Inertial Navigation System

Overview

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.

Strapdown

Strapdown systems have all their sensors mounted on a platform that changes orientation like the plane. Instead of mechanical gyros to hold it level, it has three more accurate gyros that sense the orientation of the system. Additionally, it has the same three acceleration sensors.

Whereas the gimballed system just senses the orientation of the platform to get the aircraft's attitude, the strapdown systems have three gyroscopes that sense the rate of roll, pitch, and yaw. It integrates them to get the orientation, then calculates the acceleration in each of the same axes as the gimballed system.

Due to the sensing of the rate of rotation, rather than just holding a platform level, very accurate and sensitive gyroscopes are needed. There are several sensitive gyroscope types available, but the most commonly used on is the ring laser gyro. This basically is a circular path (actually, usually triangular) that laser light travels around. This light goes both ways. When it is rotated, one direction appears to go faster than the other -- and when they get around and meet, they interfere. This creates a pattern that is picked up by sensors. These gyroscopes are so accurate the alignment for strapdown systems consists of finding true north by sensing the earth's rotation.

Strapdown systems have fewer moving parts, so they are more reliable and simpler than other systems. However, they do need more accurate gyroscopes and better computers, so they are a more recent development.

FlightGear's implementation

There is a simulation of an INS being developed for FlightGear. It is discussed in the following thread: [4], titled "INS Systems (from Unlimited Wishlist.)"

The implementation is still in development.

Done

• Basic class layout and code is working
• First XML support (will be replaced with PropertyList format)
• Lat/lon integration, very basic work
• Most properties for input/output supported

Todo

1. Convert to the PropertyList XML format
2. Fake alignment (using a timer)
3. Multiple unit mixing
4. Other small tweaks
5. Ready to use in an aircraft
6. Convert integration over to quaternions, add orientation integration.
7. Fake a drift model (a better fake than the alignment...
8. Add a faked radio updating system
9. More realistic alignment simulation (gimballed/strapdown differences, etc...)

References

(Sources linked to above)

1. A good reference on aircraft navigation
2. Detailed overview of INS systems
3. Information on the IMU on Apollo 11 and gimbal lock
4. Thread on the forum

Additional:

• [5] Wikipedia article on inertial navigation systems
• [6] everything2.com has some more information
• [7] Another source of good INS information and the sources of drift.
• [8] Some information about the F20's INS
• [9] Some information about INS, GPS, and a system for taxiing awareness using GPS (probably INS/GPS, actually.)

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