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Aviation Theory

Global Positioning System (GPS) [page 2]

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pages: 1: General & Requirements | 2: Usage & more Technical Info

    1. GPS IFR operations in oceanic areas can be conducted as soon as the proper avionics systems are installed provided all general requirements are met. A GPS installation with TSO C-129 authorization in class A1, A2, B1, B2, C1, or C2 may be used to replace one of the other approved means of long range navigation such as dual INS or dual Omega. A single GPS installation with these classes of equipment which provides RAIM for integrity monitoring may also be used on short oceanic routes which have only required one means of long range navigation.
    2. GPS domestic enroute and terminal IFR operations can be conducted as soon as the proper avionics systems are installed provided all general requirements are met. The avionics necessary to receive all of the ground based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational. The ground based facilities necessary for these routes must also be operational.
    3. The GPS Approach Overlay Program permits pilots to use GPS avionics under IFR for flying existing instrument approach procedures, except localizer (LOC), localizer directional aid (LDA) and simplified directional facility (SDF) procedures. In the future, stand alone GPS approaches will be developed and introduced into the NAS.
    4. GPS IFR approach operations can be conducted in accordance with Phase I, Phase II or Phase III of the GPS Approach Overlay Program, as appropriate, as soon as the proper avionics systems are installed and the following requirements are met. This general approval to use GPS to fly instrument approaches is limited to U.S. airspace. The use of GPS in any other airspace must be expressly authorized by the Administrator. GPS instrument approach operations outside the United States must also be authorized by the appropriate sovereign authority.
    1. Authorization to fly approaches under IFR using GPS avionics systems require that:
      1. A pilot use GPS avionics with TSO C-129 authorization in class A1, B1, B3, C1, or C3; and:
      2. The specific approach procedure to be flown must be retrievable from the airborne navigation database associated with the TSO C-129 equipment.
        NOTE: GPS avionics systems installed and operated in accordance with the AFS/AIR guidance dated July 20, 1992 are not approved for "overlay" program phase II or III.
    1. Phase I - Under Phase I, GPS avionics can be used as the IFR flight guidance system for approaches as long as the ground based NAVAID(s) required by the published procedure is operational and actively monitored while conducting the approach. Approach clearances must be requested and approved using the published title of the existing approach procedure such as "VOR Rwy 24".
    2. Phase II - Under Phase II, GPS avionics can be used as the IFR flight guidance system for an approach without actively monitoring the ground based NAVAID(s) which defines the approach. However, the ground based NAVAID(s) must be operational. In addition, the related avionics must be installed and operational but need not be turned-on during the approach. Approaches must be requested and approved using the published title of the existing approach procedure such as "VOR Rwy 24".
    3. Phase III - Phase III begins when FAR Part 97 instrument approach procedures are retitled "GPS or VOR Rwy 24". When this Phase begins, ground based NAVAIDs are not required to be operational and the associated aircraft avionics need not be installed, operational, turned on or monitored. GPS approaches will be requested and approved using the GPS title, such as "GPS Rwy 24". Pending FAA's publication of FAR Part 97 GPS approaches, stand alone GPS approaches will be developed and authorized on a case-by-case basis.
      NOTE: In each Phase, any required alternate airport must have an approved instrument approach procedure, other than GPS or LORAN-C, which is anticipated to be operational and available at the estimated time of arrival.
    1. The objective of GPS procedures design is to supply seamless navigation for all GPS equipped aircraft. GPS allows the most direct routing to the initial approach waypoints for standardized approaches at the maximum number of locations. The concepts described in this section provide the pilot and the air traffic controller with the most efficient method for routing traffic to a particular destination.
    2. GPS approaches are designed utilizing a Basic "T" concept with connecting terminal routes to the en route airway structure. The Basic "T" design normally aligns the procedure on runway centerline, with the missed approach point (MAP) located at threshold, the final approach fix (FAF) 5 nautical miles (NM) from threshold, and the intermediate fix (IF) 5NM from the FAF. Two initial approach fixes (LAF) are located 4 or 5NM from the IF (one on each side of the IF), normally at points 90 degrees to the intermediate course. Missed approaches will normally be predicated on courses and turn waypoints to the missed approach holding fix. (See FIG 1-1-27)
