Peter R. wrote:
Andrew Sarangan wrote:

That mental calculation could be easily performed by the GPS and
displayed as a glideslope. But I have not seen any GPS do that.
Certification is irrelevant.

Certification is most certainly relevant to your query, as that is most
likely what kept that feature out of the TSO C129a certified IFR GPS's.

A handheld Garmin 196 (I think that is the model a pilot-friend had with
him a couple of years ago) that we took up on a practice IFR flight did
just that. It displayed a glideslope for a non-precision approach. If the
cheaper handhelds can do it, then why don't their IFR-certified TSO C129a
big brothers do it?

But handhelds aren't certified, so there's no guarantee of correctness.


The basic requirement they don't meet is the Integrity requirement,
e.g. the guarantee a) that the error can be bounded and b) that
sufficient warning can be provided when Integrity can not be met.

129 boxes aren't certified for Vertical Guidance, so I suspect that,
even for an NPA (LNAV) approach the same would hold true.

Because it wasn't part of the certification and
therefore, regardless of their ability to provide this feature, are
restricted from doing so due to the certification.

The lack of certification is based on Standalone (Unaugmented) GPS not
being certified for Vertical guidance. This traces to the fact that the
dominant error (after SA was turned off) is the Ionospheric component
and the recevier's model (Klobuchar) is not certified to provide
sufficient Integrity for the Vertical component. With SBAS (e.g. WAAS
in the US), the Integrity requirement has been proven to be met with
sufficient Availability over the Service Volume, to approve approaches
with Vertical Guidance,.

Note that when even when the 145/6 boxes were deployed up in Alaska
(Capstone project), WAAS had yet to be commissioned, and thus the
published approaches were LNAV only.

Also note, there had been talk of building newer 129 boxes, but with
the 145/6 boxes now out, the manufs. apparently can't cost justify
upgrading a box that still wouldn't perform as well as the 145/6

--
Peter

Regards,
Jon