Page 183 - Radar Technology Encyclopedia

173 EXCITER FALSE ALARM

3. Self-correcting or intermittent failures, based on the
Exciter First transmit Second transmit nature of the time of operability disruption. A self-correcting
mixer mixer
600 MHz failure is a failure leading to a brief disruption of operability.
Waveform Power
generator amplifier This failure sometimes is referred to as a malfunction. An
transmitter 2,800 MHz
60 MHz 540 MHz 2,200 MHz intermittent failure is a frequently occurring, self-correcting
Crystal Multiplier Crystal Multiplier Duplexer
(x9) osc XO (x32) failure of the identical nature.
osc XO 1
2
4. Explicit or implicit failures, based on presence of
external signs. An explicit failure is a failure that is detected
600 MHz
60 MHz
Coherent RF immediately when it occurs without use of meters. An
video receiver
Product Second receive First receive implicit failure is a hidden failure lacking external manifesta-
detector mixer mixer
tions and that can be detected only by means of the corre-
Figure E21 Coherent-radar exciter (after Fink, 1982, sponding measurements.
Fig. 25.82, p. 25.72). The number of failures per unit of time is referred to as
failure rate, while the average value of the time of operation
between failures is called mean-time-between-failures. Since
the process of failures in a radar is random and depends on
many factors, failure-free performance time is also random
F and a series of distributions, the most widely used being the
Weibull, the exponential, and the Rayleigh distributions, are
used to describe it. AIL
FADING is “the variation of radio field intensity caused by
changes in the transmission medium, and transmission path Ref.: Fink (1982), Ch. 28; Leonov (1991), p. 10.
with time.” Typically random variations appear in the signal Mean-time-between-failures (MTBF) is an expected oper-
received via various transmission media, especially for propa- ating time between failures. It is an indicator of failure-free
gation via the troposphere or ionosphere at frequencies above performance of repaired radar elements, units, assemblies,
about 100 kHz. Usually two types of such variations are dis- and the station as a whole, usually expressed in hours. AIL
tinguished: power fading (or attenuation) that is associated Ref.: Fink (1982), p. 28.4; Leonov (1991), p. 31.
with comparatively large-scale changes in the medium, and
Mean-time-to-first-failure expected value of operating time
variable-multipath or phase-interference fading. In radar,
until the first failure. Time-to-first-failure is an indicator of
severe fading has a strong effect on the target detection per-
failure-free performance of unrepaired radar elements and
formance. The readers who are interested in details can find
units and is expressed in hours. AIL
many graphical results in Cantafio (1989), where the influ-
Ref.: Leonov (1991), p. 27.
ence of various level of scintillation severity from no scintil-
lation to worst-case Rayleigh fading on probability of FALSE ALARM. A false alarm is “an erroneous radar target
detection are cited. SAL detection decision caused by noise or other interfering signals
Ref.: IEEE (1993), p. 476; Fink (1975), p. 18.70; Cantafio (1989), pp. 97– exceeding the detection threshold.” (See also DETEC-
117. TION.) In general, it is an indication of the presence of a
radar target when there is no valid target. False alarms are
FAILURE. A failure is an event involving disruption of the
generated when thermal noise exceeds a preset threshold
operability of a radar. The term failure must be understood to
level, by the presence of spurious signals (either internal to
mean not only complete loss of radar operability, but also its
the radar receiver or from sources external to the radar), or by
deterioration due to excursion of the value of parameters
equipment malfunction. A false alarm may be manifested as a
beyond established tolerances. Failures are categorized as fol-
momentary blip on a cathode ray tube (CRT) display, a digital
lows:
signal processor output, an audio signal, or by all of these
1. Sudden or gradual failures, based on the nature of the
means. The task of the radar designer is to establish the
parameter change. A sudden failure is a failure characterized
threshold such that the radar target detection goals (expressed
by a spasmodic change in one or several parameters. It arises
in terms of a specified detection range against a target of a
randomly and unexpectedly, and cannot be corrected during
specified radar cross section, with a specified probability) can
radar maintenance. A gradual failure is a failure characterized
be met with a signal-to-noise ratio (SNR) that is consistent
by a gradual change in the values of one or several parame-
with the capabilities of the radar design. If the detection
ters. Hence, gradual failures can be forecast and averted
threshold is set too high, there will be very few false alarms,
through preventive maintenance.
but the SNR required will inhibit detection of valid targets. If
2. Dependent or independent failures, based on their
the threshold is set too low, the large number of false alarms
interconnection. A dependent failure is a failure of an element
will mask detection of valid targets, and scarce radar
caused by the failure of another element. An independent fail-
resources (e.g., time) will be expended in investigating the
ure is a failure of an element not caused by failures of other
false alarms. Solutions to the false-alarm problem involve
elements but arising for other reasons.
implementation of constant false-alarm rate (CFAR) schemes