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Navigation Database Currency (Part Two) NOTAMs Relevant to GPS and GPS Signal Availability

in Navigation

NOTAMs Relevant to GPS

There are numerous notices to airmen (NOTAMs) that apply specifically to users of navigation aids. For example, when anomalies are observed in the behavior of the global positioning system, or when tests are performed, a GPS UNRELIABLE NOTAM is issued. Similarly, published instrument procedures that rely on RNAV equipment sometimes become “Not Available” when safety concerns arise, such as ground-based interference. It is important to check all NOTAMs prior to IFR flights and, especially, GPS and WAAS NOTAMs before flying. Remember, when talking to a flight service station (FSS)/automated flight service station (AFSS) briefer, you must specifically request GPS/WAAS NOTAMs.

GPS Signal Availability

GPS-based RNAV equipment that uses the DOD GPS relies on adequate signal reception throughout the course of a flight. Signal reception becomes especially critical during instrument approaches when signal reception criteria become more stringent. Signal reception is generally predictable, and you can request information on likely signal reception for the destination airport in the preflight briefing from Flight Service. Many GPS RNAV units include a feature called receiver autonomous integrity monitoring (RAIM) that allows you to view predictions about future signal reception at specific locations. WAAS-enabled receivers do not have this restriction or limitation due to the error corrections available from the WAAS. WAAS is a form of differential GPS (DGPS) providing enhanced position accuracy. Each Wide Area Reference Station (WRS) provides correction data to a Wide Area Master Station (WMS), which computes a grid of correction data to be uplinked to a geostationary satellite (GEO) from a Ground Earth Station (GES). The geostationary satellite transmits the correction data (and also navigation data) to the user on the L1 GPS navigation frequency (1575.42 MHz). The user GPS receiver uses the downlink WAAS data to correct received navigation data. The goal of WAAS is to obtain at least a 7-meter horizontal and vertical accuracy.

Local Area Augmentation System (LAAS), when it becomes available, is another DGPS mode which is designed to provide 1-meter accuracy for precision approaches. It uses a local error VHF transmitter near the runway providing a direct link from the sensor to the aircraft GPS receiver.


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