The 777 INS has two reference systems. They are the ADIRU and SAARU.
The ADIRU is the Air Data Inertial Reference Unit and the SAARU is the
Standby Airdata Attitude Reference Unit. The ADIRU utilized laser
ring gyros and accelerometers while the SAARU uses lower cost fiber
optic ring gyros and accelerometers.

The ADIRU is a fault-tolerant design, so there is internal redundancy
within the system itself. The SAARU acts as a safety backup for the
ADIRU in case something takes out the entire ADIRU defeating its
fault-tolerant design.

The FMS (Flight Management System) function within AIMS (Airplane
Information Management System) utilizes data from the ADIRU and SAARU
as well as from navigation radio sources including GPS, DME, VOR, and
NDB to update the intertial navigation solution to maintain an
accurate position fix.

Dean Wilkinson

Gerald Sylvester wrote in message link.net...
I got a cockpit tour of a United 767-300ER in year 2000 that was used for
overseas international flights. Crew said they have 3 INS units on board.

my friend just got bumped down off the 767 with UA. I remember him
saying it had 3 INS units. He used to fly the 777 and said, "we
weren't even told how many INS it has" apparently due to the systems
on the 777 having so many redundencies (this is from memory). I think
he also said that the 777 has 9 hydraulic systems. If 8 of them
fail, the remaining system can power the other 8.

Gerald