Inertial Navigation System

The inertial navigation system is a system of aircraft navigation still used today in many large aircraft as the primary navigation system. Instead of using radio navigation aids or satellites, this system is entirely self-contained within the aircraft, computing the aircraft's position by sensing its acceleration and orientation. It, in essence, is a real accurate dead-reckoning computer.

There are several names for it, some with slightly different uses:

Inertial Navigation System (INS)

Inertial Reference System (IRS)

Inertial Navigation Unit (INU)

Inertial Reference Unit (IRU)

Inertial Measurement Unit (IMU, the name used by NASA in many cases -- look at the Pheonix lander now on Mars.)

Dead-reckoning is a navigation technique where you know where you started, what direction you flew, how fast you flew, and how long you flew. You can then, on a map, trace your position.[1] The inertial navigation system (abbreviated "INS") uses accelerometers to find your velocity and direction, and computers compile in the time and an entered start position to calculate your current position.

Two Types

There are two primary types of inertial navigation systems, gimballed and strapdown.[2]

Gimballed

Gimballed systems have a platform in the device that is mounted in gimbals. This device has 2 or more mechanical gyroscopes (not likely there are more than 3) that keep this platform level. On the platform, in addition to the gyroscopes, are usually three accelerometers, one in each direction. This was the earlier type of INS. It does not need accurate gyroscope orientation sensing, they only need mechanical gyroscopes to keep a platform level -- a much less demanding task for the gyroscopes. Additionally, since the accelerometers are already oriented (usually north/south, east/west, and up/down) the actual integration to obtain velocity and then position can be done by simpler, analog electronics.

However, they do have their disadvantages. One is reliability -- the spinning gyros and gimbals all move, so you have wear and tear and they can fail or lose their accuracy. However, a more prominant issue is with "gimbal lock."[3] This is when two of those three gimbals align. Since they both function about one axis, and the other only does one axis too, you only have two axes -- any rotation about the last axis cannot occur, so the platform is swung and misaligned with rotation about that axis. There are two main solutions: navigate around it or a fourth gimbal. On Apollo 11 there were only three gimbals, so they planned their maneuvers around gimbal lock. Their computers told them where not to go to keep two gimbals from aligning. The second, but more complex, solution is a fourth gimbal. This gimbal is motorized to keep it always oriented away from the other gimbals, so you keep three independent axes at all times. However, this is mechanically more complex. The fourth-gimbal system is used more in more recent gimballed inertial navigation systems.

References

Page under construction