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3.1.2.1.5.2 Mode S interrogation. The Mode S interrogation shall consist of three pulses: PI, P2 and P6 as shown in
Figure 3-4.
Note.- P6 is preceded by a PI - P2 pair which suppresses replzes from Mode AK transponders to avoid synchronous
garble due to random triggering by the Mode S interrogation. The sync phase reversal within P6 is the tuning mark for
demodulation of a serzes of time intervals (chips) of 0.25 microsecond duration. This series of chips starts 0.5 microsecond
after the sync phase reversal and ends 0.5 microsecond b&fore the trailing edge of Pg- A phase reversal may or may not
precede each chip to encode its bznary information value.
3.1.2.1.5.2.1 Mode S side-lobe suppression The P5 pulse shall be used with the Mode S-only all-call interrogation
(UF = 11, see 3.1.2.5.2) to prevent replies from aircraft in the side and back lobes of lhe antenna (3.1.2.1.5.2.5). When
used, Y5 shall be transmitted using a separate antenna pattern.
Note I.- The actiorz of P5 is azltomatic. Its presence, if of suficie~zta mplitude at the receiving location, masks the sync
phase reversal ofP6.
Note 2.- The P5 pulse may be used with other Mode S interrogations.
3.1.2.1.5.2.2 Sync phase reversal. The first phase reversal in the P6 pulse shall be the sync phase reversal. It shall be
the timing reference for subsequent transponder operations related to the interrogation.
Chapter 3 Annex 110 - Aeronautical Telecommunications
' Mode AICIS all-call: 1.6 ps
Mode AIC-mly all-call: 0.8p
Figure 3-3. Intermode interrogation pulse sequence
1-7 SLS control I
transmission; ' 1
r 2 . 0 ST 2.77 p s 3 y , 20.55 ppss Guard
4 1.25 ps
p2
0.4 ps* -
SLS control p5
transmission
Figure 3-4. Mode S interrogation pulse sequence
Annex 10 - Aeronautical Tekco~nmunications Volume ZV
3.1.2.1.5.2.3 Duta phase reversals. Each data phase reversal shall occur only at a time interval (N times 0.25) plus
or minus 0.02 microsecond (N equal to, or greater than 2) after the sync phase reversal. The 16.25-microsecond P6 pulse
shall contain at most 56 dala phase reversals. The 30.25-microsecond P6 pulse shall contain at most 112 data phase
reversals. The last chip, that is he 0.25-microsecond time interval following the last data phase reversal position, shall be
followed by a 0.5-microsecond guard interval.
Note.- The 0.5-microsecond guard interval follonling the last chip prevents the trailing edge of P6 from irztorfcring
with the demodulation process.
3.1.2.1.5.2.4 Intervals. The pulse interval belween P, and P, shall be 2 plus or minus 0.05 microsecond. The interval
between the leading edge of P2 and the sync phase reversal of P, shall be 2.75 plus or minus 0.05 microsecond. The
leading edge of P6 shall occur 1.25 plus or minus 0.05 microsecond before the sync phase reversal. P5, if ~ansmitteds,h all
be centred over the sync phase reversal; the leading edge of P5 shall occur 0.4 plus or minus 0.05 microsecond before the
sync phase reversal.
3.1.2.1.5.2.5 Pulse amnplitl~tdesT. he amplitude of P2 and the amplitude of the first microsecond of P6 shall be greater
than the amplitude of PI minus 0.25 dB. Exclusive of the amplitude transients associated with phase reversals, the
amplitude variation of P6 shall be less than 1 dB and the amplitude variation betwccn successive chips in P6 shall be less
than 0.25 dB. The radiated amplitude of P5 at the antenna of the transponder shall be:
a) equal to or grealcr than the radiated amplitude of P6 from the side-lobe transmissions of the antenna radiating P6;
and
b) at a level lower than 9 dB below the radiated amplitude of P6 within the desired arc of interrogation.
3.1.2.2 RWLY SIGNALS-IN-SPACE CHAFSCTERISTICS
3.1.2.2.1 Reply carrier frequency. The carrier frequency of all replies (downlink transmissions) from transponders
with Mode S capabilities shall be 1 090 plus or minus 1 MHz.
3.1.2.2.2 Reply spectrum. The spectrum of a Mode S reply about the carrier frequency shall not exceed the limits
specified in Figure 3-5.
3.1.2.2.3 Polarization. Polarization of the reply transmissions shall be nominally vertical
3.1.2.2.4 Modulation. The Mode S reply shall consist of a preamble and a data block. The preamblc shall be a 4-
pulse sequence and the data block shall be binary pulse-position modulated at a 1 megabit per second data rate.
3.1.2.2.4.1 Pulse shapes. Pulse shapes shall be as defined in Tablc 3-2. All valu~sa re in microseconds.
3.1.2.2.5 Mode S reply. The Mode S reply shall be as shown in Figure 3-6. The data block in Mode S replies shall
consist of either 56 or 112 information bits.
3.1.2.2.5.1 Pulse intervals. All reply pulses shall start at a defined multiple of 0.5 rnicrosccond from the first
transmitted pulse. The lolerance in all cases shall be plus or minus 0.05 microsecond.
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