GPS and new ATC technology

[David Farber <dave () farber net>]

Begin forwarded message:

From: "Mike O'Dell" <mo () ccr org>
Date: September 1, 2007 12:30:52 PM EDT
To: David Farber <dave () farber net>
Subject: GPS and new ATC technology

GPS is wonderful, when it works, but it doesn't
work everywhere all the time for various reasons,
and not just because of hostile actions. urban
canyons, natural canyons, heavy tree canopy,
tunnels, and under areas with metal roofs, for
a few examples. and it isn't that hard to jam
a GPS signal, or at least desensitize a GPS receiver
so it can't hear the real, very tiny signals from
the satellites.

An FAA study has concluded that before we make GPS the
primary guidance system for commercial aircraft, there
*must* be a reliable backup system which has very different
failure modes so the likelihood of simultaneous outages
(or "loss of precision") is extremely unlikely.

the only existing alternative is LORAN-C, ideally augmented
with "eLoran" capabilities.  the existing LORAN-C infrastructure
has recently been updated. new solid-state transmitters have
dramatically reduced the phase noise on the signals and new
cesium clocks have dramatically increased the stability.

eLoran is a receiver-side technology which exploits all the
work developing "all satellites in view" algorithms for GPS
receivers to provide "all chains in view" capability. a modern
eLoran receiver can track multiple 10s of Loran stations
simultaneously, and by using quadrature antennas and knowledge
of the extremely precise surveyed locations of the Loran
stations, an eLoran receiver can also provide absolute heading
information *without* requiring platform motion (required by
single-receiver GPS for path-over-ground).

further, an inexpensive eLoran receiver can also integrate
a GPS receiver in the same housing, and together they can
provide the absolute positioning ability of GPS ("go to
Lat/Lon xxx/yyy on the other side of the world within
so many meters") with the very high repeatability of Loran
("return to *this* spot precisely") and the ability to
generate static absolute heading. such eLoran receivers are
already down to circa $750 street price (SiTex eLoran) and
that's without high volumes.

The only problem is that in spite of all this, LORAN-C is
an unloved stepchild with continued funding in serious
jeopardy. Only a few million dollars *per year* are required
to maintain and operate the just-upgraded LORAN-C infrastructure, but
nobody can seem to find the funds. the US Coast Guard, who
paid for much of the upgrade, is all but running bake sales
to fund their existing "coastal defense" mission and doesn't
have the money, and while LORAN-C is being used by the trucking
industry for vehicle tracking, not to mention the identified
need as a backup for GPS-based Air Traffic Control, Dept of
Transportation cannot find any money under the couch cushions,
either.

so LORAN-C is scheduled to be shut down because they cannot
maintain the system, and a navigation system that gives wrong
answers is worse than no system at all.

again we have an astonishing example of "penny-wise and pound  
foolishness"
that just boggles the mind.

yeah, i now - take a number.

        -mo


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