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  Tuesday 21 February 2017 23:22 GMT  

Aviation Theory

Global Positioning System (GPS)

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


There are more and more GPS approaches being added to the IAPs (Instrument Approach Procedures) on almost a daily basis. At this time, in real world flying, the first of the precision GPS approaches are being put into service and tested in the area of Oshkosh, Wisconsin. There are already of approved non-precision GPS approaches in use around the US. Some say that the GPS system will mean the end of the VOR navigation system. Well, that's what "they" said about NDBs when the VOR was introduced and we've still got NDBs.


    1. The GPS is a United States satellite based radio navigational, positioning, and time transfer system operated by the Department of Defense (DoD). The system provides highly accurate position and velocity information and precise time on a continuous global basis to an unlimited number of properly equipped users. The system is unaffected by weather and provides a worldwide common grid reference system based on the earth fixed coordinate system. For its earth model, GPS uses the World Geodetic System of 1984 (WGS-84) datum.
    2. GPS provides two levels of service: Standard Positioning Service (SPS) and Precise Positioning Service (PPS). SPS provides, to all users, horizontal positioning accuracy of 100 meters with a probability of 95 percent and 300 meters with a probability of 99.99 percent. PPS is more accurate than SPS; however, this is limited to authorized U.S. and allied military, federal government, and civil users who can satisfy specific U.S. requirements.
    3. GPS operation is based upon the concept of ranging and triangulation from a group of satellites in space which act as precise reference points. A GPS receiver measures distance from a satellite using the travel time of a radio signal. Each satellite transmits a specific code, called a course/ acquisition (CA) code, which contains information on the satellite's position, the GPS system time, its clock error, and the health and accuracy of the transmitted data. GPS satellites have very accurate atomic clocks in order to calculate signal travel time. Knowing the speed at which the signal traveled (approximately 186,000 miles per second) and the exact broadcast time, the distance traveled by the signal can be computed from the arrival time.
    4. The GPS receiver matches each satellite's CA code with an identical copy of the code contained in the receiver's database. By shifting its copy of the satellite's code, in a matching process, and by comparing this shift with its internal clock, the receiver can calculate how long it took the signal to travel from the satellite to the receiver. The distance derived from this method of computing distance is called a pseudo-range because it is not a direct measurement of distance, but a measurement based on time. Pseudo-range is subject to several error sources; for example, an ionospheric delay, and time disparities between the atomic clocks in the satellites and the GPS receiver.
    5. In addition to knowing the distance to a satellite, a receiver needs to know the satellite's exact position in space; this is known as its ephemeris. Each satellite's signal transmits ephemeris information about its exact orbital location. The GPS receiver uses this information to precisely establish the position of the satellite.
    6. Using the calculated pseudo-range and the position information supplied by the satellite, the GPS receiver mathematically determines its position by triangulation. The GPS receiver needs at least three satellites with timing corrections from a fourth satellite to yield an unaided, unique, and true three-dimensional position (latitude, longitude, and altitude) and time solution. The GPS receiver computes navigational values such as distance and bearing to a waypoint, ground speed, etc., by using the aircraft's known latitude/longitude and referencing these to a database built into the receiver.
    7. The GPS constellation of 24 satellites is designed so that a minimum of five are always observable by a user anywhere on earth. The receiver uses data from the best four satellites above its horizon, adding signals from one as it drops signals from another, to continually calculate its position.
    8. The GPS receiver verifies the integrity of the signals received from the GPS constellation through receiver autonomous integrity monitoring (RAIM) by determining if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; thus, RAIM needs 5 satellites in view, or 4 satellites and baro-aiding to work. RAIM needs 6 satellites in view (or 5 satellites with baro-aiding) to isolate the corrupt satellite signal and remove it from the navigation solution. Baro-aiding is a method of augmenting the GPS solution equation by using a nonsatellite input source. Baro-aiding uses the pressure altitude corrected for the local barometric pressure setting to provide accurate altitude information to the GPS receiver.
    9. The Department of Defense declared initial operational capability (IOC) of the U.S. Global Positioning System (GPS) on December 8, 1993. The Federal Aviation Administration (FAA) has granted approval for U.S. civil operators to use GPS equipment as a primary means of navigation in oceanic airspace and certain remote areas. GPS equipment may be used as a supplemental means of IFR navigation for domestic enroute, terminal operations, and certain instrument approach procedures (IAPs). This approval permits the use of GPS in a manner that is consistent with current navigation requirements.
    1. General Requirements: Authorization to conduct any GPS operation under IFR requires that:
      1. The GPS navigation equipment used must be approved in accordance with the requirements specified in TSO C-129, or equivalent, and the installation must be made in accordance with Notice 8110.47 or 8110.48, the equivalent Advisory Circular or the Flight Standards/Aircraft Certification (AFS/AIR) joint guidance memorandum dated July 20, 1992. Equipment approved to TSO C-115a do not meet the requirements of TSO C-129.
      2. Aircraft using GPS equipment under IFR must be equipped with an approved and operational alternate means of navigation appropriate to the flight. Active monitoring of the alternative navigation equipment is not required if the installation uses RAIM for integrity monitoring. For these systems, active monitoring by the flightcrew is only required when the RAIM capability of the GPS equipment is lost.
      3. Procedures must be established for use in the event that the loss of RAIM capability is predicted to occur. In situations where this is encountered, the flight must rely on other approved equipment, delay departure, or cancel the flight.
      4. The GPS operation must be conducted in accordance with the FAA-approved aircraft flight manual (AFM) or flight manual supplement.
      5. Aircraft navigating by GPS are considered to be RNAV aircraft. Therefore, the appropriate equipment suffix must be included in the ATC flight plan.
      6. Prior to any GPS IFR operation the pilot should review the appropriate NOTAMs. NOTAMs will be issued to announce outages. Pilots may obtain these NOTAMs from FSS briefers upon request.
      7. Air carrier and commercial operators conducting GPS IFR operations shall meet the appropriate provisions of their approved operations specifications.


... page 2: Usage & more Technical Info

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