User's Manual

SYSTEM DESCRIPTION AND INSTALLATION MANUAL
T
3
CAS/Part No.9005000
(b) Mode Select
Air traffic density has significantly increased since the inception of
ATCRBS. Aircraft within the same approximate range and azimuth from
the interrogator may interfere with each other’s replies. Much verbal
communication is also needed to coordinate ATC, causing more traffic
on communication channels. This also places a heavier burden on the air
traffic controllers.
Mode Select (Mode S) has been designed as an evolutionary addition to
ATCRBS to supply enhanced surveillance as well as data
communication capability for ATC, with greater degrees of automation in
mind. Ground-air-ground data link communications can be
accommodated with the surveillance interrogations and replies,
permitting use of the Transponder for a number of different ATC
functions. Messages can be either 56 or 112 bits in length. The same
transmit and receive frequencies are used as for ATCRBS.
1 Ground and Airborne Installations
To facilitate the introduction of Mode S into ATCRBS, both ground
and airborne Mode S installations include full ATCRBS capability.
Mode S interrogators supply surveillance of older ATCRBS-equipped
aircraft, and Mode S Transponders reply to ATCRBS interrogators.
Mode S interrogators are able to command Mode S Transponders
not to reply to compatible ATCRBS-only interrogations. They are
also able to solicit only Mode S replies from Mode S Transponders
which minimizes RF transmissions.
Another unique aspect of Mode S is that each aircraft equipped with
a Mode S Transponder is assigned a unique 24-bit address. This
address appears in either a coded or clear form in every Mode S
reply. This not only improves aircraft identification by ATC, but also
permits selective interrogation once the aircraft has been acquired by
an ATCRBS/Mode S or Mode S-only All-Call interrogation. This is
aimed toward reducing RF channel loading.
Another way to minimize RF traffic is the capability of locking out the
transponder from replying to All-Calls, from either all interrogators or
from specific ones, for retriggerable 18-second intervals. Including
the station’s identity code in the interrogation message enables this
lockout. Probability-based replies make it possible to separate
transmissions from aircraft that would otherwise be garbled when a
group of aircraft might answer a single interrogation.
All transmissions for surveillance or data communications, from the
ground up to the aircraft (uplink) and from the aircraft down to the
ground (downlink), are protected by a 24-bit Cyclic Redundancy
Code (CRC) error detection scheme, also referred to as parity. In
addition, ground interrogators can perform error correction on
received downlink messages.
Pub. No. 8600200-001, Revision 004
34-45-29
1-112
04 Nov 2014
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