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 &amp; 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"?
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.