The 95 percent confidence figure seems bizarre. So you die every twentieth
approach down to minimums?


"John Collins" wrote in message
...
Wyatt,

The HFOM and VFOM are defined as horizontal and vertical figure of merit
and
are 95% confidence numbers. They are required to be displayed to the
pilot
by TSO-146a and RTCA DO-229c. They are related to the HPL and VPL values
except that they don't take exclusion into account and are based on two
standard deviations verses 5 standard deviations for HPL and VPL. They
are
also given in feet verses meters for HPL and VPL. Doing a little math, one
can convert between HPL or VPL and HFOM or VFOM as long as you assume
there
are no satellites being excluded. The bottom line is that a VFOM less
than
about 60 feet will be required in order to conduct an LPV approach with a
DH
250 ft or higher without it downgrading to LNAV minimums and not providing
vertical guidance. If an LPV approach has a DH lower than 250 feet, such
as
the new 200 ft minimums, the VFOM will have to be below 40 feet. Usually
the value of HFOM will not have any operational consequences. I typically
see VFOM values around 20 to 23 feet in NC.

Regards,
,

John D. Collins
4317 Old Saybrook Ct
Charlotte, NC 28211
(704) 364-3696 Tel/Fax
(704) 576-3561 Cell

"Wyatt Emmerich" wrote in message
. ..
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