"Jay Maynard" wrote in message
...
On 2008-09-05, Thomas Borchert wrote:
In the US, the tail number is assigned an 8-digit octal number to be
used as
the flight ID algorithmically by the FAA.
Hmm. Do you have a source for that?

Not directly, but I got curious to see how it was assigned, and started
poking at the FAA registration database (which includes the assigned
code).
The algorithm winds up assigning 50000001 to N1, 50000002 to N1A, 50000003
to N1AA, 50000004 to N1AB,... 50000032 to N1AZ, 50000033 to N1B, 50000034
to
N1BA,... 50001131 to N1ZZ, 50001132 to N10, 50001133 to N10A, and so on. I
never got to the point of writing C code that would generate the code, but
it would be fairly straightforward. The algorithm depends on the rules for
assigning N numbers, and works left to right, with the letters in order
from
A to Z (skipping I and O), then 0-9, taking all of the letter combinations
in order before expanding the number field.

There's no need as it's already been done and put online.

http://www.airframes.org/

Since every airline flight using a flight number rather than a tail
number
for the flight ID field in the mode S data packet would break that
algorithm, it doesn't make much sense to use it in the first place.

It saves having an application process for a code separate from the
registration process. They have to come up with the number from somewhere,
and since they have a block big enough to accommodate every possible N
number, it works out easiest that way.

This is exactly correct. The registration number and the MODES ICAO ID are
one and the same.

The ICAO ID can also be used to selectively address the transponder (the S
in ModeS stands for "select"). So one particular transponder may be
interrogated exclusively by using the unique ICAO ID. Part of the reason
for this is the FAA was concerned about transponder saturation in areas that
have a high density of sensors. The theory was that a ModeS transponder
could be selectively interrogated by one sensor, and that sensor would share
that position data with other sensors within range. This technology was
never implemented in the US for a few reasons, but that capability still
exists within the transponder standard.