Back up Flight Recorder for US contests

Marc Ramsey wrote:


I'm actually suggesting spend less, go with the units that lack pressure
sensors, at least that way you (and the OLC) know exactly what you are
getting....

No argument -- 76, 76S, whatever does your job. Spend the money on tows.
As long as the IGC file is good, I'm happy.

But if I read you correctly you are saying that the altimeter in our
gliders are likely to be more accurate than GPS altitude throughout
their range. Is that your contention?


Jack

Jack wrote:
No argument -- 76, 76S, whatever does your job. Spend the money on tows.
As long as the IGC file is good, I'm happy.

But if I read you correctly you are saying that the altimeter in our
gliders are likely to be more accurate than GPS altitude throughout
their range. Is that your contention?

That is a different issue. GPS altitude and pressure altitude aren't
measuring the same thing. If you want to know your actual elevation
above the geode (or even "sea level"), GPS altitude, with all of its
fluctuations, will be far more accurate than any altimeter at anything
above a few thousand feet. If you want to know your pressure "altitude",
calibrated to a pressure gradient from an idealized model of the
atmosphere at a specific sea level temperature and pressure, then an ISA
calibrated barometric altimeter is the way to go.

The problem with the Garmin "S" models is that it is never clear exactly
what they are measuring. In auto-calibration mode, they use some sort
of long period (20 minutes or so) tracking algorithm to continuously
recalibrate the altimeter against GPS altitude. Works pretty well most
of the time, but I've seen the displayed altitude go off on a tangent at
times, for no apparent reason. In any other mode, it's not clear
whether they are using ISA or some other curve, whether there is any
temperature compensation, etc. So, either way, you end up with
something like "altitude" recorded, it's just never very clear exactly
what it is...

Marc

Marc Ramsey wrote:
The problem with the Garmin "S" models is that it is never clear exactly
what they are measuring. In auto-calibration mode, they use some sort
of long period (20 minutes or so) tracking algorithm to continuously
recalibrate the altimeter against GPS altitude. Works pretty well most
of the time, but I've seen the displayed altitude go off on a tangent at
times, for no apparent reason. In any other mode, it's not clear
whether they are using ISA or some other curve, whether there is any
temperature compensation, etc. So, either way, you end up with
something like "altitude" recorded, it's just never very clear exactly
what it is...

Marc

I second Marc's point. I borrowed an S last year, and I also didn't
see any advantage; in fact, it was confusing. For post-flight
analysis, I'd much rather know that I'm using one standard (ie. WGS84)
rather than some hybrid standard (ie. pressure altitude kinda/sorta
adjusted to WGS84). The US contest rules (fairly) indicate that they
have to take the least favorable interpretation in terms of altitude,
and I wouldn't want there to be any debate about what was shown in the
log.

There are about 40 GPSMap76 units listed on eBay right now. If you
have some discipline, you should be able to get one for around $150.

P3

Please look at my website "used" pages
http://www.wingsandwheels.com/page8.htm and
http://www.wingsandwheels.com/page20.htm
I typically have a used CAI loggers and sometimes other types taken on
trade.
tim
Wings & Wheels
www.wingsandwheels.com


"Udo Rumpf" wrote in message
.. .

I need help interpreting the 2006 rules regarding the above subject.
The rules do not discuss a back up flight recorder.
What options exist for a backup flight recorder that would be
excepted across the board in US contests for none team use.
Udo
PS. If you have a used GPS Nav 20 or 25 let me know.