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.