On the MD-80 there are 2 compass systems driven by flux valves. These provide outputs to the RMIs, HSIs, VORs and DFGCs.
The AHRS also gets a heading reference from the Compass 1 or 2 system - then the AHRS becomes a highly accurate heading and attitude reference using 2 gyros and 3 accelerometers. There are 2 AHRS units in each aircraft.
Newer IRS units generally get their references internally by measuring the earth's movement over a period of time during initial start up and computing the location by calculating the minute lateral accelerations, which would vary predictably according to your lat/lon. Heading inputs assist in speeding the initial calculations, but are not required for NAV use with an IRS. These modern IRSs take a few minutes to initialize (longer at higher latitudes) - while the MD-80 AHRS system is up and ready in under 1 minute.
Last edited by vapilot2004; 23rd April 2006 at 08:44.