LPV versus LNAV/VNAV versus LNAV+V

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.

--

Wyatt Emmerich
President, Emmerich Newspapers
601-977-0470

PO Box 16709, Jackson MS 39236
Shipping: 246 Briarwood Drive, Suite 101, Jackson MS 39206

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.

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

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
President, Emmerich Newspapers
601-977-0470

PO Box 16709, Jackson MS 39236
Shipping: 246 Briarwood Drive, Suite 101, Jackson MS 39206
"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

Wyatt Emmerich wrote:
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?


I can't give you the technical answers. The FAA experts tell me that
the WAAS vertical path on LPV onlykeeps increasing in accuracy from the
PFAF to the threshold, just like an ILS G/S.

On Dec 14, 10:58 am, "Wyatt Emmerich" wrote:
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?

FOM = Figure of Merit

Not sure if Garmin is truly using HFOM/VFOM as defined; if so, these
are bounds on the expected accuracy values *assuming there's no
satellite failure*.

HPL/VPL (Protection Level) are part of the Integrity function being
provided by the receiver. They are sometimes referred to as
"containment" values. These are based on the receiver being able to
detect (FD in the case of TSO129) and exclude (FDE in the case of
TSO145) a faulty measurement (ranging errors).


Regards,
Jon