MIL-STD-291C
5.10 Dead time. Dead time is defined as the total time following a decoded pulse pair (from an interrogation or noise) during which there will be no response to subsequent interrogations or noise. The dead time shall not normally exceed 60 µsec and is not usually extended by the echo suppression circuits, but or special cases where the geographical site of the transponder is such
as to produce undesirable reflection problems, a longer echo suppression time may be specified;
this will tend to increase the average dead time. In extreme cases, this may require
compensating pulses to maintain azimuth accuracy. The spacing between any two consecutively transmitted random pulse pairs that are not replies shall be not less than 60 µsec.
5.11 Composite 15 and 135 Hz variable bearing signal. The bearing signal shall be generated by rotating a directional pattern which produces, at a point in space, a composite amplitude modulation of the r.f. pulse signals at 15 and 135 Hz. Viewed from above, the direction of pattern rotation shall be clockwise. For X-mode operation the amplitude modulation shall be synchronized with the 15 Hz main reference group so that at a point due south of the beacon the 10th pulse of the main reference group will coincide with the positive slope point of inflection of the 15 Hz component of the bearing signal and at points geographically spaced by
all multiples of 40 degrees from due south, the 12th pulse of each auxiliary reference group shall coincide with the positive slope point of inflection of the 135 Hz component. For Y-Mode operation the coincidence points shall be midway between the 6th and 7th pulses of the 15 Hz reference group, and shall be at the 12th pulse of the 135 Hz reference group. The main reference group shall occur when the maximum of the composite signal is directed due east of
the transponder. In the absence of harmonics, the modulation envelope of the detected r.f. pulse signal shall follow this formula:
y = 1.0 Asin (2 π ft ± θ -γ) + Bsin (18 π ft ± 0 - 9γ) Where:
(1) y = normalized composite 15- 135 Hz signal amplitude
(2) A = represents modulation of 15 Hz component
(3) B = represents modulation of 135 Hz component
(4) θ = deviation in electrical degrees from coincidence of the 15 Hz modulation and the main reference group.
(5) 0 = deviation in electrical degrees from coincidence of the 135 Hz modulation and the auxiliary reference group.
(6) γ = bearing to the ground or shipboard station from the point of observation. (7) f = pattern rotation frequency in Hz.
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