wrote:
The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
according to the online flight display.

Or is it the straight 76 that you bought?

I bought the GPSMAP 76 (there's also a straight GPS 76 but it's not
suitable for our purposes: not enough trackpoints, for one thing). The
GPSMAP 76S does have the baro sensor (as well as an electronic compass
sensor) but I'm not sure whether it's evident from the track log
whether the user locked out the auto-calibrate feature that slowly
adjusts the altimeter to GPS altitude to account for pressure changes.
I'm also not sure whether the track log records both pressure and GPS
altitudes. If the answer to these questions is yes (i.e., it's evident
if auto calibration is turned off and it records pressure altitude),
then the extra money for the GPSMAP 76C might be worth it. It's still
not an IGC $ecure flight recorder but it's the next best thing...at 1/5
or 1/6 the price.

Chip Bearden
ASW 24 "JB"

The answers to the two questions a

- No, the 76S unit records only 1 altitude. All of the recreational
Garmin units (at least through early this year) logged a fairly limited
subset of the available NMEA data (understandably, since the average
recreational user needs only time, position, and altitude).

- As to which altitude it is storing, that depends. The basic
"altitude" in the 76S is the barometric altitude. The auto-calibrate
feature compares the barometric altitude to the GPS altitude and
periodically adjusts the barometric altitude. Exactly how it does
that (ie. average delta over n number of fixes, how it determines
whether to discard a fix, etc.) is not public domain. But, I have had
correspondence with Garmin engineers who pointed out that the algorithm
means that you essentially end up with GPS altitude but smoothed by the
barometric altitude to take care of any individual GPS altitude fixes
that might have been degraded (at least that's my current
understanding). If you turn off auto-calibrate, then only the
barometric altitude is stored. There is no way to tell from the
output record whether auto-calibrate was on or off.

Given the above, I'm not convinced yet that the 76S is necessarily
"better", though I do think it might actually be a long-term solution
if we can come up with a complete understanding of the auto-calibration
algorithm(s).

P3