Any Unknown or Unlisted Aircraft Manufacturer
Other / Unknown
Aircraft Equipment Problem Less Severe
General None Reported / Taken
Aircraft; Equipment / Tooling
We need to understand the true implications of the GPS being unreliable. The current idea is that the GPS will be deferred if the symptoms arise despite the underlying condition being ever present.(GPS could fail; regardless of the predeparture integrity check Passing)This risk needs a control; whether that be placing the KNOWN DEFECTIVE units on deferral; placing a restriction on the flight planning predication on GPS availability; or (unlikely) updating the RAIM check to consider the defect in our MMR.Honeywell iMMR has a confirmed issue causing it to not being able to receive a specific satellite in the GPS constellation. This causes a loss of GPS signal when the iMMR is unable to receive an adequate number of satellites due to the defect. In pre-departure stage of flight planning Dispatch runs a RAIM check to determine whether GPS signal will be available during the course of the flight. However this RAIM check is not considering the defect in our receivers; thus rendering the check invalid. While the check has passed reality is that the GPS may not be available due to the defective unit.The greatest risk would be during the landing phase where a flight was dispatched predicated on utilizing a GPS approach. However again due to the inaccurate RAIM check the GPS receiver is not available to resolve the satellite constellation and the flight must divert due to no available approaches. This situation is worsened if instead of the destination the alternate airport was planned to use a GPS approach; after diverting from their destination the flight now has no approach available at their alternate due to the; invalid RAIM check at the time of flight planning.In less severe cases we are still planning on having GPS coverage; despite knowing the MMR receiver is faulty and could 'fail at any time' (technically it could be forecast when they would fail).
Pilot described the need to understand all implications resulting from the failure of GPS.