Re: Automated data transfeer for ground SCU

["Charles Freeman" <cfreeman@alum.mit.edu>]

John,

I understand what you have been working with and I will follow your
developments with interest.

As you have pointed out, the problem with the APRS system you are proposing
is one of infrastructure. VHF is subject to nearly the same line-of-sight
propagation characteristics as UHF.  Thus with a ground-based VHF solution,
the aircraft would have to be line-of-sight to a ground repeater station in
order to be in contact. Unfortunately, the moment when we are most concerned
about what our aircraft are up to is as they land. This puts their antenna
at zero height and reduces their effective coverage area to a small spot,
especially if they are landing in a canyon or valley floor. A large
infrastructure of ground communications sites will be required. The existing
systems which use this technique have not proven very cost effective or
reliable in areas of non-flat terrain.

The solution is to use a satellite as the location of the repeater. This
allows you to build a system without the large ground infrastructure.
However, satellites themselves are expensive, so to be affordable you need
to piggyback onto existing systems.

There are essentially five satellite based systems out there right now using
three technologies.

The first technology is the LEO, or Low-Earth Orbit satellite. The advantage
of these is they are low, so you don't need a lot of transmit power or fancy
antennas to talk to them. The box in the aircraft can be small and
inexpensive. The disadvantage of a low satellite is that much like cell
towers you need a lot of them to provide 100% coverage, and you need several
earth stations to make sure the satellite can see both you and the earth
station at the same time. This makes the system expensive to build and
operate, so in our current fiscal climate the financial viability of these
systems is questionable. GlobalStar and Orbcomm are two competing companies
using this technology.

The second technology is Iridium. These are also LEO satellites but they
operate at a higher altitude than Orbcomm or Globalstar, so fewer satellites
are required. They are also very sophisticated, relaying messages from
satellite to satellite so only one ground station is required. The Iridium
system went bankrupt, but was bailed out by the US Government and right now
is in fair financial shape. This technology bears watching.

The last technology is the geostationary satellite. These operate in high
earth orbit and stay fixed over one spot on the earth's surface. The
advantages are that very few satellites and very few earth stations are
required to maintain contact. These systems are very fast and reliable as a
result. They use a technology to communicate which allows for a very large
number of stations to be in operation without congesting the system. The
disadvantage is that a more powerful and sophisticated transceiver is
required in your aircraft, raising the hardware acquisition costs. However,
considering the cost of an EMS aircraft they are still very affordable.
Inmarsat C and Outerlink are two examples of such technology that have shown
to be financially viable.

Two of these existing satellite technologies have marketed themselves
specifically to the aviation market. Echo Flight is a company which has
built aviation-specific products using the Orbcomm LEO technology. These are
cheap but somewhat low-performance. Outerlink has built a very high-quality
system which is rapidly gaining clients in the Air Medical market.

Without trying to advertise in this forum, I'll mention briefly that
Innovative Engineering has built dispatch, mapping, and flight tracking
software which integrates to both of these technologies. Our AeroMap and
Dispatch modules provide the base end for two way textual communication as
well as flight tracking using either of these systems. These systems are
available now off the shelf. For more information please contact me or go to
our website, www.aeromed-software.com.

I hope that this has helped you assess the existing state of the art in
text-based aircraft communications technologies. I will be interested to see
how your APRS systems develop.

Charlie Freeman
Innovative Engineering




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