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the presence and determine the address of Mode S-equipped aircraft using their Mode S acquisition squitters (DF = 11)
or extended squitters (DF = 17).
Note I.- It is acceptable to acquire individual aircraft using either acquisitiorz or extended squitters (DF = I I or
DF = 17), and to rnonitor for both squitters. However; ACAS rnust monitor for acquisitiorz squitters because, at any tirne,
not all aircraft will transmit the extended squitter:
Note 2.- in the)dture, it becomes permitted jor ainrafi not to transmit the acquisition squitter; relying instead on
continuul transmission of the exten.ded squitter; it would become essential for all ACAS ~irzits to muriitor for both the
acqzrisition and th,e extelrded squitters.
Annex 10 - Aeronautical Telecommunications Volume IV
4.3.7.1.2.2 Surveillance interrogations. On first receipt of a 24-bit aircraft address from an aircraft that is
determined to be within the reliable surveillance range of ACAS based on reception reliability and that is within an
altitude band 3 050 m (10 000 ft) above and below own aircraft, ACAS shall transmit a short air-air interrogation
(UF = 0) for range acquisition. Surveillance interrogations shall be transmitted at least once every five cycles when this
altitude condition is satisfied. Surveillance interrogations shall be transmitted each cycle if the range of the detected
aircraft is less than 5.6 km (3 NM) or the calculated time to closest approach is less than 60 s, assuming that both the
detected and own aircraft proceed from their current positions with unaccelerated motion and that the range at closest
approach equals 5.6 km (3 NM). Surveillance interrogations shall be suspended for a period of five cycles if:
a) a reply was successfully received; and
b) own aircraft and intruder aircraft are operating below a pressure altitude of 5 490 m (18 000 ft); and
c) the range of the detected aircraft is greater than 5.6 krn (3 NM) and the calculated time to closest approach exceeds
60 seconds, assuming that both the detected and own aircraft proceed from their current positions with unaccelerated
motion and that the range at closest approach equals 5.6 km (3 NM).
4.3.7.1.2.2.1 Range acquisition interrogations. ACAS shall use the short air-air surveillance format (UF = 0) for range
acquisition. ACAS shall set AQ = 1 (Chapter 3, 5.1.2.8.1.1) and RL = 0 (Chapter3, 3.1.2.8.1.2) in an acquisition
interrogation.
Note 1.- Setting AQ = 1 results in a reply with bit 14 of the RIfield eqi~alt o 1 and serves as an aid in distinguishing
the reply to own interrogation fronz replies elicited from other ACAS units (4.3.7.'1.2.2.2).
Note 2.- In the acquisition interrogation RL is set to 0 to command a short acquisition reply (DF = 0).
4.3.7.1.2.2.2 Tracking interrogations. ACAS shall use the short air-air surveillance format (UF = 0) with RL = 0 and
AQ = 0 for tracking interrogations.
4.3.7.1.2.3 Surveillance replies. These protocols are described in 4.3.11.3.1.
4.3.7.1.2.4 ACAS broadcast. An ACAS broadcast shall be made nominally every 8 to 10 s at full power from the top
antenna. Installations using directional antennas shall operate such that complete circular coverage is provided nominally
every 8 to 10 s.
Nore.- A broadcast causes other Mode S transponders m accept tlze interrogation without replying and to present
the interrogation content containing the Mufield at the transponder output duta interface. The UDSl = 3, UDS2 = 2
combination ideiztfles the data as an ACAS broadcast containing the 24-bit address of the interrogating ACAS aircrafl.
This provides each ACAS with a means nf determining the number of other ACAS within its detection range for limiting
interference. The format of the MU field is described in 4.3.8.4.2.3.
4.3.7.2 AIR-AIR COORDINATION PROTOCOLS
4.3.7.2.1 Coordination interrogatiolzs. ACAS shall transrnjt UF = 16 interrogations (Chapter 3, 3.1.2.3.2,
Figure 3-7) with AQ = 0 and RL = 1 when another aircraft reporting RI = 3 or 4 is declared a threat (4.3.4). The MU
field shall contain the resolution message in the subfields specified in 4.3.8.4.2.3.2.
Note I.- A UF = 16 interrogation with AQ = 0 and RL = I is intended to cause a DF = 16 reply ,from the other
aircraft.
Note 2.- An aircraft reporting RI = 3 or RI = 4 is an aircraft equipped with an operating ACAS wlzich has vertical
only or vertical and horizontal reso[utr'on capability, respectively.
4.3.7.2.7 Coo~dinntin~rezp ly These protocols are described in 4.3.1 1.3.2.
Chapter 4 Annex I 0 - Aeronautical Telecommunications
4.3.7.3 PROTOCOLFOSR ACAS COMMUNICATION WITH GROUND STATIONS
4.3.7.3.1 RA reports to Mode S ground stations. These protocols are descrjbed in 4.3.1 1.4.1
4.3.7.3.2 RA broadcasts. RA broadcasts shall be transmitted at full power from the bottom antenna at jittered,
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