IEEE Geoscience and Remote Sensing Magazine - December 2022 - 26

DATA STREAMS AND MEMORY MANAGEMENT
Up to now, it has been detailed how multiple echoes
are collected simultaneously through multiple SCORE
beams and azimuth channels. In the following, the correspondence
between the samples of the echo lines and
the (buffer) memory is reviewed, as it is crucial for understanding
the onboard data processing.
Let the first pulse, Tx 1, be transmitted at time t0; the
near-range echo will arrive after 2Rc /near
second and
last for Techo second. During this time, the signal from a
changing subset of the feed elements is sampled, weighted,
and summed up. The resulting Nrb element vector of
azimuth channel naz (known as the range line) is
b .
1,naz
The data stream is written into a memory block sufficient
to store N2 rb
counts for the complex-valued echo data; thus, one real
and one imaginary value are stored per sample), where
each sample occupies Mbit bits of memory.
The number of samples Nrb, as given by (4) in [1], is
for continuous data acquisition during the complete echo
time Techo. In this case, the blocked data samples still occupy
memory space. This is necessary when data samples
are later to be recovered through an onboard interpolation
process, as is the case in the staggered SAR operation mode
detailed in part 3 of the tutorial, to be published in a later
issue of IEEE Geoscience and Remote Sensing Magazine. Otherwise,
no memory allocation for the samples lost during
the blockage times is necessary, and the number of stored
samples per range line may be reduced. Taking into account
the previously mentioned blockage times, the (reduced)
number of samples per range line becomes
NTecho () .fN f1
-
rb=-x
sb beam
s
(13)
The echo of Tx 1 is received and processed by all Naz
azimuth channels simultaneously; each azimuth channel
naz produces the output
1,naz
tT ,0pri
+
the second Tx 2 pulse is transmitted; its echo
tT
00
++ 1 ++ (14)
pri
22 Techo
Rnear
c
t
Rnear
c
Tpri
arrives before the end of the first echo, and thus,
()
data streams are written simultaneously into memory. At
time
real-valued samples (the factor 2 acwhich
indicates that there is an overlap time during which
two echoes b1
and b2
from different range positions
are received simultaneously. Since the reflector focuses
these echoes on different sets of elements, they can be
separated by DBF to produce separate, i.e., independent,
data streams. The preceding expression is equivalent to
TT ,
pri 1 echo which has already been stated previously as a
particularity of ultrawide-swath imaging. As before, the
data streams b ,n2 az
memory blocks, each of NM bits.
with nN1az
2·rb
= g az are assigned to Naz
bit
◗ Parameter Values: In conventional SAR systems (last
generation), the number of ADC bits Mbit per (real)
sample is typically 8 bit. State-of-the-art SAR instruments
use a larger number of bits, on the order of
10-14 bit per sample, due to the increased system dynamic
range. In DBF SAR, this is further increased for
the internal SCORE beamforming operations such
that Mbit becomes 16-20 bit.
As the echo signals from subsequent transmit pulses Tx i
are received, more SCORE beams are added, up to a maximum
of Nbeam
data streams. When the echo of the first
transmit pulse reaches the far swath limit, the SCORE
beam will be pointing to far range Rfar, and the number
of SCORE beams is reduced by one until a new near-range
Rnear echo arrives, requiring reassigning the data stream
b ,n1 az
to that echo.
DATA RATE AND AZIMUTH OVERSAMPLING
The digital system architecture and design are driven by the
high data rates of DBF SAR instruments. In the following,
expressions for the data rate after SCORE, i.e., applied on the
b ,knaz
any arbitrary PRI time interval Tpri
=- beamecho
b
. In total, for each echo, Naz 1pri
Nbeam beams are acquired.
three segments:
1) During () econd,
TT NT1s
2) DuringTN TT s,b2pribeam
streams are acquired.
3) During the remaining xb s,
the data from
=-x - echo econd Nbeam 1- data
econd no data are acquired
(blockage).
Summing the samples of each interval, dividing by
and collecting terms yields an average of
T ()
22f
T1 T2
Techo
τb
FIGURE 20. The timing of (overlapping) ground return echo signals
from successive (color-coded) transmitted pulses for N
beam 3=
simultaneous SCORE beams. The average data rate is computed
considering the interval
T1, T2, and xp .
26
T ,pri which is divided into three segments:
Time
fss
T
pripri
echo
T --bx Nbeam
1
T ,pri
data given in (5), are derived. Referring to Figure 20,
may be divided into
(15)
(real) samples per second per azimuth channel. The first
term accounts for the sampling of the echo signal (note that
TT /
echo
pr 1i2 in a multibeam system), while the second term
reduces the number of samples in proportion to the blockage
duration.
Quantizing at Mbit bits/sample and considering all Naz
azimuth channels, the total average data rate is obtained
from (15) after substituting
fT1prfpri
=
/:
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE DECEMBER 2022
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IEEE Geoscience and Remote Sensing Magazine - December 2022

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