108
5 Performance Characteristics of Radar Location …
a significant wider angular zone, measured by tenths and hundreds of degrees, a
necessity arises to shift a directional pattern in one or two angular coordinates. Such
shift of radio beam, providing by mechanical movement of antenna, electronic beam
scanning or antenna relocation due to movement of radar platform, provides a radar
air surveillance.
Observing of a given range within limits of D min − D max is carried out in period of
time, incomparable shorter, then an air scan time in angles within limits of α min −α max
and β min − β max . For example, at observing of a target at distance 150 km and scan
time of assigned air sectors in 1 s, single scan of the whole distance is conducted
in time
2D max
c
= 10
−3 s, i.e., to thousand times faster, then at angle. It is practically
possible to consider a scan of assigned range as an instantaneous one, in consequence
of which a term of radar surveillance is applied in a greater degree for air observing
in angular coordinates.
Methods of radar surveillance are divided into single-beam, multi-beam, instantaneous (parallel) and sequential. There is also mixed-mode method of radar
surveillance.
Instantaneous surveillance is used in radars by one fixed beam, measuring only
a range and, consequently, have no use for beam scanning. For instance, airborne
radio altimeters, range-finding radio-beacon stations. Instantaneous surveillance is
possible at measuring of angular coordinates, but for this purpose, a directional
pattern (DP) of radar antenna should be multi-beam and instantly cover the whole
surveillance zone of radar. Such instantaneous multi-beam surveillance is otherwise
called a parallel surveillance.
In multi-beam systems of instantaneous surveillance, the antenna beams remain
stationary, but a number of simultaneously operating receiving beams should provide
an overlapping of the whole surveillance zone. In these conditions, a number of
beams equals to a number of angularly positioned elements, containing in radar
surveillance zone. Each of beams is connected with its receiving unit, due to which
a target angular coordinate is defined by a number of a receiver, at output of which
a maximum signal is fixed. Range is still defined by reflected signal dwell time
relatively to a sounding one. All target coordinates are measured simultaneously
and almost instantaneously. Main disadvantage of sequential method, a longtime
of surveillance, collapses. However, multi-element aerials, multi-channel receiving
units, complex information-processing devices, necessary for multi-beam radars,
make them a complicate in manufacturing and in operation.
Single-beam surveillance with scanning is called a sequential surveillance, since it
requires a sequential illumination of all zone elements. There is a variety of sequential
surveillance.
If one of two angular coordinates is measured, e.g., in aviation onboard surface
surveillance radars or ground-based surveillance radars, then an antenna beam is
imparted with wide width in a plane of non-measured angular coordinate in order
to overlap the whole surveillance zone per one rotation (one sweep) of antenna.
Such surveillance is called a circular, or sector surveillance. In Fig. 5.19, variants of
circular and sector radar surveillance are illustrated.
5 Performance Characteristics of Radar Location …
a significant wider angular zone, measured by tenths and hundreds of degrees, a
necessity arises to shift a directional pattern in one or two angular coordinates. Such
shift of radio beam, providing by mechanical movement of antenna, electronic beam
scanning or antenna relocation due to movement of radar platform, provides a radar
air surveillance.
Observing of a given range within limits of D min − D max is carried out in period of
time, incomparable shorter, then an air scan time in angles within limits of α min −α max
and β min − β max . For example, at observing of a target at distance 150 km and scan
time of assigned air sectors in 1 s, single scan of the whole distance is conducted
in time
2D max
c
= 10
−3 s, i.e., to thousand times faster, then at angle. It is practically
possible to consider a scan of assigned range as an instantaneous one, in consequence
of which a term of radar surveillance is applied in a greater degree for air observing
in angular coordinates.
Methods of radar surveillance are divided into single-beam, multi-beam, instantaneous (parallel) and sequential. There is also mixed-mode method of radar
surveillance.
Instantaneous surveillance is used in radars by one fixed beam, measuring only
a range and, consequently, have no use for beam scanning. For instance, airborne
radio altimeters, range-finding radio-beacon stations. Instantaneous surveillance is
possible at measuring of angular coordinates, but for this purpose, a directional
pattern (DP) of radar antenna should be multi-beam and instantly cover the whole
surveillance zone of radar. Such instantaneous multi-beam surveillance is otherwise
called a parallel surveillance.
In multi-beam systems of instantaneous surveillance, the antenna beams remain
stationary, but a number of simultaneously operating receiving beams should provide
an overlapping of the whole surveillance zone. In these conditions, a number of
beams equals to a number of angularly positioned elements, containing in radar
surveillance zone. Each of beams is connected with its receiving unit, due to which
a target angular coordinate is defined by a number of a receiver, at output of which
a maximum signal is fixed. Range is still defined by reflected signal dwell time
relatively to a sounding one. All target coordinates are measured simultaneously
and almost instantaneously. Main disadvantage of sequential method, a longtime
of surveillance, collapses. However, multi-element aerials, multi-channel receiving
units, complex information-processing devices, necessary for multi-beam radars,
make them a complicate in manufacturing and in operation.
Single-beam surveillance with scanning is called a sequential surveillance, since it
requires a sequential illumination of all zone elements. There is a variety of sequential
surveillance.
If one of two angular coordinates is measured, e.g., in aviation onboard surface
surveillance radars or ground-based surveillance radars, then an antenna beam is
imparted with wide width in a plane of non-measured angular coordinate in order
to overlap the whole surveillance zone per one rotation (one sweep) of antenna.
Such surveillance is called a circular, or sector surveillance. In Fig. 5.19, variants of
circular and sector radar surveillance are illustrated.