Page 318 - Radar Technology Encyclopedia

POWER power, effective radiated (ERP) 308

/
lent to the expenditure of one joule of energy per second. One tt is called the radar duty factor D . For a CW radar, the
u
r
7
joule = 10 ergs in Gaussian units. The term power is used in peak and average power are identical. PCH
several different ways in radar; this entry will deal with those
Center-line power in the frequency spectrum of a waveform
most common. PCH
is the power in the central line, or carrier frequency, f . If the
0
Power-aperture product is the product of a radar’s average signal amplitude at frequency f is a, the center-line power is
0
2
transmitter power and the effective aperture area (i.e., power- equal to a /2. PCH
aperture product = P A ). The antenna effective aperture area Power combining refers to adding the outputs of separate
av r
is equal to
power sources, especially as related to radar radiated power.
2 RF power combining of two or more radar transmitter tubes
G l
r
A = Ah = ------------
r a 4p to meet radar power requirements beyond the capability of
any single tube has been practiced since the 1950s. With the
where A is the physical area, h is the aperture efficiency, G r development of solid-state transmitters, power combining has
a
i
is the antenna gain, and ls the radar wavelength. That the become widely practiced and necessary, since the individual
detection range of a radar is directly proportional to the transmitters are, relative to RF tubes, low-power devices. In
power-aperture product can be seen in the search radar equa- addition to achieving the total radar power required, combin-
tion: ing the outputs of several RF transmitters has the advantage
4 of increasing total radar system reliability. When large num-
4 P A t sF
av r s
R m = ------------------------------------ bers of solid-state transmitters are combined, as in an active
4pkT D 1 () L
s 0 s array radar, the additional benefit of “graceful degradation”
may be claimed, in that the random failure of a few transmit-
where t is the radar frame time, s is the target’s RCS, k is ter modules will have little effect on the performance of the
s
Boltzmann’s constant, D (1) is the detectability factor (or overall array or of the radar in general.
0
required SNR) for a single pulse, and L is the total search In an alternate approach to the combination of complete
s
loss. (See APERTURE, antenna; RANGE EQUATION.)
transmitter modules in an active array, the outputs of individ-
PCH
ual solid-state power generating devices, such as gallium-ars-
Ref.: Barton (1988), pp. 12–26.
enide (GaAs) IMPATT diodes, may be combined within a
Available power is (1) the maximum output power at the single resonant cavity called the combiner, which then serves
transmitting antenna terminals, after all losses between the as the power module of a central transmitter for a radar sys-
output of the transmitter and the antenna input terminals have tem using a conventional (passive) antenna. To date, consid-
been incurred. Typical losses include ohmic or dissipative eration of this approach to solid-state power combining has
losses such as line loss, which attenuates the signal as it trav- been restricted to airborne applications having severe power-
els through the waveguide, and insertion loss when the trans- aperture limitations, such as radar missile seekers. (See
mitted signal passes through a circulator or T/R switch TRANSMITTER, solid-state.) PCH
designed to protect the radar receiver from excess power Ref.: Ostroff (1985), Ch. 5.
leakage during transmission. In some cases the actual power
Power density is the distribution of power over a surface
at the antenna input terminals may not be equal to the maxi- 2
area, W/m . For an isotropic radiator with power P , the
t
mum available power (e.g., when a three-dimensional radar 2
power density at range R from the source will be I = P /4pR .
t
s
uses power management to allocate transmitter power as a
If a transmitting antenna with power gain G is used in place
t
function elevation beam position). (See also LOSS, trans-
of the isotropic antenna, the power density along the axis of
mission-line.) 2
the beam now will be P G /4pR . A spherical target of radius
t t
(2) Available power may also refer to the average level of 2
a
a will have a projected area s = p . If the target is on the
electrical prime power, in watts available at the output of an
radar beam axis at range R, the target will intercept a fraction
electrical battery, generator/inverter, or line source used to
of the radiated radar power given by P = I s. This power will
s
i
supply the original source of electrical power. (See primary
be reradiated isotropically by the spherical target and the
power.)
power density of the return signal at the radar I will be
r
(3) From a signal generator (e.g., as used to determine 2 2
P G s/(4pR ) . The signal after capture by a radar antenna
t t
the properties of a receiver), or from any linear network, the
with effective aperture A will have the power:
r
available power is the maximum power that can be taken
from the network by a suitably adjusted load. PCH P G A s
t r
t
S = I A = ---------------------
r r
2
Average power in radar is the average transmitted power P av ( 4pR )
over the pulse repetition interval (PRI) or some longer period.
PCH
If P is the peak pulse power, t the pulse width, and f the Ref.: Johnson (1984), p. 1.6; Barton (1988), p. 10.
t
r
pulse repetition frequency (PRF), then P = P t f . Since the
av
r
t
PRF is the reciprocal of the PRI t , P = P (t/t ). The quantity Effective radiated power (ERP) is the product of radar
r
av
r
t
transmitted power and antenna gain, P G , in watts. Here,
t t