Page 319 - Radar Technology Encyclopedia

309 power, effective radiated (ERP) power programming

radar power is generally expressed in terms of peak pulse Ref.: IEEE (1993), p. 988; Skolnik (1980), p. 225.
power; for a CW radar, the values of peak and average power Power handling capability [capacity] describes the maxi-
are identical. Similarly, the ERP of a jammer is P G and the mum power that can be carried by a transmission line or other
j j
jammer effective radiated power density (ERPD) is P G /B , j RF component in the transmitter path. This value is deter-
j j
where B is the bandwidth over which the jammer signal is mined by the dimensions of the component, spacing of com-
j
transmitted. PCH ponents subject to high field strengths, and presence of
Ref.: Schleher (1986), p. 422; Eaves (1987), p. 691. atmospheric pressure or pressurized gases.
Power efficiency may be calculated by dividing the average Instantaneous power, when a voltage E = E sin w is
m
t
power output of a device or system, by the average power applied to a resistance R, is equal to EI, where I = E sin wt/R
m
required at the input. For a complete radar system, the total = I sin wt. E is the instantaneous voltage, E is the maxi-
m
m
power efficiency would be measured by dividing the average mum amplitude of a sine wave of frequency w, and I is the
2
transmitted RF power, P , by the primary power supply input instantaneous current. Since EI = E I sin wt, and by substi-
av
m m
(watts/watt). Power-added efficiency is a term sometimes tution,
used to describe the efficiency of a particular subsystem or 2 1 1
component (e.g., a solid-state transmitter amplifier or a trans- ( sin wt )= --- – --- cos 2wt
2
2
ceiver module). PCH
the instantaneous power is also equal to
Power factor is the average real power drawn, in watts, by a E I E I
m m
m m
electrical load, divided by the rms voltage-amperes drawn by ------------- – ------------- cos 2wt
2
2
the same load.
The average value of this power can be seen to be E I /
m m
Ref.: IEEE (1993), p. 987.
2, and in radar engineering this value, equal to one-half of the
Power fluctuations, as related to the primary power supply maximum instantaneous power, is called the peak power P of
t
of a radar, are variations in line voltage with time due to tran- a radar transmitted pulse. PCH
sients (voltage spikes), ripple effects, or load changes. Con- Ref.: Skolnik (1980), p. 52; Hovanessian (1984).
trol of power fluctuations is especially important to maintain
noise power (see NOISE).
proper transmitter operation. In tube-type transmitters, regu-
lators are used to ensure that the output of the radar high-volt- peak power (see instantaneous power).
age power supply (HVPS) is constant within a few percent;
Primary [prime] power is the source of electrical power
the type of regulator used depends on the requirements of the
used to operate a radar. For land-based civil radars and some
particular transmitter power tube. Solid-state transmitters also
fixed-site military radar systems, prime power is provided by
require control of power fluctuations, and special electronic
the commercial power grid (land lines); in airborne radar sys-
conditioning circuits are usually designed into the transceiver
tems, prime power is usually provided by electrical genera-
module to regulate the power, which is at much lower voltage
tors driven by the aircraft engines. Field army radars are
than that used in tube-type transmitters. PCH
generally powered by portable diesel generators. Radar mis-
Ref.: Skolnik (1990), p. 5.16; McQuiddy, David N., et al., “Transmit/Receive sile seekers have short operational lives, and in this case,
Module Technology for X-Band Active Array Radar,” Proc. IEEE 79,
no. 3, Mar. 1991. electrical batteries serve as the source of prime power. PCH
Power gain is “in a given direction and at a given point in the Power programming is the variation of radar transmitted
far-field, the ratio of the power flux per unit area from an power in accordance with different radar mode requirements.
antenna to the power flux per unit area from an isotropic radi- For example, an agile-beam phased-array radar may use a
ator at a specified location with the same power input as the “burnthrough” waveform against noise jammers that uses
specified antenna.” It is a measure of the ability of an antenna higher power (and longer dwell times) than the normal vol-
to concentrate energy in a preferred direction. Numerically, ume search waveform; it may also program a lower average
power gain G (generally referred to simply as the antenna power waveform on targets already under track than to targets
gain), may be calculated as 4p times the maximum power being transitioned from search to track mode. Some types of
radiated/unit solid angle divided by the net power accepted by transmitter tubes, such as the crossed-field amplifier (CFA),
the antenna. Equivalently, power gain is equal to the maxi- are more amenable to stepped programming than others, and
mum radiation intensity from the subject antenna divided by solid-state active array radars have an inherent capability for
the radiation intensity from a lossless isotropic source with transmitted power programming via computer control of indi-
the same power input. Gain is related to the effective aperture vidual transceiver modules. Power programming can be
2
A by G = 4pA /l . The power gain of an antenna is always implemented via a software template, wherein the radar
r
r
less than the directive gain, since it includes antenna dissipa- resources are allocated by radar operational cycle, or it may
tive losses. It is the power gain G that should be used in the be adaptively implemented using more sophisticated algo-
radar range equation. (See ANTENNA gain; APERTURE, rithms that respond to the sensed environment. PCH
effective; RANGE EQUATION.) PCH power spectrum (see SPECTRUM).