IEEE Geoscience and Remote Sensing Magazine - September 2021 - 11

◗ Parameter values: Reflector SAR systems are capable
of imaging swathes on the order of 300-400 km;
the diameter of the reflector antenna is typically
.
212m The feed array consists of NN antenna
el # az
elements in the elevation (cross track) and the azimuth
(along track), respectively. Typically, Nel
be as large as 28-36, while Naz
than four.
The radiation patterns of the feed elements are commonly
referred to as primary patterns, which illuminate the
reflector surface generating the secondary patterns. If the
main lobe of the primary pattern illuminates an area that
is larger or smaller than the reflector area, then the reflector
is over- or underilluminated, respectively. The term antenna
element should, more generally, be replaced by subarray, thus
taking into account that the feed array may be divided into
subarrays (see Figure 1), each consisting of several radiators.
For example, in Figure 2, a passive feed network combines
two patch elements into a subarray. Since the focus is on
the digital part, it is mostly irrelevant how many radiators
constitute a subarray, thus motivating the lax use of the term
antenna element here. In terms of system execution, however,
these are key aspects that have an essential impact on the
beamforming and therefore on performance, such as the
ambiguity-to-signal ratio and the cross-polarization cancellation
[30]. In general, it can be said that, if a subarray's
primary pattern underilluminates the reflector, then there is
a loss in the signal-to-noise ratio because the aperture area
is not fully utilized. On the other hand, a strong overillumination
is an indication that more
antenna elements can be combined
into a subarray, thereby reducing the
primary beamwidth and, by this, the
spillover loss [31]; this also reduces
the number of digital channels.
BASIC (SINGLE-BEAM) SCANON-RECEIVE
IMAGING MODE
Consider an SAR instrument that
successively transmits frequencymodulated
pulses (chirps) of fixed
duration p
x (see Figure 3), illuminating
a wide swath on the ground. The
pulses are transmitted at a regular
rate every Tpri
seconds, which is commonly
known as the pulse repetition
interval (PRI). The minimum pulse
repetition frequency (PRF) of a SAR
system,
f ,prf
is inversely proportional
to the size of the antenna, given here
by the diameter of the reflector D,
and may be approximated by
f = $
1
prf
T
pri
a D
v
2
sat
,
(1)
FIGURE 4. A reflector SCORE transmitter illumination. All feed array elements are active,
generating a narrow primary beam that underilluminates the reflector, thus generating a wide
secondary beam that illuminates the complete swath to be imaged.
SEPTEMBER 2021 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
11
Limiter
· ··
may
is usually smaller
where Tpri
is the PRI, vsat
is the satellite platform velocity,
and a is a proportionality constant, which depends
on the illumination of the reflector by the feed array, the
operation mode, and the processed Doppler bandwidth,
i.e., the azimuth SAR image resolution. The expression in
(1) is derived by taking into account a radiation pattern
beamwidth that is inversely proportional to the electrical
antenna aperture size. The proportionality constant a
mainly depends on the reflector illumination (or the amplitude
taper in a planar antenna), with values in the range
.
a 122.
to 1.5. Typical values for Tpri
T ,pri
of a few tens of microseconds.
On transmit, all NN·el
are, then, 250-500 µs,
corresponding to a PRF of 2-4 kHz. The duty cycle is
around 4-10% of the
resulting in a pulse duration p
x
az antenna elements of the feed
array are activated with a maximum amplitude (a uniform
amplitude taper); this generates a narrow primary beam,
which illuminates a small part of the reflector; i.e., the feed
is underilluminating the reflector, which, in turn, generates
a wide secondary beam to illuminate the complete swath, as
illustrated in Figure 4. A phase taper is applied to optimize the
beam shape, thus requiring phase control on transmit, which
is typically achieved through setting an appropriate TRM [32].
1
Techo
Tpri
2
3
4
5
Time
FIGURE 3. The transmit pulse sequence versus time. The duration
of the ground echo return is Techo.
Flight
Direction
Feed Antenna
Elements
PhaseShift
HPA
Tx In
LNA
Rx Out
TRM
Front End
Slant
Range
Feed
(Primary)
Beam
Reflector
Reflector Antenna (Secondary ) Tx Beam

IEEE Geoscience and Remote Sensing Magazine - September 2021

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