called gyroscopes which measure my rotation. By processing the accelerating signal and the rotation signal, combining them with some pretty clever algorithms, I can work out at any point in time where I am. This technology was pioneered, was driven by the Cold War that existed at that particular point in time.

This is the first inertial navigation system; it was a massive piece of kit and it was able to a guide bomber from the east coast to the west coast of the United States, very, very big piece of equipment, and there was an underwater version of this which was used to accurately navigate submarines for long periods of time underwater, so they could accurately get from one place to another just using this navigation technology.

Another instance of this inertial navigation technology is what got the Apollo space craft to the moon, so that they’re able to miniaturize this massive inertial navigation system and shrink it down to a box that was about one foot, cubed, and that’s what got the vehicle to the orbit around the moon and also got the vehicle then from the lunar orbit down on to the surface of the moon and back up again, and this was in 1968.

So, phenomenal technological progress from an idea to a massive sensor through to a relatively small sensor that could be used for landing men on the moon.

Today inertial navigation system is almost a commodity. It’s reduced to the size of a single chip, and these inertial navigation chips, which contain accelerometers and gyroscopes are embedded into devices like our phones; this is how our phone knows whether it’s in portrait mode or in landscape mode. It’s how we use it to play games just by moving the phone around, these are all done with inertial navigation technology. Smart toys, like this quadrotor here, use inertial navigation sensors to keep the vehicle balanced in space and to keep it stable.

They’re used in many, many, many, many technologies and many, many applications. So, in this particular lecture, we’re going to talk about how inertial measurement systems work.