Question ATPL - Airline Transport Pilot license - Radio Navigation

What is the procedure to be followed if, on a flight under IFR conditions using the NAVSTAR/GPS satellite navigation system, the number of satellites required to maintain the RAIM (Receiver Autonomous Integrity Monitoring) function are not available? - ATPL

The flight has to be continued under VFR conditions

The flight may be continued using other certificated navigation systems

The flight may be continued as planned if at least 4 satellites are available and the pilot monitors the GPS-System manually

+/- 2NM or better for 75% of the flight time