From: "John Barrett" <2thman@olympus.net>
To: "Lancair Mail List" <lancair.list@olsusa.com>
Subject: GPS Nav
Date: Tue, 12 Jun 2001 18:37:51 -0700
Message-ID: <NEBBLABMMLJGBIFPPHBBOEJECCAA.2thman@olympus.net>
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Below is the text of an email I received from a representative of the FAA
concerning my letter of concern about alternatives to WAAS.  He sent
attached a document written by his Canadian counterpart (Andrew Graham).
This document concludes that raw GPS vertical data is accurate enough for
precision approaches.  I have uploaded it to my web site (see below).  It is
named "Raw GPS for VNav".

Regards,

John Barrett
www.carbinge.com

>>>
John-

It was a pleasure talking with you yesterday.  As I described on the phone,
WAAS is an augmentation to GPS that will improve the accuracy and
reliability of GPS to the point that it is as accurate as ILS.  Without this
augmentation, the vertical errors from GPS can be significant and would
impact safety if they were not otherwise mitigated.  Highway-in-the-sky (HITS)
is a type of display, and can be used to make aircraft control more intuitive
and reduce pilot-related errors.  Therefore, it is difficult to compare these
two technologies.

You also mentioned the potential to use baro-related data for vertical
reference, as a means of mitigating the GPS vertical error. There is a major
FAA initiative to do just that: the service level is called "LNAV/VNAV". 
The basic idea is to use GPS for lateral guidance, and barometric altitude
(or a combination of barometric altitude and GPS altitude) for vertical
guidance.  Many air transport category aircraft have been equipped with this
capability for years, using the criteria of AC 20-129.  The aviation industry
recently updated these requirements, and they can be found in RTCA/DO-236A
(available at www.rtca.org).

This type of vertical guidance was identified as a key safety improvement as
part of the Safer Skies initiative.  The ability to fly a constant-descent
rate, stabilized approach substantially increases safety over a non-precision
approach.  About 100 approach procedures have already been published that
support this capability: the procedures are published as "RNAV" approaches
and have a minima line called "LNAV/VNAV".  Using this technology, the FAA will
increase the number of runway ends with a vertically-guided approach by 10%
in 2001, further increasing in future years.  The approach criteria for this
type of approach is published in Order 8260.48.

While this technology has been generally available in the transport comunity
for years, it has historically not been available to the general aviation
community due to the cost of an air data computer.  Recent technology developments
(turning GPS selective availability off, high-resolution baro encoders,
etc.) may bring this capability to the GA community.  The FAA is pursuing these
concepts in an Ad Hoc working group of the Satellite Operational Implementation
Team (SOIT).  Please find attached a copy of a draft working paper written
by my co-chair in this activity, Andrew Graham (NavCanada).

If you have any additional questions or ideas, please do not hesitate to
call me.  We are all eager to provide vertical guidance to as many aircraft as
possible at the lowest possible cost.

Bruce DeCleene
Navigation Program Manager
Aircraft Engineering Division, Aircraft Certification


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