A
multi-sensor system is one that accepts several inputs for computing
navigational solutions and usually refers to IRS, ground based Radio Navaids
and GPS.
For example, there are several UK airlines still operating short-haul aircraft
that do not utilise GPS and rely on DME , VOR and IRS for their inputs. When
within range of Radio Navaids these are used as primary navigational inputs
butout of range of Navaids, e.g. over water, distant from land, IRS will be
used.
A major disadvantage of this is that you can incorrectly enter your position
and whilst the FMS will ask you to confirm your position you can still get it
wrong. Whilst on the ground the system cannot use Navaids to check the position
so accepts the input data and uses it as a start position. However, when it
automatically tunes the Navaids it expects to find after take-off (based on the
input data) it cannot find them and fails - on one well known occasion the
whole system shut down and the crew had to return to the departure aerodrome to
reset everything on the ground (as you cannot re-align the Inertials in flight)
at considerable cost to the airline.
A multi-sensor system using GPS trilaterates its position from 'visible',
'healthy' satellites. Provided the GPS RAIM function is working this is used as
the primary navigational aid.