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.


The WAAS vertical guidance component is the same for all three
approaches. What is different is how the WAAS G/S is ultized in the
procedure.

In the case of LPV, the computations of the anchor points take into
account very precise measurements of the earth's curvature and other
ILS-like factors. Also, lateral obstacle clearance tapers down in the
final segment

With LNAV/VNAV the final approach segment's obstacle environment
surfaces are still treated someone like ILS but without all the precise
measurements of the curvature of the earth and other precise anchor
points. In other words the WAAS G/S is emulating a BARO VNAV G/S but
without the temperature errors. There is no taper down of lateral
obstacle clearance.

With LNAV+V there is no vertical guidance provided for in FAA procedure
design. It is strictly a Jeppesen add on, and if done correctly, will
not violate any stepdown fixes in the final approach segment. But, it
is just plain old non-precision obstacle clearance down the final; i.e.,
as little as 250 feet of ROC, unlike the other two, which have greater
obstacle clearance in the earlier portion of the final segment. LPV,
like ILS, goes to less than 250 of obstacle clearance close in, but
LNAV/VNAV does not.

There are many more esoteric factors, but trying to explain those would
become TERPS 101 and 202.