Basically the IRS is an upgrade of the INS, instead of using the gyros and accelerometers to align the platform that they are based on, it basically calculates how far out of alignment said platform is and inputs it into the database. The advantages of the ring laser gyro allow the IRS to have a shorter "spin up" time and a longer life. You still have to input a latitude for the system to work out the earth rate of the gyros.