To clarify:

The Dell Streak doesn't have a pressure transducer, to my knowledge. So it's using GPS altitude to figure final glides. GPS altitude is quite accurate, AFAIK, certainly more accurate than pressure altitude ("accurate" referring to altitude as measured with an extremely long string, that is).

My old GPS/NAV, LNAV, Compaq Aero/GNII combo has always done a great job of helping manage final glides and continued to do so at Nephi. But...the pressure altitude was 1,000+ feet lower than the GPS altitude when up high. And I was gliding through air that was less dense than a standard atmosphere. So I decided I was better off trusting the pressure altitude (used by the Cambridge system) than GPS altitude (used by the Dell Streak/Top Hat system). At the extreme, if the density altitude were extremely high, the glider would hardly glide at all; it would plummet despite the robust GPS altitude because the wings wouldn't generate much lift. At least that was my reasoning.

So it's not which system I trust, or how to manage final glides, or which polar to input, or which computer calculation to rely upon, or even whether the static pressure is measured precisely or has changed slightly since launch. I've been doing final glides since 1968 using a variety of methods starting with a cardboard whiz wheel. The question is which altitude to use. From 50 miles out, 13,000 ft on the altimeter/Cambridge (which agreed closely) gave me a 33 glide ratio to the 5,000 ft Nephi airport (ignoring the finish gate floor). The GPS altitude of 14,000 ft. at the same position yielded a 29.3 glide ratio. My feeling was that the 29 glide ratio was overly ambitious. But I'm eager to hear the discussion, hoping to avoid digging out my old engineering books.

Chip Bearden
ASW 24 "JB"