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From: dwl@hou4b.UUCP (D Levenson)
Newsgroups: net.aviation
Subject: Re: IFF/SIF Modes - (nf)
Message-ID: <1223@hou4b.UUCP>
Date: Thu, 13-Dec-84 08:55:47 EST
Article-I.D.: hou4b.1223
Posted: Thu Dec 13 08:55:47 1984
Date-Received: Fri, 14-Dec-84 06:50:25 EST
References: <1200006@uokvax.UUCP>
Organization: AT&T Information Systems Laboratories, Holmdel, NJ
Lines: 19

Civilian transponders are capable of replying to two types of
interrogations, mode A and mode C.  The difference between the
interrogations is detectable by the spacing of the two pulses which make
up the transmission.  The transponder answers mode A by sending the
code selected by its operator.  In answers mode C by sending the
pressure-altitude, encoded by a Gray code.

Most ATC facilities alternately interrogate modes A and C, thus
requesting identity and altitude on alternate polls.  Polls occur at
a rate of about 100 per second, during the interval that your
transponder is illuminated by the major lobe of the ground station's
antenna.

Military mode 3, I think, corresponds to Civil mode A.  I'm not sure
what military mode corresponds to Civil mode C, or what the other
modes (Civil B, D; Military 1, 2, etc.) are for.

-Dave Levenson
AT&T-ISL, Holmdel