My Garmin 530W has something called HFOM and VFOM and the numbers are
usually higher than a couple of feet, which you describe as the practicable
accuracy of a WAAS.

Can you explain this for me?

--

Wyatt Emmerich
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PO Box 16709, Jackson MS 39236
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"J.Kahn" wrote in message
...
Wyatt Emmerich wrote:
Searching the newsgroup archives, I have been unable to find the
technical distinction between LPV, LNAV/VNAV and LNAV+V approaches. I do
not mean the legal and procedural differences. I mean what are the
technical differences of the GPS that makes one more accurate than the
other. In other words, is there some extra GPS data on an LPV approach
that makes it more accurate? Or are they all just as accurate in terms of
location precision and deviation.


LPV requires a WAAS certified GPS receiver that meets the accuracy
requirements of LPV precision approaches, which is something like +/- 10
feet horizontally and vertically thanks to Wide Area Augmentation System
(a ground station network measures satellite signal propagation errors and
calculates and broadcasts correction data to WAAS receivers), and can
duplicate the function of a traditional ILS down to an ILS-ish DH.

Whereas VNAV approaches, which don't require WAAS accuracy, are just a way
of providing ILS-like follow-a-glide path convenience to non precision
step-down approaches (but the glide path can't go below the non-precision
MDA). You can duplicate the function more or less simply by estimating
an applicable descent rate and letdown point and holding the decent rate
down to MDA that allows you to just clear each step in the approach.
Interestingly, with VNAV you are supposed to treat the intersection of
glide path with MDA as the MAP, the way you would with a precision
approach, which may be a quarter mile short of the runway with the MDA at
4 or 5 hundred feet. If you fly the traditional step down and level at
the MDA you can go all the way to the normal MAP at the runway threshold.

I believe the +/- 10 accuracy is a max allowable cert requirement and WAAS
units are in practice accurate to a couple of feet. Non WAAS receivers
are supposed to have an accuracy of +/- 50 feet and in practice are
accurate to around 10-20 feet.

John