I recently experimented with flying a GPS approach using the GPS
reported altitude as the "altimeter"... First, I flew the GPS approach
track making notes as to the altimeter reading versus the GPS reported
altitude on two GPS units in the cockpit...

Now, knowing the average local offset between the GPS datum circle and
the local ground altitude as reported by the local barometric pressure
altimeter the official pair of altimeters in the pilots lounge... I
wound up with 79 feet as my averaged offset between the GPS datum plane
and the local altimeter... Notice I am emphasizing the word 'local'
here... Your findings will differ with your area and with what they are
doing down inside that Colorado mountain at any given instant...... A
local WAAS transmitter and a WAAS receiver in my plane would, of
course, narrow that altitude difference... Also note the altitude
delta I found is almost within allowable altimeter error...

Anyway, the second trip around I concentrated on flying the altimeter
and IVSI on the
Garmins instead of the altimeter - jeez, are they twitchy; according to
them I never did establish level flight, they always showed either
climb or descent, albeit usually at a rate of ~10 or 20 fpm... Trying
to fly for real with an altitude instrument this twitchy would be
exhausting.. The descent phase on the various legs of the approach was
interesting but doable...

So, my little experiment shows that one can do a GPS approach - after a
fashion - using the GPS altitude reporting... Knowing in advance what
the local differential between the GPS and the cold hard ground would
be nice...

I intend to next fool around with using the GPS IVSI for flying an ILS
in place of the glide slope... I have done this with the barometric
IVSI and had the radar guys watch me... Two times out of three they
said they could not see any divergence... The one time they did see it
I was definitely off the glide slope as I got sloppy on speed control
and wound up too fast...

Fun stuff to do as opposed to droning around in circles...

denny

Newps wrote:
"Mxsmanic" wrote in message
...


No, it is not. It's especially dangerous for RVSM flight, but it's so
inaccurate that it should never be used for anything,

Your complete misunderstanding of the GPS system is showing. I have two
GPS's in the plane, one panel mount and one portable. It is rare that
either one is ever off by more than 100 feet in altitude. Using GPS for
your altitude for VFR flight would be perfectly fine.