6.1 Detection of Radio Signals
139
a target. Consequently, an envelope extractor (some type of envelope detector) is
usually used, which eliminates the signal dependence at processing unit output from
a random phase. However, envelope detector leads to loss of some certain part of
signal and this should be considered by introducing the demodulation (detecting)
loss coefficient α dm .
In those cases when a time gate (selector) is applied, gating a signal in a moment
of signal maximum from processing unit output (or from output of envelope detector)
one should consider a duration of time strobe. Maximum position y out is unknown
precisely and detection threshold exceed moment by a signal y out can be within limits
of some signal delay range in time (usually within time resolution interval). Time
selector should be open at this time interval. Due to possible passing of additional
noises, the losses arise, considered by gate loss coefficient α gt .
Consequently, at target detection in the described non-perfect system (Fig. 6.8),
under otherwise equal conditions, a signal-to-noise ratio at input of threshold unit
Q sn is happen to be considerably lower than required value R d in perfect system.
However, in this case a probability of detection P cd also turns out lower of the
specified (required), since:
Q sn =
R inp
α f lu α mt α d α gt
(6.25)
To obtain the specified value of target detection probability P cd , it is necessary that
value Q sn should be less than R d , and this requires a signal energy increase, reflected
from a target, and correspondingly increase of input signal-to-noise ratio R inp . Value
R inp should be more or equal to product of R d to resulting losses coefficient α ls =
α f lu α mt α d α gt :
R inp ≥ R d α f lu α mt α d α gt
(6.26)
If we substitute R inp into (6.26) formula, then we obtain Q sn ≥ R d , that is
required for detection with specified value P cd . R inp value in this case we designate
as a detection parameter in non-perfect radar R dnr . Increase of signal energy at radar
input can be provided by different ways, e.g., by increasing of radiation power or
by time increase of target illumination (dwelt time). Usually, an energy increase is
achieved just by distance reducing to a target.
Certainly, at examining of detection conditions in real radar systems the losses
can arise due to other reasons o factors (e.g., losses, connected with displaying or
radar operator actions etc.). These losses record is possible at examining of certain
types of radars.
139
a target. Consequently, an envelope extractor (some type of envelope detector) is
usually used, which eliminates the signal dependence at processing unit output from
a random phase. However, envelope detector leads to loss of some certain part of
signal and this should be considered by introducing the demodulation (detecting)
loss coefficient α dm .
In those cases when a time gate (selector) is applied, gating a signal in a moment
of signal maximum from processing unit output (or from output of envelope detector)
one should consider a duration of time strobe. Maximum position y out is unknown
precisely and detection threshold exceed moment by a signal y out can be within limits
of some signal delay range in time (usually within time resolution interval). Time
selector should be open at this time interval. Due to possible passing of additional
noises, the losses arise, considered by gate loss coefficient α gt .
Consequently, at target detection in the described non-perfect system (Fig. 6.8),
under otherwise equal conditions, a signal-to-noise ratio at input of threshold unit
Q sn is happen to be considerably lower than required value R d in perfect system.
However, in this case a probability of detection P cd also turns out lower of the
specified (required), since:
Q sn =
R inp
α f lu α mt α d α gt
(6.25)
To obtain the specified value of target detection probability P cd , it is necessary that
value Q sn should be less than R d , and this requires a signal energy increase, reflected
from a target, and correspondingly increase of input signal-to-noise ratio R inp . Value
R inp should be more or equal to product of R d to resulting losses coefficient α ls =
α f lu α mt α d α gt :
R inp ≥ R d α f lu α mt α d α gt
(6.26)
If we substitute R inp into (6.26) formula, then we obtain Q sn ≥ R d , that is
required for detection with specified value P cd . R inp value in this case we designate
as a detection parameter in non-perfect radar R dnr . Increase of signal energy at radar
input can be provided by different ways, e.g., by increasing of radiation power or
by time increase of target illumination (dwelt time). Usually, an energy increase is
achieved just by distance reducing to a target.
Certainly, at examining of detection conditions in real radar systems the losses
can arise due to other reasons o factors (e.g., losses, connected with displaying or
radar operator actions etc.). These losses record is possible at examining of certain
types of radars.