Capstone

"Scott Moore" wrote in message
newstttc.5331$eY2.1195@attbi_s02...
Bob Noel wrote:


Mode-S doesn't have a provision for "anonymous." And changing
the specs for Mode-S would be, ahem, challenging. It's not
actually an FAA issue.

Well, I do understand the issue, but not why anonymous is so hard. The
shop programs the N number, right ? What is to prevent them from declaring
a "universal" N number (the equivalent of 1200) and just programming all
"protestants" with that on request ?

You're confusing the "Flight ID" variable with the ICAO 24-bit ID. For most
GA aircraft, both are programmed at install. The flight ID is set to the
N-number and the 24-bit ID is set. In air transport A/C, the Flight ID can
be set from the flight deck to reflect the current Flight ID.

The difference is that the Mode-S system doesn't care what the N-number or
Flight ID is (tho the controller might), it's just a variable to be
transmitted. The ICAO 24-bit ID is a different story. It is the "network"
address for your transponder. It is embedded in all Mode-S communications
to and from your transponder. For the system to work, the 24-bit IDs have
to be unique. For comparison, you can think of it as a hard-coded IP
address.

If you've done much network support, you'll know what kind of problems occur
when there are duplicate addresses in the system. To insure the system
works, ICAO assigned blocks of the 24-bit addresses to each member state,
who is then responsible for assigning them to specific aircraft. The FAA
assigns one to an aircraft when it is registered (whether it has a Mode-S
transponder or not). Look in the registry database if you want to know
yours.

The problem with 'anonymous' IDs is managing them so that you can have a
randomly assigned ID without a chance of duplication. You could conceivably
create some Mode-S equivalent to DHCP, but that is a) non-trivial, and b)
wouldn't necessarily provide anonymity. In the end, it would be a lot of
work for a feature that would just make the units more expensive to satisfy
a small vocal segment of the customer base.

Gerry