The US FAA has a long history of requiring certain avionics and then
declaring it obsolete in a few years. The long history of slowly reducing
VHF comm radio channel separation is an example.

Ian is right that a GPS based ATC system is the best way to go. I fear it's
just too simple an answer for the bureaucrats to get their minds around.
There's an old joke that says the most terrifying words in the English
Language are, "I'm from the government and I'm here to help."

Bill Daniels

"Ian Strachan" wrote in message
oups.com...
Gilbert Smith wrote:

I manage an airstrip close to a CTR and close to the final approach
path to the international airport inside the CTR. We have an agreement
with the airspace authority which confines our flight paths to a safe
area.

A visiting pilot was given a transponder code and told to keep it
selected on his departure scheduled for 15 minutes after landing,
which he duly did. This caused a TCAS alert on a landing passenger
jet. Our agreement now specifies transponders switched off (not even
squawking standby) within 5 miles of our strip.

?Gilbert, could you give some more details. Particularly, is your
strip in the UK? Is your agreement a local one with the airport, or is
it with (or known to) the National Air Traffic or Regulatory body?

On the general matter of aircraft location and proximity warning
systems, radar is essentially a product of World War II technology
whereas ADS-B is the future. ADS-B will provide air traffic
controllers and pilots with much more accurate information that will
help keep aircraft safely separated.

Those words come not from me, but from the US FAA.

As I understand it, ADS-B (Automatic Dependent Surveillance-Broadcast)
is a system based on Satellite Navigation that automatically transmits
GPS (or GLONASS or Galileo) position and other data from an aircraft to
other receivers in ATC units and/or other aircraft. It is being tested
by the USA FAA and also in Australia and certainly seems to be the
system of the future. Here is an extract from the FAA Fact Sheet dated
2 May 2006

"ADS-B is the future of air traffic control. Instead of using radar
data to keep aircraft at safe distances from one another, in the
future, signals from Global Positioning Satellites will provide air
traffic controllers and pilots with much more accurate information that
will help keep aircraft safely separated in the sky and on runways.

Although radar technology has advanced, it is essentially a product of
1940s World War II technology. Radar occasionally has problems
discriminating airplanes from migratory birds and rain "clutter."
Secondary surveillance systems can determine what objects are because
they interrogate transponders; however, both primary and secondary
radars are very large structures that are expensive to deploy, need
lots of maintenance, and require the agency to lease real estate to
situate them.

ADS-B, on the other hand, receives data directly rather than passively
scanning for input like radars, so does not have a problem with
clutter. ADS-B ground stations are inexpensive compared to radar, and
are the size of mini refrigerators that essentially can go anywhere, so
they minimize the required real estate. In addition, ADS-B updates once
a second and locates aircraft with much more precision.

ADS-B also provides greater coverage, since ADS-B ground stations are
so much easier to place than radar. Remote areas where there is no
radar can now have precise surveillance coverage."

--------- end of FAA quote ---------

So why are some Authorities trying to impose expensive and power-hungry
transponders on people who fly mostly in unregulated airspace? Also,
there are many types of aircraft that do not have electrical
generators, such as gliders, hang gliders, para gliders and many motor
gliders (turbos for instance).

Surely, a future system based on Satellite Navigation would be much
better all round. GPS is now being carried in most GA and many sport
aircraft worldwide. A smart avionics engineer should be able to design
a special low-powered transmitter that would take an NMEA or other
output from existing GPS equipment and automatically transmit the data
on (electronic) request. This could be a practical step towards the
full ADS-B system of the future and would not involve the fitting of
transponders to such classes of aircraft, Mode S or otherwise. It
seems very similar to what is already part of ADS-B link technology,
the Universal Access Transceiver (UAT), for which development
(according to the FAA web site) started in the mid-1990s.

Ian Strachan
Lasham Gliding Centre, UK