IRS - Inertial Reference System
INS Inertial Navigation System
INS is the older type system
IRS is the newer system
Both pieces of kit do the same thing, by sensing the movement of the aircraft, and continuously calculating the aircraft's position, speed etc.
The systems sense movement according to the following:
INS uses mechanical gyros, which move physically within the system itself, and with the use of accelerometers. This suffers cumulative errors as detailed in previous messages.
IRS uses laser ring gyros, that have no moving parts, and sense the difference in the frequency of the laser beam reflected in the system, which in turn allows the computers to calculate, position, movement, speed etc. THE IRS system suffers LESS cumulative errors than INS - hence the difference in terminology - Hope this expains the pioint well!