IEEE Geoscience and Remote Sensing Magazine - September 2021 - 12

Flight
Direction
Feed Antenna
Elements
Limiter
· ··
and facilitates better suppression of
range ambiguities. The transmit antenna
beam still needs to be wide
to illuminate the complete imaged
swath. SCORE is especially attractive
if implemented onboard and
thus does not result in any substantial
increase of the data rate
(indeed, SCORE can be considered as
a lossless data compression process
along the elevation [27]). Viewed
in terms of the trade space, SCORE
decouples the imaged swath width
from the receiving antenna height
[13], [38] allowing for a wider swath,
a higher SNR, and a better range
ambiguity suppression.
At this point, it is sufficient to
consider a single column N 1el # of
FIGURE 5. The SCORE geometry and operation. A narrow, high-gain receive beam is directed
toward the echo signal traversing the ground (note that the antenna beam is shown to converge,
emphasizing the narrow beamwidth, while the actual beam diverges).
FUNCTIONAL SCAN-ON-RECEIVE DESCRIPTION
In 1981, Blyth [33] suggested a basic approach for analog
beam steering using variable phase shifters such that
the receive beam of a planar antenna moves across the
swath in accordance to the position of the echo. About
20 years later, his idea found a more detailed description
and justification in the independent and almost contemporary
works by Kare [34] and Suess and Wiesbeck [35].
The former extends the idea to reflector antennas, while
the latter-for the first time-presents DBF techniques
in conjunction with time-varying receive beam steering
in the elevation. The technique, known as scan on receive
(SCORE), uses multiple channels in the elevation
to generate a receive beam that follows the SAR signal
echo while it traverses the ground [36], [37]; see Figure 5.
SCORE enables an effective increase in the receive antenna
elevation dimension to generate a narrow and
high-gain beam; this yields a higher signal-to-noise ratio
◗ Parameter values: The reference L-band SAR considered
throughout part 1 of this tutorial operates
at 1.24 GHz and has a D 15m-=
sa 700 km,
t =
the receive array; this simplification
is dropped later in parts 2 and 3 of
the tutorial. The transmitted pulse
traverses the ground with the scattered
signal, causing an echo that
is reflected back to the sensor. The
duration of the echo signal depends on the swath width
and is given by
T
where Rnear
echo =
and R arf
2 RR
c
()
-
farnear
+xp
,
swath, respectively, c is the velocity of light, and p
(2)
are the near and far limits of the
x is
the duration of the transmit pulse. For a spaceborne
SAR, the ground pulse extension, which is the area on
the ground that is simultaneously contributing to the
echo received at any time instance [39], is smaller than
the swath width; its value
der of a few kilometers, where i
|x h ) is on the orh
is the incidence angle.
gr pic 2 sin
=
/(
Nevertheless, the pulse extension has a major effect on
SAR, as detailed later.
◗ Parameter values: The near and far slant ranges
of the reference SAR are R
near 812km=
diameter reflector
to image a 70-km swath. For an orbit height of
h
the swath may be positioned between
the look (off-nadir) angles nearj = 28.7° and
far
j = 32.7°, respectively. The receive pattern, scanning
the ground, has a beamwidth of 1.1º. On transmit,
the reflector is underilluminated such that the
secondary pattern beamwidth of 4° is sufficient to
illuminate the complete swath.
12
T
echo 03ms.=
and Rfar
852km, respectively. With a pulse duration of p
=
x =
29 s,n the echo duration, according to (2), becomes
.
A particularity of parabolic reflector antennas, when
compared to direct-radiating planar antennas, is that the
energy of the incoming wave is focused to different regions
(of the feed array), depending on its angle of arrival. A
proper reflector/feed design for SCORE operation focuses
90% of the power on approximately two feed elements,
in which case, for any angle of arrival, two or three feed
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE SEPTEMBER 2021
Slant
Range
Rx Score Beam
Reflector
Phase Shift
HPA
Tx In
LNA
Rx Out
Front End
TRM

IEEE Geoscience and Remote Sensing Magazine - September 2021

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - September 2021