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
wyatt@northsidesun.com
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,

The vertical guidance component is the same in the three approach types. Of
course, the LPV and LNAV/VNAV have a DH, whereas the LNAV+V has an MDA. The
primary technical differences are in the lateral guidance and where the Full
Scale Deflection (FSD) of the CDI is defined. In both cases, the FSD is
angular, typically +/- 2 degrees inside the FAF. The LNAV or LNAV/+V has the
angle set by the GPS and a fixed minimum course width of +/- 350 feet at the
threshold whereas the LNAV/VNAV or LPV has the angle and course width set by
the database.

Another difference between the lateral aspects of the approaches is based on
the required accuracy and integrity of the GPS signal at the time of the
approach. The Horizontal and Vertical Protection Limits (HPL and VPL) are
roughly defined as the 99.99999% certainty that the actual horizontal and
vertical positions fall within the protection limits. The HPL and VPL are
continuously calculated by the WAAS GPS. If the HPL exceeds 40 meters, then
an LPV approach may not be conducted. For an LNAV/VNAV or LNAV/+V or LNAV,
HPL must not exceed 556 meters. For approaches with a DH at 250 ft or
higher, the VPL must be below 50 meters or vertical guidance will not be
provided and the GPS will downgrade to LNAV minimums. For approaches with a
DH below 250 ft, the VPL must not exceed 35 meters.

The area that the approach design must protect against obstacles varies
based on the type of approach. The obstacle protected area is largest for
an LNAV approach due to the allowed drive and dive method of flying and the
allowed horizontal integrity. It is somewhat lower for the LNAV/VNAV
approach due to the glide path and much lower for the LPV due to both the
glidepath and the required integrity of the horizontal position. In fact
the protection area is the same for an LPV and an ILS Cat I approach.

The specifications for WAAS GPS equipment is defined in RTCA DO-229D and TSO
C146a. A very good book on the subject is Instrument Flying Update by John
C. Eckalbar.

Regards,

John Collins

--

Regards,

John D. Collins
4317 Old Saybrook Ct
Charlotte, NC 28211
(704) 364-3696 Tel/Fax
(704) 576-3561 Cell
johncollins@carolina.
"Wyatt Emmerich" <wemmerich@> wrote in message
news:NsKdnZyDcI6orv3anZ2dnUVZ_qelnZ2d@. ..
> 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
> wyatt@northsidesun.com
> PO Box 16709, Jackson MS 39236
> Shipping: 246 Briarwood Drive, Suite 101, Jackson MS 39206
>