# Why does Boeing 737 use 2 Inertial Reference Systems (IRS) and GPS?

According to this website the Boeing 737 aircraft use 2 IRS and 2 GPS: IRS L & IRS R. Why? Apparently they are totally independent of each other.

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

Follow-up question: this same website states: "POS INIT is used to enter the aircraft position into the IRS's for alignment". What is meant by "alignment"?

• Related question Mar 9, 2017 at 11:25
• For the same reason they use two engines: Redundancy. Aug 2, 2017 at 20:35
• Or at least two of everything, and everyone, too. Jan 30, 2018 at 22:49

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

So that if one fails, the other can still be used to complete the flight.

Airbus even have 3.

What is meant by "alignment"?

Initialization of the system and solving the "heading problem" through a magnetometer. But this is a different question.

• Thx for your answer. But if it is only for redundancy and they do not communicate amongs eacht other. How do they compensate the differences in measurements of both systems? 3 isn't that overkill? Mar 9, 2017 at 9:42
• @traducerad there is usually a third system that performs what is called "voting and monitoring"
– Federico
Mar 9, 2017 at 9:43
• @traducerad, The use of 3 identical systems is what is known as triple redundancy. Let's say you have only have 2 systems, and 1 of them begins to show different readings than the other. How do you know which to believe? With 3 systems in total, if 1 begins to show different numbers, you will know that the 2 other ones are more than likely the "correct" ones. Mar 9, 2017 at 14:46
• @traducerad - redundancy is what keeps the plane safe should something go wrong with a single system. I'm sure that manufacturers & airlines would love to do away with redundancy because it adds weight, complexity and cost which take away from cargo capacity, range and profit. However, they prefer to not have the bad publicity of planes falling out of the sky due to a failure in a single system. There are probably many single-system failures every day that have essentially zero impact on a flight arriving safely due to the redundant systems. Mar 9, 2017 at 15:12
• @Federico, no, it is easiest to solve at Equator, because there the axis of rotation lies in the local horizontal plane. The methods are actually both used, because magnetometer gives magnetic heading, and that is used for heading indicator, while the gyroscopic method (gyrocompass is a gyro with attached weight so the axis is horizontal, but I expect IRS to simply spin up all axis gyros, measure the rotation and align the coordinate system accordingly) gives true heading, which is needed by the inertial navigation system. Aug 2, 2017 at 22:09

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

Because airliners falling out of the sky or disappearing without a trace is not acceptable from either a political or financial standpoint. Stuff fails and so backups are needed to keep the flight on-track when it does.

Having two systems of each type makes it easier to automatically detect failures, having four systems total means there is something to fall back on when a a system fails.

Follow-up question: this same website states: "POS INIT is used to enter the aircraft position into the IRS's for alignment". What is meant by "alignment"?

Inertial navigation relies on a double integral. The sensors measure acceleration, the acceleration must be integrated numerically to get velocity, and velocity must be integrated numerically to get position. Gravity and rotation complicate the picture even more.

The trouble with integrals and particularly double integrals is they tend to compound errors over time. A constant error in acceleration translates to a linearly growing error in velocity, which translates to a quadratically growing error in position.

So, to maintain acceptable accuracy, intertieral naviation systems must be frequently re-set to the aircraft's real position while the aircraft is staionary on the ground.