Fellow TruTrak Owners,
I had an interesting conversation with Chuck Bilbe at TruTrak yesterday at
SNF about the new Sorcerer and GPSV. I have a CNX80 (Garmin GNS480) which
will fly all the GPS approaches with all the vertical guidance. But my
TruTrak DFC 250 isn't compatible with GPSV. But the new Sorcerer is, sort
of, according to TruTrak.
I got a crash course (not the best name, I guess) is autopilots
yesterday. GPSS, which is the horizontal guidance that allows an autopilot
to fly courses very accurately, do things like turn anticipation, and on my
unit, fly holding patterns, is driven by a 32 bit data stream that gives a huge
amount of information like distance from the fix on the approach (important if
you want to adjust the autopilot's gain as you get closer to the fix), rate of
closure on the course, etc. Far more than a CDI "needle" gives you, so you
can accurately fly a fast airplane without zig-zagging back and forth across the
course.
Apparently the GPSV (vertical signal) coming from my UPSAT/Garmin unit is
simple "needle" information. TruTrak says that was they really need is 32
bit data for the vertical axis so it can keep from ballooning up-and-down across
the vertical course. TruTrak says that the Sorcerer will fly just fine
when the GPSV signal is a 3 degree glideslope course, but when you have a step
down type of approach it can't hold the vertical as well. Apparently, Stec
claims to be stable, but they only have the "needle" information too, and they
aren't very stable either in fast airplanes.
My contact at Garmin, a former UPSAT guy, said that they are aware of the
issue, but resources aren't available right now to write the software to drive
the 32 bit GPSV signal. I'm not sure weather I should upgrade to the
Sorcerer or not, even though TruTrak would update my unit when the new 32 bit
data is available from Garmin. I'm curious why the "standard" data format
wasn't established by the ARINC people, or whoever decides this stuff.
More to come when I dig a little deeper.
Mike Easley
Colorado Springs
ES