Curiosity: Navigation in Space
I was watching Apollo 13 (Tom Hanks, Kevin Bacon, Bill Paxton) for the 20th time recently. It’s an amazing story of triumph in the face of crippling catastrophe.
However, one scene in particular piqued my curiosity. The Command Module’s oxygen tanks had just blown up and the inertia from the venting gas is throwing the ship around in circles. In Houston at Mission Control, the Guidance, Navigation, and Controls Systems Engineer (GNC) says to the Flight Director, “They’re all over the place. They keep going close to gimbal lock”.
The word “gimbal” comes up a few more times in the film, often as a central conflict between crew members (“I’m going to need your gimbal angles, Jack! Before you shut down the computer!” -Lovell). It also gets a few close-ups on camera. If you don’t know what it is, it looks like this:
As I wondered why this was so important, I assumed it was how they navigated. Then I thought, “How do astronauts navigate in space?! It’s not like they have a compass. How do gimbals work?”
So here I am.
After Googling “How to navigate in space” and stumbling through some inconclusive links and blogs, I eventually came across what I was looking for: A wiki article explaining a “Inertial navigation system” and how it works.
It works based off of the technique of dead reckoning. After getting an initial position, orientation, and velocity (speed AND direction), a computer uses motion sensors (accelerometers) and rotation sensors (gyroscopes) to measure the position, velocity, and orientation of the vehicle continuously. It’s kinda like if you were to blindfold someone and lead them around, they would be able to tell how far they walked and maybe where they were in relation to where they started; only, these instruments are WAY more accurate.
This gyroscope is the “gimbal” that they are referring to in the film, which helps them figure out which direction they are pointing. The “gimbal lock” they are referring to is when 1 of the 3 axis of the gyroscope is restricted from moving (because your ship is spinning around so much?), so (I think) it can’t give any data in 1 axis. Which is a problem if you need it to know where you are.
I think it’s called “dead reckoning” because you don’t need any external measurements or references to tell where you are and where you’re heading. That way, no other data needs to be taken. Usually, the navigation data (speed, position, orientation) is checked regularly throughout the journey. But, if accurate initial data is taken, it can be surprisingly accurate. However, if the initial data is off, then each subsequent dead reckoning will be off more and more.
Overall, it is used to give pretty accurate navigation data, but they also check their position with stars manually if they need to make a correct.ion.
Note: This is WAY more complicated than this, and they use a ton of other instruments and algorithms, etc. But this is the basic theory behind it. They also use it on submarines and they us it every few hundred miles on planes to check their wind drift and stuff.