RAIM Issue Brief

July 16, 2009

RAIM Issue Brief

Overview

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:

  • T routes (RNAV routes below 18,000 ft)
  • Q routes (RNAV routes above 18,000 ft)
  • RNAV SIDs (standard instrument departures)
  • RNAV STARs (standard terminal arrival routes)
  • RNAV ODPs (obstacle departure procedures)

*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.

Eligibility

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.

  • GPS 155, GPS 165, GNC 300, GPS 155XL, GNC 300XL
  • GPS 400, GNC 420, GNC 420A, GNS 430, GNS 430A, GPS 400W, GNC 420W, GNC 420AW, GNS 430W, GNS 430AW
  • CNX80, GNS 480
  • GPS 500, GPS 500 TAWS, GNS 530, GNS 530 TAWS, GNS 530A, GNS 530A TAWS, GPS 500W, GPS 500W TAWS, GNS 530W, GNS 530W TAWS, GNS 530AW, GNS 530AW TAWS
  • G1000, Embraer Prodigy, Cirrus Perspective, G950, and G900X Systems
  • KLN-89B, KLN-90A, KLN-90B, KLN-94
  • Apollo 2001, Apollo 2101System
  • Apollo SL50, Apollo SL60, Apollo SL65
  • Apollo GX50, Apollo GX55, Apollo GX60, Apollo GX65

Baro-aiding

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.

RAIM

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.

Obtaining a RAIM Prediction

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.

  • The FAA provides a RAIM prediction website  for high speed internet users which quickly and easily provides a graphic depiction of predicted RAIM outages. The most important aspect of this website is selecting the baro-aiding option if applicable. AOPA also recommends checking the box for interference. This will display areas where reception may be affected by other events such as Department of Defense testing.
    • There are two views: summary and playback. AOPA recommends the playback mode. If using the summary view, be sure to check the applicable time, ie, don’t assume a red area is for your entire flight. It is likely for a very short time.
  • Flight Service: As part of your standard briefing, in addition to requesting GPS notams as usual, pilots should also request an enroute and terminal RAIM prediction. You will need to inform the briefer whether or not you have baro-aiding, and you may provide airports for the terminal prediction.  You will also need to provide FSS with airports along your route of flight to execute the RAIM checks.
    • Please note, this change is also new for flight service, and as the FAA and Lockheed explore development of an integrated RAIM prediction tool, please bear with your briefer as he or she works to obtain your prediction, as this may take a few minutes.
  • Use your GPS receiver’s RAIM prediction capability. On units such as the Garmin 430, pilots can manually perform RAIM prediction for departures and approaches. For enroute procedures using most GPS units, pilots will need to enter airports along their intended route to forecast coverage.  However, there are several complex problems that can be encountered when using this method. For example, if a GPS satellite is unexpectedly unavailable and is NOTAM’ed out of service, pilots will need to know if this information has been incorporated into their GPS’s RAIM prediction. They can do this by checking the satellite status page to see if that satellite has been excluded. If not, a pilot must manually deselect the out of service satellite within the RAIM prediction software in their receiver if possible.  Some GPS receivers do not allow pilots to deselect satellites, in which case non-GPS means of navigation should be used.
  • Other means of obtaining the required RAIM prediction exist, such as PC-based software provided by GPS manufactures, but often they are more complex and are not recommended for most pilots.