Use of the Global Positioning System (GPS) as a Substitute for Non-Directional Beacons (NDB) and Distance Measuring Equipment (DME) Purpose: This Notice is issued to clarify the authorized uses of GPS in the U. S. National Airspace System (NAS). It is intended to amplify guidance contained in Advisory Circular (AC) 90-94, "GUIDELINES FOR USING GLOBAL POSITIONING SYSTEM EQUIPMENT FOR IFR EN ROUTE AND TERMINAL OPERATIONS AND FOR NONPRECISION INSTRUMENT APPROACHES IN THE U.S. NATIONAL AIRSPACE SYSTEM." The 14 Code of Federal Regulations (CFR) Part 91 requires air navigational equipment to be "appropriate to ground facilities to be used." The current U. S. NAS is based on NDB, Very-high frequency Omni-Range (VOR) and VOR/DME ground facilities. Therefore, depending on ground facilities to be used, in addition to VOR equipment, Automatic Direction Finding (ADF), and/or DME equipment OR an area navigation (RNAV) system which provides navigational performance equivalent to ADF and/or DME equipment must be installed in an aircraft and must be operable for IFR flight operations in the NAS. Properly installed IFR-certificated GPS equipment, operated in accordance with AC 90-94 and this Notice, provides navigational performance equivalent to ADF or DME equipment, except for flying NDB instrument approach procedures. Operations: Subject to the restrictions below, operators in the U. S. NAS are authorized to use GPS equipment certified for IFR operations in place of ADF and DME equipment for the following operations: Determining the aircraft position over a DME fix. Flying a DME arc. Navigating to/from an NDB. Determining the aircraft position over an NDB. Determining the aircraft position over a fix made up of a crossing NDB bearing. Holding over an NDB. Restrictions: These operations are approved for GPS avionics approved for IFR, including multi-sensor systems with GPS sensor. This equipment must be properly installed and the provisions of the applicable FAA approved Aircraft Flight Manual (AFM), Flight Manual supplement, or Approved Operations Manual should be met. The required integrity for these operations is provided by Receiver Autonomous Integrity Monitoring (RAIM), or an equivalent method. For air carrier operations, operations specification approval is required to use GPS. Waypoints to be used for these operations must be retrieved from the GPS airborne database. The database must be current. The GPS system must be operated within the guidelines contained in the AFM, Flight Manual Supplement or Approved Operations Manual. The Course Deviation Indicator (CDI) must be set to terminal sensitivity when tracking GPS course guidance in terminal areas. The NDB or DME ground facility, which supports the charted requirement, may be temporarily out of service. Charting will not change to support these operations. Except for use as the primary instrument approach navigational source, charted requirements for ADF or DME can be met using the GPS system. NOTE- An aircraft is not authorized to fly any IFR approach using GPS unless that instrument approach procedure is retrievable from the airborne database. Approach procedures that are omitted from the database can not be legally flown using GPS navigational equipment. Guidance: The following is provided as interim guidance until the new Advisory Circular 90-94 or the next issue of the Aeronautical Information Manual (AIM) is published. It is general and not specific to any particular GPS system. For specific guidance for your system refer to the approved AFM, Flight Manual Supplement or Approved Operations Manual or contact the manufacturer of your system. To determine the aircraft position over a DME fix: If the fix is identified by a five letter name which is contained in the GPS airborne database, you may select, from the airborne database, the named fix as the active GPS waypoint or you may select, from the airborne database, the facility providing the DME for fix make up as the active GPS waypoint. If the fix is identified by a five letter name which is not contained in the GPS airborne database, or if the fix is not named, you must select, from the airborne database, the facility providing the DME for fix make up as the active GPS waypoint. If you selected the named fix as your active GPS waypoint, you are over the fix when the GPS system indicates you are at the active waypoint. If you selected the DME providing facility as the active GPS waypoint, you are over the fix when the GPS distance from the active waypoint equals the charted DME value. To fly a DME arc: You must select, from the airborne database, the facility providing the DME arc as the active GPS waypoint. Maintain position on the arc by reference to the GPS distance in lieu of a DME readout. To navigate to/from an NDB: Select terminal CDI sensitivity in accordance with the AFM, Flight Manual Supplement or Approved Operations Manual if in the terminal area. Select the NDB facility from the airborne database as the active waypoint. Select and navigate on the appropriate course to/from the waypoint. To determine the aircraft position over an NDB: Select the NDB facility from the airborne database as the active waypoint. You are over the NDB when the GPS system indicates you are at the active waypoint. To determine the aircraft position over a fix made up of a crossing NDB bearing: A fix made up by a crossing NDB bearing will be identified by a five-letter fix name. You may select the named fix, from the airborne database, as the active GPS waypoint or you may select, from the airborne database, the NDB facility providing the crossing bearing for fix make up as the active GPS waypoint. If you selected the named fix as your active GPS waypoint, you are over the fix when the GPS system indicates you are at the waypoint as you fly the prescribed track from the non-GPS navigational source. If you selected the NDB facility as the active GPS waypoint, you are over the fix when the GPS bearing to the active waypoint is the same as the charted NDB bearing for the fix as you fly the prescribed track from the non-GPS navigational source. To hold over an NDB: Select terminal CDI sensitivity in accordance with the AFM, Flight Manual Supplement or Approved Operations Manual if in the terminal area. Select the NDB facility from the airborne database as the active waypoint. Select non-sequencing mode and the appropriate course in accordance with the AFK Flight Manual Supplement or Approved Operations Manual. Hold using the GPS system in accordance with the AFM, Flight Manual Supplement or Approved Operations Manual. NOTE- If the NDB is a compass locator charted with a collocated fix of the same name, use of that fix, from the airborne database, as the active waypoint in place of the NDB is authorized. Planning: You should plan ahead before using GPS systems as a substitute for ADF or DME. You will have several alternatives in selecting waypoints and system configuration. After you are cleared for the approach is not the time to begin programming your GPS. In the flight planning process you should determine whether you will use the equipment in the automatic sequencing mode or in the non-sequencing mode and select the waypoints you will use. When you are using your GPS system to supplement other navigational systems, you may need to bring your GPS control panel into your navigation scan to see the GPS information. Some installations will present localizer information on the CDI whenever a localizer frequency is tuned, removing the GPS information from the CDI display. Good advance planning and intimate knowledge of your navigational systems is vital to safe and successful use of GPS in lieu of ADF and DME. 6/25/98 (ATA-100) Top Posted Monday, February 08, 1999 9:25:13 AM ©1996-1999, Aircraft Owners and Pilots Association. Contact AOPA