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"?