What action should be taken if RAIM is lost before reaching the FAF?

When RAIM (Receiver Autonomous Integrity Monitoring) is lost before reaching the Final Approach Fix (FAF), the appropriate action is to go missed approach. This is crucial because RAIM is essential for ensuring the reliability and integrity of GPS signals used during the approach phase of flight. When RAIM is lost, the aircraft's ability to determine its position accurately is compromised, which could lead to navigating to a point where the aircraft may not be safely aligned with the runway.

Opting to go missed approach at this point prevents the pilot from continuing the approach with potentially unreliable navigation data. This decision prioritizes safety, as it allows the pilot to regain a stable position on a published missed approach procedure, which is designed to safely guide the aircraft away from the runway environment while ensuring that all necessary procedures and protocols are adhered to.

Continuing to the destination, switching to visual flight rules, or reporting to ATC for a new heading do not adequately address the lack of navigational integrity posed by the loss of RAIM. These options could expose the flight to risky situations due to inadequate position verification, particularly in low visibility conditions or in proximity to terrain. Therefore, executing the missed approach procedure is the safest and most reliable action to take under such circumstances.