IEEE Geoscience and Remote Sensing Magazine - June 2015 - 17

From an instrument point of view each mode is characterized by the activated elements on transmit, and the selection of the digital data streams which are stored on receive.
In addition, radar parameters such as pulse repetition frequency, polarization channels etc. are adapted to the specific mode. The basic modes which are being considered are
shown in Table 2.
A. AntennA ConCept And operAtion prinCiple
The acquisition of very wide swaths with sufficient sensitivity requires large antennas to be deployed into space. Representing the state of the art for communication satellites, large
mesh reflectors are recently considered for spaceborne SAR
missions. In contrast to direct radiating planar array antennas, a reflector is illuminated by an array of feed elements,
which in turn deflects the electromagnetic wave towards the
Earth surface. A peculiarity of such array-fed reflector antennas is that each feed element receives echo signals from an essentially non-overlapping angular domain. A 15-m reflector
design for Tandem-L is shown on the left in Figure 9, where
the feed array is almost horizontally aligned. A wide swath is
illuminated by all elements simultaneously transmitting the
chirp signal, as indicated by the broad blue beam. On receive a
narrow high gain beam is generated by adopting only a small
subset of feed elements in concordance with the direction of
the echo signal. By this the receive beam is scanned across the
swath from near to far range. This scanning operation, in the

Reflector

2D Feed
Array

Rx Beam

Tx Beam

ParaMeters

descriPtion

B1/B2

350 km swath, 10 m
azimuth, Single/Dual-pol.

Staggered SAR, linear Tx

175 km swath, 10 m
azimuth, Quad-pol.

Staggered SAR, circular Tx

D1/D2

350 km swath, 3 m azimuth, Single/Dual-pol.

Stripmap, monostatic
image (with gaps), bistatic
image (no gaps), linear Tx

175 km swath, 3 m azimuth, Quad-pol.

Stripmap, monostatic
image (with gaps), bistatic
image (no gaps), circular Tx

SAR literature denoted as SCORE (SCan-On-REceive) [21],
[22] can be efficiently implemented by means of digital feed
array hardware [24]. Such a concept is presented on the right
side of Figure 9. Each of the 36 elevation elements consists of
two dual-polarized microstrip patches. On receive the signal
is routed through an individual transmit-receive (TR) modules module for horizontal (H) and for vertical (V) polarization and digitized in the corresponding DBF unit. Typically,

ADC

1a 1b

ADC

2a 2b

ADC

36a 36b
ADC

H-Pol Data

V-Pol Beamforming Unit
Rx BF & CTRL Rx BF & CTRL
Rx BF & CTRL

Nadir

Mode

Rx BF & CTRL Rx BF & CTRL
Rx BF & CTRL
H-Pol Beamforming Unit

Flight
Direction

table 2. basic Modes of oPeration for tandeM-l.
Variations of these iMaging Modes are Possible,
by altering the range and aziMuth resolution,
corresPonding to the chirP and doPPler bandwidth, resPectiVely. for the high-resolution Modes
d1/d2 and d4 the oPeration in staggered sar Mode
is being inVestigated.

V-Pol Data
Figure 9. Left: Scan-On-Receive operation principle where the entire swath is illuminated with all elevation feed elements active, while on

receive a narrow high gain beam follows the pulse on ground. For high azimuth resolution acquisitions the feed array is extended in flight
direction by a total of three columns. Right: Receive chain of an individual feed array column, consisting of 36 dual polarized patch pairs
(1a...36a, 1b...36b). Each polarization port signal (H, V) is routed through a transmit-receive (TR) module and digitized in the respective
beamforming unit. ADC stands for analog-to-digital converter, Rx for receive, BF for beamforming, CTRL for control.
june 2015

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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - June 2015