Page 104 - Radar Technology Encyclopedia

94 control, radar converter, analog-to-digital (ADC)

automatic gain control (AGC) (see GAIN). Positional converters contain a transparent disk with an
axis, which is rigidly connected to the antenna drive, with
automatic noise-level control (see CFAR).
cyclic Gray code marked on it in the form of alternating trans-
control, radar. In modern multifunction radars, operations parent and opaque sectors. The sectors are arranged in rings,
are, to a large extent, automated. The role of the human oper- the number of which is equal to the number of bit positions of
ator is one of monitoring, selection of scenario-dependent the count. The angular dimension of each sector is equal to
software, and override in case of radar malfunction or at the the reference angle. On one side of the disk is a slotted pulse
direction of higher authority. Virtually all radar functions, source of light, and on the other, photo-detectors, one for each
such as transmitted waveform selection and scheduling, and bit position. The code is read when a target pulse is applied to
antenna beam formation and steering, are controlled by a set the light source. The signal of the photo-detectors is con-
or sets of computer programs or preestablished templates. verted to binary code.
Surveillance, multiple-target tracking, target identification, Cumulative converters were used with older radars. In
and, if required, weapon guidance and control, are all per- them the count is made with angle marks uniformly arranged
formed automatically. Radar action scheduling conflicts are about the circumference of a disk or drum (for example, illu-
typically resolved by special algorithms that establish priori- minated openings in an optical converter). They have a draw-
ties and provide override responses. Radar control can extend back associated with the necessity to input the initial values,
to the radar receiver and signal processor, which may be and the accumulation of errors when malfunctions occur. IAM
adaptively reconfigured in response to the sensed environ- Ref.: Vasin (1977), p. 216; Kazarinov (1990), p. 414.
ment of weather, surface clutter, and electronic countermea-
An analog-to-digital converter (ADC) is “a device that con-
sures.
verts a signal that is a function of a continuous variable into a
Whether the radar system control is implemented by a
representative number sequence.” Typically, an ADC consists
central computer or with multiple distributed “processors” is
of a sampling unit, a quantization unit, and a coding unit
a matter of radar architectural preference and economics, but
(Fig. C40).
with either approach, redundancy is generally built in to
enhance the control system’s reliability.
analog Sampling Quantization Coding digital
The preceding discussion is most applicable to a multi- unit unit unit
waveform code
function phased-array radar (e.g., the U.S. Army’s Patriot),
where computer control is a necessity. For example, the
actions required to control the radar beam alone are clearly
beyond the response capability of any human operator. The t t t
importance of the phased-array antenna to multifunction Figure C40 Analog-to-digital converter.
radar has led some radar developers to limit use of the term
“radar controller” to the computer and software associated The basic configurations of ADCs are the simultaneous
with its control. In its more general interpretation, however, a converter (Fig. C41), and the sequential converter (Fig. C42).
“radar controller” includes the entire ensemble of computers, The simultaneous converter performs all operations simulta-
data and signal processors, and all the software required to neously. In this case, an array of 2:1 comparators indepen-
operate the radar in its various modes. dently convert the analog voltage into quantized magnitudes,
A radar controller can also direct the operation of non- and 2:1 input logic converts the quantized amplitude into an
phased-array radars, including reflector antenna radars, those N-bit parallel digital code. Such a converter has the highest
with planar arrays, as well as various hybrid configurations. conversion speed. The sequential converter can use N identi-
The Westinghouse-developed ARSR-4, a long-range surveil- cal cascaded stages, and a delay line is often introduced to
lance radar shared by the FAA and the U.S. Air Force, is an compensate for the time required for each quantization opera-
example of an array-fed reflector antenna radar that is tion. This converter is slower than the simultaneous converter
designed to operate under automatic control and is com- but is often simpler and cheaper. Other configurations are
pletely unmanned. PCH modifications of these two basic types (e.g., successive-
approximation and ripple-through converters).
sensitivity time control (see SENSITIVITY).
Modern ADCs typically use integrated circuits for high-
CONVERSION, CONVERTER speed simultaneous conversion. One advanced type is the
optical-electronic converter using optical-electric interaction.
An angle-to-code (digital) converter is a device that
Laser light goes to the input of an electrical-optical modula-
changes the angular coordinates of a target into digital code.
tor, whose other input receives the analog signal. The change
In a surveillance radar, when the antenna rotates at constant
in the voltage of the analog signal is converted into a distribu-
speed, the converter converts the time interval proportional to
tion of characteristics of the optical signal (into amplitude or
the angle of rotation of the antenna into digital code. Because
phase distribution). A binary representation of the analog
of insufficient stability of the speed of rotation of the antenna,
value is formed, usually by means of simultaneous electronic
indirect converters of angle into digital code are used. These
comparison of the signal amplitude received from the photo-
are subdivided in to cumulative and positional types.