7 Non-coherent and Pseudo-coherent Radar Systems
151
NCRS receiver (detector) refers to superheterodyne type and is usually with one
frequency transformation (conversion). Due to high-frequency instability of oscillations formed by magnetron, to maintain the continuity of intermediate frequency
(IFq) of receiver, the automatic frequency control (adjustment) system is used.
Construction particularities of NCRS devices are defined by parameters of input
signals and characteristics of output equipment. Input signal, in general, represents a
random process with broad amplitude range (up to 60 dB and higher). This stipulates
a necessity to provide a large dynamic range of receiving path. Besides, a dynamic
range of displaying units happens to be considerably lower than variation range of
input signal amplitudes.
To solve this problem, the process of signal amplification in receiving device
is divided between intermediate-frequency pre-amplifier (IFPA) and directly
intermediate-frequency amplifier (IFA). There is a function of manual adjustment
of amplification gain factor in NCRS, realized in IFPA, and IFA with logarithmic
characteristic prevents limitation of powerful signals and permits to maintain its
specifics.
Another distinctive feature of receivers is application of temporal automatic gain
control (TAGC), which provides reducing of receiver amplification right after transmitter pulse sending and smooth rising of amplification with the passage of time.
TAGC circuit is adjusted in a way that after 50–60 µs after pulse radiating of
transmitter, an amplification reaches its ordinary value. Such adjustment provides
reducing of signal amplification, reflected from close-spaced targets that prevents
intense strobing of screen by these signals.
Considerable disadvantage of magnetron transmitters is non-coherence of forming
pulses sequence; in each new radio pulse, the initial phase is a random value. Coherence ensuring is possible at signal processing. Burst processing coherence is ensured
by memorizing of initial phases of sounding pulses at a period of its repetition.
In some radar application cases, it is necessary to solve a problem of “wind shear”
and turbulence detection. Doppler principle is taken as a basis of wind shear detection,
concluding in frequency difference of radiating and reflected from target pulses. To
solve this problem, it is necessary to apply a coherent processing of radiated and
received signals. It is impossible to solve the problem through the abovementioned
NCRS.
NCRS composition, including relatively effective and at the same time inexpensive transmitting units, characterizing by high pulsed power, has led to application of
pseudo-coherent method of signal processing in order to detect the most hazardous
for flight moisture targets. The major advantage of this method is that it permits
to extract signals with frequency increment due to Doppler effect at reflection of
sounding signals from moving objects, which represent the hydrometeors, presenting
at phenomenon of “wind shear” and turbulence.
Functional diagram of pseudo-coherent radar system (PCRS) is depicted in
Fig. 7.2.
The main components of the chart are the following: antenna with stabilization
and control unit, intended for formation in air space of specified directional pattern
and control of its shift independently on AV trajectory irregularities. The purpose of
151
NCRS receiver (detector) refers to superheterodyne type and is usually with one
frequency transformation (conversion). Due to high-frequency instability of oscillations formed by magnetron, to maintain the continuity of intermediate frequency
(IFq) of receiver, the automatic frequency control (adjustment) system is used.
Construction particularities of NCRS devices are defined by parameters of input
signals and characteristics of output equipment. Input signal, in general, represents a
random process with broad amplitude range (up to 60 dB and higher). This stipulates
a necessity to provide a large dynamic range of receiving path. Besides, a dynamic
range of displaying units happens to be considerably lower than variation range of
input signal amplitudes.
To solve this problem, the process of signal amplification in receiving device
is divided between intermediate-frequency pre-amplifier (IFPA) and directly
intermediate-frequency amplifier (IFA). There is a function of manual adjustment
of amplification gain factor in NCRS, realized in IFPA, and IFA with logarithmic
characteristic prevents limitation of powerful signals and permits to maintain its
specifics.
Another distinctive feature of receivers is application of temporal automatic gain
control (TAGC), which provides reducing of receiver amplification right after transmitter pulse sending and smooth rising of amplification with the passage of time.
TAGC circuit is adjusted in a way that after 50–60 µs after pulse radiating of
transmitter, an amplification reaches its ordinary value. Such adjustment provides
reducing of signal amplification, reflected from close-spaced targets that prevents
intense strobing of screen by these signals.
Considerable disadvantage of magnetron transmitters is non-coherence of forming
pulses sequence; in each new radio pulse, the initial phase is a random value. Coherence ensuring is possible at signal processing. Burst processing coherence is ensured
by memorizing of initial phases of sounding pulses at a period of its repetition.
In some radar application cases, it is necessary to solve a problem of “wind shear”
and turbulence detection. Doppler principle is taken as a basis of wind shear detection,
concluding in frequency difference of radiating and reflected from target pulses. To
solve this problem, it is necessary to apply a coherent processing of radiated and
received signals. It is impossible to solve the problem through the abovementioned
NCRS.
NCRS composition, including relatively effective and at the same time inexpensive transmitting units, characterizing by high pulsed power, has led to application of
pseudo-coherent method of signal processing in order to detect the most hazardous
for flight moisture targets. The major advantage of this method is that it permits
to extract signals with frequency increment due to Doppler effect at reflection of
sounding signals from moving objects, which represent the hydrometeors, presenting
at phenomenon of “wind shear” and turbulence.
Functional diagram of pseudo-coherent radar system (PCRS) is depicted in
Fig. 7.2.
The main components of the chart are the following: antenna with stabilization
and control unit, intended for formation in air space of specified directional pattern
and control of its shift independently on AV trajectory irregularities. The purpose of