Most of the above is correct but let's just clear up a few definitions.
In the acronyms IRS and INS the "S" is for "System" These are inertial systems that contribute to the aircraft navigation in assorted ways. An IRS uses an "IRU" where the "U" is for Unit. The same applies to an "INU" - the unit at the core of an inertial navigation system. An IRS may be an INS if it includes a navigational computer. More about this later...
"Reference" in the inertial reference system applies to the fact that the system uses inertial means to produce the aircraft roll, pitch and yaw alignment references used for the attitude and directional instruments. There is then no need for vertical gyros, directional gyros or magnetic compass systems.
"Navigation" in the inertial navigation system refers to the fact that the system uses the inertial data to compute the aircraft position and provide other outputs such as groundspeed or track.
Older INS systems had a mechanical inertial navigation unit, using mechanical gyros to keep an inertial platform horizontal and referenced to true north. Accelerometers on the platform sensed acceleration along the aircraft axes and a digital computer used the accelerometer outputs to calculate the navigation parameters and provide outputs to display them.
Newer Inertial Reference Systems use sensors, usually "Laser Ring Gyros" to measure the rotational rates around the aircraft axes. A digital computer uses these rotational rates to calculate the aircraft attitude. The crew insert the current location at start-up and the computer resolves the earth rates sensed by the gyros to deduce the aircraft alignment in inertial space. Once alignment is complete the system provides constant monitoring of aircraft attitude in roll, pitch and yaw as well as true heading. A database of the earth's magnetic variations then allows the system to calculate and display magnetic heading. The platform with the laser gyros and navigation accelerometers is bolted to the airframe and doesn't move (which is why such systems were once referred to as "strap-down" systems.) The inertial position is calculated by comparing the attitude data and the linear accelerations along the aircraft axes. Inertial Reference Systems are generally linked to a Flight Management System (FMS) to perform navigation, with the FMS constantly monitoring nearby DME stations to provide automatic position updates to the inertial position calculated by the IRS.
Honeywell built a "LaserNav" system that provided navigation data through a Control and Display Unit (CDU) that could accept enough waypoints to fly a long sector without reprogramming. It was thus an Inertial Navigation System with a built-in Laser Inertial Reference Unit providing the inertial data. No FMC was required. I assume that the LaserNav System is still in production.
So, an IRS provides attitude and direction display data by inertial means and provides inertial position for navigational use, either through an FMC or by its own built in computer/CDU interface.
An INS uses inertial means to calculate and display navigational data and can navigate an aircraft through a set of waypoints entered by the crew. It may or may not provide attitude and directional information too; an IRS may thus be an INS if it has its own navigational computer and CDU.
Hope that helps with the differences between INS and IRS...
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Through difficulties to the cinema
[This message has been edited by Blacksheep (edited 23 April 2001).]