Page 13 - Volume 10 Number 8

Figure 1: A typical FMS/GPS unit’s predictive GPS (or RAIM) page. Note that it displays the flight’s destination (top left), ETA (top right), and the predicted availability of terminal/ approach level GPS coverage at the ETA and plus/minus
5, 10, and 15 minutes of the ETA (the green “Y” indicating “yes”). A RAIM check of a specific waypoint (other than the destination) could be manually selected, as well.
terminal operations at the ETA. RAIM predictions can be acquired a variety of ways, including from Flight Service, the FAA website, or via the RAIM Prediction feature built into many IFR-certified GPS and FMS units. While WAAS GPS receivers perform RAIM checks continuously, non-WAAS units only perform an automatic RAIM check prior to commencing an approach. For non-WAAS users, the FAA recommends pilots perform manual RAIM checks before departure and as often as feasible before flying a GPS approach procedure (Figure 1). Additionally, non-WAAS GPS users must perform RAIM prediction checks prior to flying T and Q-Routes (GPS-based airways), or RNAV Arrival and Departure procedures (SIDs, STARs and ODPs). WAAS users are exempt from those requirements, assuming they are operating in WAAS coverage areas. Not only do WAAS-certified GPSs check RAIM automatically (within WAAS coverage areas), but they will also annunciate any RAIM-related problems. In the event a RAIM check fails, GPS approach procedures are not approved and the pilot must resort to visual or other means of approach navigation (VOR, LOC, etc.). Assuming the flight was planned legally, there should always be a non-GPS approach available at the destination and/or alternate airport (or VFR conditions forecast) to ensure the flight can be completed in the absence of available GPS navigation. For more specific details on GPS navigation and RAIM, refer to the Aeronautical Information Manual (AIM), 1-1-19.
GPS Downgrades or Component Failures
Degraded GPS signals can and do cause RAIM warnings. However, RAIM is generally black or white, in that it is (or is predicted to be) within acceptable levels or not. If the prediction is outside acceptable RAIM level, that would immediately eliminate the option of conducting any GPS terminal or approach procedures. But, when operating with WAAS, multiple
AUGUST 2016
KING AIR MAGAZINE • 11