Plane & Pilot
Tuesday, November 1, 2005

How High Are We Now?


We all fly at erroneous altitudes—even when accompanied with a GPS. Here’s how to determine and understand the best way to get the most precise reading.


How High Are We NowIf you have a GPS and a blind encoder in your panel, you may have three independent ways to determine your altitude. But which one is most accurate? We all grew up on baro altitude, so after a short review, we’ll plunge into the GPS world of the WGS84 datum, your height above ellipsoid (HAE) and mean sea level (MSL) altitudes.

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In fact, all three Garmin IFR navigators (430, 530 and 480) make that correction. So, when you see “GPS ALT” on the Garmin 480, or “ALT” on the satellite page in the Garmin 430 and 530, these are MSL estimates using the GPS solution and the geoid correction. With WAAS, this altitude estimate is probably the most accurate one you have in your aircraft.

With an appropriate GPS and WAAS receiver, you can shoot GPS approaches with precision vertical (APV), as the LPV and Lnav and Vnav approaches are called. For now, only the Garmin 480 is certified for these approaches. The Chelton EFIS has a GPS with WAAS as well, but it’s certified only for Lnav operations since it solves for position once each second rather than five times per second as required under TSO-146 for APV operations.

For APV operation, the vertical leg is tilted downward at the angle shown in the approach profile, and it must terminate at the correct MSL height above the runway. For those GPS approaches, the airport HAE is in the database, so the difference in HAE and MSL altitudes for that airport is the local correction applied to the GPS solution.

On vertical legs, the Garmin 480 knows where you are both horizontally and vertically from that line and will display localizer and glideslope information on both your HSI and on the NAV page (an artificial HSI in the unit). You can use many autopilots in the NAV (or analog) mode to couple to this approach, and digital vertical output and digital autopilots are on the near horizon. The decision height still is determined by baro altitude.

GPS altitude also is used in TAWS-B systems, sometimes in combination with baro altitude. On the Garmin 530 unit, the TAWS page shows “MSL(G)” in the upper right-hand corner, meaning its your GPS-derived MSL altitude.

In the future, the altimeter may be replaced with a simple GPS with WAAS capability that shows a digital display of your altitude. Meanwhile, have a glass of wine, get slightly high and dream on.





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