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FAA Glossaries

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Barometric Aiding

I have been flying a Cessna 172SP with an old-school Bendix stack and unlike the Garmin GPS and S-Tec autopilots that I am used to flying, I need to enter the altimeter setting into the KLN 94 GPS and the KAP 140 Autopilot. Most of the GPS systems in GA aircraft use Barometric Aiding to assist in detecting integrity anomalies. Most Garmin units interface directly with the altitude encoder to provide baro-aiding, but other units require the pilot to enter the local altimeter setting. AOPA has an article that lists non-WAAS GPSs that have baro-aiding. WAAS-enabled GPS units do not rely on RAIM so they do not rely on baro-aiding.

Barometric aiding is an integrity augmentation that allows a GPS system to use a non-satellite input source (e.g. the aircraft pitot-static system) to provide vertical reference and reduces the number of required satellites from five to four. Baro-aiding requires four satellites and a barometric altimeter input to detect an integrity anomaly. The current altimeter setting may need to be entered into the receiver as described in the operating manual. Baro-aiding satisfies the Receiver Autonomous Integrity Monitoring (RAIM) requirement in lieu of a fifth satellite.

As usual, John D Collins has an excellent discussion of this.

The RAIM algorithm can be improve upon by using the aircraft pressure altitude, as the vertical position, although less accurate than the lateral position, can substitute for one of the satellites. This is called Baro Aiding and probably all IFR GPS installations use this. With Baro Aiding, only four satellites are needed to determine a value of RAIM.

With WAAS, the calculations are determined by the WAAS system as each ground station that measures its position via the GPS satellites is at a know location, and the error can be determined one satellite at a time. This integrity data is uplinked to the WAAS satellites and is broadcast as part of the WAAS correction messages. So the WAAS system itself is determining the integrity information via explicit measurement and passing it on to the WAAS receiver to determine the HPL.

The autopilot has an altitude capture and hold function where you can select an altitude and climb or descend rate and the autopilot will adjust the elevators to capture the rate and altitude. You still need to set the appropriate power for a climb or descent. You need to enter the altimeter setting so it knows where to stop or hold.

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