Backup Flight Data Recorder?

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

Papa3 wrote:
I opted to go a slightly different route and purchased a Garmin GPS
Map 76. For well under $200 street price, you get a fully functional
navigation system that is completely self-contained, including the
ability to run on 2 AA batteries for more than 8 hours (I use two
2500mAh rechargables which I swap out after every flight; have recorded
long ridge missions of over 8 hours without any problems). It has
it's own display and good recording and downloading capabilities.
Plus, it doesn't take up any precious panel space or require additional
displays or wiring. Mine is mounted on a home-built bracket, but RAM
and others provide some very slick mounting options. In the US, it
is acceptable as contest documentation. The only problem would be for
a badge flight, where the FAI has not approved it for documentation.

My flight recorder has failed several times (for trivial but fatal
reasons unrelated to installation or power supply) so I'm absolutely
committed to having a backup. Having borrowed Erik's Garmin GPS MAP 76
several times for contest backup, and having evaluated the
alternatives, I bought my own recently ($160 delivered on eBay). It's
completely self contained, produces an IGC file (using SeeYou)--albeit
not a secure one--that is usually indistinguishable from the one in my
primary flight recorder, allows me to upload the contest database, and
has a nice display, with a built in U.S. roads and towns database.

Yeah, it's not IGC approved so you'll need something else as your
primary recorder. But as a backup, it's exactly what I need. It will
also provide a NMEA input to varios and PDAs for a backup moving
map/flight computer, so long as GPS altitude is OK.

The last point is the only real concern I have: if my primary recorder
fails, I'll be relying on GPS altitude for starts, special-use airspace
clearances, etc. So long as I know about it, I just have to pay
attention to the GPS readout. But GPS and pressure altitude vary
anywhere from a few feet to many hundreds of feet.

Garmin makes a similar model with a pressure altitude sensor but it's
unclear whether that would be secure enough to meet the requirements at
a [U.S.] contest...and it costs more.

My PDA-and-primary-flight-recorder-based moving map froze at the Uvalde
Std. Nats. a few weeks ago just as I was maneuvering to turn and tap
the last cylinder as I flew by it and head for the finish. I punched up
the TP on the Garmin and used it to navigate while I fumbled with the
PDA, eventually doing a soft reset. I was very happy I had the Garmin!

Chip Bearden
ASW 24 "JB"