Graphics Programs Reference
In-Depth Information
j
φ
Σ()
S
2
=
(
1
+
e
)
(9.20)
The phase error signal is computed from the ratio
∆Σ
⁄
. More precisely,
j
φ
∆
---
1
e
φ
--
=
------------------
=
j
tan
(9.21)
j
φ
1
+
e
which is purely imaginary. The modulus of the error signal is then given by
φ
--
∆
Σ
------
=
tan
(9.22)
This kind of phase comparison monopulse tracker is often called the half-angle
tracker.
9.4. Range Tracking
Target range is measured by estimating the round-trip delay of the transmit-
ted pulses. The process of continuously estimating the range of a moving target
is known as range tracking. Since the range to a moving target is changing with
time, the range tracker must be constantly adjusted to keep the target locked in
range. This can be accomplished using a split gate system, where two range
gates (early and late) are utilized. The concept of split gate tracking is illus-
figure. The early gate opens at the anticipated starting time of the radar echo
and lasts for half its duration. The late gate opens at the center and closes at the
end of the echo signal. For this purpose, good estimates of the echo duration
and the pulse center time must be reported to the range tracker so that the early
and late gates can be placed properly at the start and center times of the
expected echo. This reporting process is widely known as the Ðdesignation pro-
cess.Ñ
The early gate produces positive voltage output while the late gate produces
negative voltage output. The outputs of the early and late gates are subtracted,
and the difference signal is fed into an integrator to generate an error signal. If
both gates are placed properly in time, the integrator output will be equal to
zero. Alternatively, when the gates are not timed properly, the integrator output
is not zero, which gives an indication that the gates must be moved in time, left
or right depending on the sign of the integrator output.
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