Beginning September 28, 2009, pilots using non-WAAS-equipped IFR GPS units will need to perform preflight Receiver Autonomous Integrity Monitoring (RAIM) checks prior to flying T-routes as well as advanced RNAV arrival and departure procedures that are typically found only at large airports. This will enable pilots to know if a GPS outage is forecast for a flight planned before they encounter the outage. Specifically affected are:
*RNAV (GPS) instrument approach procedures are not impacted by this change. IFR GPS units must automatically perform a RAIM check before beginning an approach. However, performing a RAIM check prior to leaving the ground will better enable pilots to plan ahead and is recommended specifically for pilots without baro-aiding (see below).
The change is due to the expiration of a NOTAM that previously exempted RAIM checks for RNAV 1 and RNAV 2 operations as described in Advisory Circular 90-100A. The GPS constellation has been performing above its minimum required levels for some time, but satellites are aging and some delays in their replacement may result in reduced availability in the next few years.
Pilots using WAAS-equipped GPS units in areas of WAAS coverage are not required to check RAIM pre-flight but should continue checking WAAS notams as usual.
Pilots with non-WAAS-equipped GPS units that qualify for 90-100A (see below) procedures that are planning for or would like to be able to accept the above procedures can get current RAIM information from a flight service station briefer, the Internet at www.raimprediction.net, or RAIM prediction tools built into some GPS units such as the Garmin 430/530 series. Please refer to your GPS manual for more information.
The following GPS units qualify for at least some 90-100A procedures and are commonly found in GA aircraft. A complete list can be found on the FAA’s Web site.
Baro-aiding is a type of GPS integrity augmentation that basically allows your GPS to use your static system to provide a vertical reference and reduce the number of satellites required. GPS units that have baro-aiding are much less likely to experience outages. Some units require manual entry of the altimeter setting for baro-aiding. If your GPS unit prompts you for current altimeter setting, be sure to enter it each time when relying on baro-aiding.
The units below require baro-aiding as part of their installation, and therefore if your GPS was installed properly you have baro-aiding.
GPS 155, GPS 155XL, GPS 165, GNC 300, GNC 300XL, GPS 400, GNC 420, GNS 430, GPS 500, GNS 530, G1000 (pre-TSO C146a versions), Apollo GX50, Apollo GX60, Apollo GX65
For other units, pilots need to contact their installer or manufacturer to verify if they have baro-aiding. Most likely, if your GPS is connected to your altitude encoder, it has baro-aiding capability.
More information on RAIM can be found in the AIM, section 1-1-19.
Pilots are also reminded that many handheld and VFR GPS units do not provide RAIM monitoring or alerts. Loss of the required number of satellites in view, or the detection of a position error, cannot be displayed to the pilot by such receivers. In receivers with no RAIM capability, no alert would be provided to the pilot that the navigation solution had deteriorated, and an undetected navigation error could occur. A systematic cross-check with other navigation techniques would identify this failure, and prevent a serious deviation.
Pilots can check for predicted areas predicted to have continuous loss of RAIM for more than 5 minutes through several methods. If a forecast outage is found, pilots should plan to use non-GPS procedures, re-route their flight where RAIM requirements can be met, or delay their flight. Often RAIM predictions are fairy short, and delaying a takeoff for 5 minutes can enable a pilot to avoid flying into areas without GPS coverage.