    3. When terrain, airspace, or air traffic requirements preclude use of standard Basic "T" approach construction, the flexibility of GPS allows alternatives that exploit the benefits of seamless navigation. Design possibilities include:
      1. Modification to the Basic "T" by eliminating one of the IAFs, changing the length of the initial legs, or the angle at which they intercept the intermediate segment.
      2. Use of an approach design which consists of intermediate and final approach segments. This variation requires terminal routes to an IAF at the IF; and, depending on the angle of arrival at the fix, may require a holding pattern course reversal to align the aircraft with the final approach course. (See FIG 1-1-28).
      3. Air Traffic radar vectors to a published segment of the GPS approach procedure. Where terminal routes are not published the approach will be annotated with the note: "RADAR REQUIRED."
    4. Recent applications of "free flight" concepts advance seamless GPS procedure design philosophy, eliminate terminal routes, and use the Basic "T" enhanced by the Terminal Arrival Area (TAA). The objective of the TAA is to identify an area suitable for GPS navigation from the en route structure direct to the Basic "T." To the maximum extent possible, except where restricted by limitations imposed by air traffic control, surrounding terrain limitations, etc., this design is used for new or revised stand-alone GPS SIAPs. The TAA is composed of three areas; the straight-in area, the right base area, and the left base area. These areas establish minimum terminal area IFR altitudes for procedural entry.
    5. The straight-in area is semi-circular with a 30 l NM radius arc beyond the top of the basic "T" centered on the IF. The arc boundary of the straight-in area is equivalent to an IAF. After crossing the boundary or when released by ATC within the TAA, aircraft are expected to proceed on a direct course to the IF. The left base and right base areas are bounded by the top of the basic "T" and the extension of the intermediate segment course, with 30 NM radius arcs centered on the IAF on each side of the "T." (See FIG 1-1-29). The arc boundaries of the base areas are equivalent to feeder fixes. After crossing the TAA boundary from the en route phase of flight or when released by ATC within the TAA, aircraft are expected to proceed on a direct course to the appropriate corner IAF, depending on their geographic position. Course reversals normally are not required with a TAA because routing through the "T" IAFs provides flexibility and standardization in maneuvering to the final approach course. When circumstances require a course reversal it will be established as a holding pattern in lieu of procedure turn at the IF which will be designated as an IF (IAF).
    6. Normally, the TAA will be designed with only one minimum altitude. There will be locations, especially in mountainous areas, where it will be necessary to sectorize the TAA to provide for lower altitudes prior to reaching the IAFs. Sectors may be delineated using wedges of the circle and/or GPS nautical mile step-down arcs. (See FIG 1-1-30). Where a step-down arc or sector is not practical to establish lower altitudes at the IAFs, a holding pattern course reversal will be provided. If certain parts of the TAA encompass terrain or airspace which does not allow for the establishment of an IAF within descent criteria, it may be necessary to eliminate a portion of a quadrant (or even eliminate the entire TAA). When a portion of the Basic "T" or TAA is unusable, it may be necessary to establish a terminal route to the center IF (IAF) for either a straight-in (NoPT) approach or a holding pattern course reversal. Additionally, modifications to the Basic "T;" e.g., leg length, or angle of turn to the intermediate segment, may also be used when obstructions or airspace restrict use of the basic approach design. (See FIG 1-1-31)
    7. Whenever a complete TAA is charted, the minimum safe altitude (MSA) depiction found in the plan view of the approach will be omitted as the TAA altitudes provide the minimum obstacle clearances of the MSA.
    RAIM information is an on request item during preflight briefings. GPS Receiver Autonomous Integrity Monitoring (RAIM) Aeronautical Information can be obtained for a period of 3 hours (ETA hour and 1 hour before to 1 hour after the ETA hour) or a 24 hour time frame at a particular airport. Briefers will provide RAIM information for a period of 1 hour before to 1 hour after the ETA, unless a specific time frame is requested by the pilot.


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