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