What does inertial navigation mean?

Navigation actually solves the problem of where to go. We can always think of a compass for this.

But there is a classic joke. A man got lost with a compass: "I know where North is, but where am I?"

Therefore, to complete navigation, we need to know where I am, where the north is, and if there is a destination, we need to know where the destination is, so as to tell the user the way to the destination. Among them, where I am is very important.

The floor is paved with square bricks. You know which brick you are on from the beginning, then take three steps to the left, five steps forward, four steps backward, four steps backward, two steps left, and so on. Each step is the length of a brick.

Tell this to someone who is not in the room. He draws on a piece of paper and knows which brick you should be on and where you are facing without looking at you.

This is the basic principle of inertial navigation and other navigation methods.

You know your initial position, your initial position (posture), how you change your position at every moment, and how you walk relative to it at every moment. Add these together and continue to push step by step. Without considering all kinds of errors, the result should be exactly your current position and position.

But how do you know how your direction and position have changed? Different navigation systems use different sensors and different methods. For example, the odometer uses the revolutions of the wheels on the vehicle, and the Doppler log emits sound waves to the bottom of the water like a bat ... and inertial navigation is called inertial navigation because it uses inertial devices, namely accelerometers and gyroscopes.

Accelerometers use the principle of a=F/M to measure acceleration and "inertia force" of objects.

Gyroscope measures angular velocity, which is a device that I personally find very interesting. When I first realized its principle, I was very surprised.

If you put a gyro on the table and gently push the upper part of its shaft, it will fall off; But if you turn the top around and stand on the table, and then push it like this, it will stagger vertically forward, as if something had stopped the top from falling.

The same principle can also explain why once a bicycle is ridden, it is not as easy to fall down as slowing down or standing in place.

Regarding the principle of gyroscope, you can watch the video of Shenshi Space Teaching:

Shenshi Space Teaching: Gyro Rocking Forward Video

In this way, we have the basic gyroscope and accelerometer, and we know the initial position. Can we get their data safely, and then integrate and push them to get jobs?

But wait, why are inertial devices called inertial devices? Because it outputs data in relative inertial space. On the earth, it can be roughly considered as outputting data in a relative universe.

What is this concept? Don't forget, the earth is round and still spinning!

When we navigate, we need data relative to the east, north and sky.

This is easy to understand. If you don't do this, but directly use the data of the relative universe to see the navigation output, you will stand still here. Twelve hours later, the navigator will tell you that you are "down" (actually, it depends on the latitude where you are standing, not necessarily with your head down), which will confuse users.

In terms of displacement, the displacement data relative to the universe will truly reflect the rotation of the earth. The earth is really sitting on the ground and walking 80,000 miles a day. You just want to know how much you have gone east and how much you have gone north. Where are you going now and where are you going next?

So we need to convert the data of inertial system into the data of navigation system (generally geographic system, that is, the northeast sky), that is, we need to subtract the angle change caused by the rotation of the earth and the latitude and longitude change on the earth. This process is realized by the physical platform that always tracks its position in the northeast sky in platform inertial navigation and a series of formulas and calculations in strapdown inertial navigation.

Whether it is a physical platform or a mathematical platform, when you have this platform, you can first determine the initial position, velocity and attitude, and then add the output integral of inertial devices step by step to get the position, velocity and attitude information of the carrier. Of course, if we really do this, we will face many new problems and need to solve them one by one.

The above is my summary of inertial navigation principle [