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,
i'or a typical 708 operating duty, m SNR N
peak powsr conaumption of 1.3 to 4 KW
could bo fittad fron puro anergy
c 3ns id@ PCP t ions. Neverthe lson , th k a woil ld
aply to rely heavily on battarlam. Woro
conservntlvely, a MI powor to SNR
allocation in the 0.5 to 1.9 KM rplngo wan
retainad for system trade-ofPR. Concerning
data rates, an upper technology bound of
200 ~bit/cec. was also assurnad. In viow of
the abovo, a SAR payload mann allocation
in the 150 to 250 Kg range reoulto, ear a
500 to 000 Kg spacecraft launch mass
range.
4.2. SAR SENSORS F~LIG11TSAT~-
We rovlew the main trade-offs impacting
tho SAR desiqn in presence of conetraitits.
Tho SAR will normally operate in the
STRIPPVIP node: the swath will be
electronically repositioned, inside the
access onglc, by beam steering in the
elevation plane only. Thin operating mode
wa-s found to be adequate for tho intended
hiqh raoolutim missions, given the
lightsats constraints. Other SA$ operating
nodes, ouch as SCANSAR, are also available
if required by the mission. I
4.2.1 xmnqe interpretationj tarqet
cha racto+?iel.: ice, a&S/N
!
For extenckd targets hinh 'rasolution'
images intcarpretation is nore rolatad to
pixol sizo than to radiometric renolution.
A singlo look S/N of 5 dB was choaon for
.;q'. dimensioning. SAR opernting at
fi!.rrsnt frequencies respond difforently
i > '.he physical chaidctcristica of tho
Earih surface, in terms o€,bachocntterinq
coofficiont vs. the incidenco angle. For
tho OB~O backscattor coeificlonte and
swath width tho average tranmitted RF
power incressos fcurfold from S to X band.
Ncvertholess the average backscottsr from
typicel ground surfaccs also increase by 6
dR from S to X band, so that the two
effects conpensate nnd it could bo
possible, in principle, to transmit the
same power at both bands. For discrete
targ~ts, however, the determining factor
is the contrast ratio against the speckled
clutter. The relative merits of X vs. S
band are still unresolved duo to th0 large
variety of possible scenarioe and the
scarcity of experimental data at X band.
Assumimql frequency independent ,target
Radar Cross Sections, the SAR ohould be
denigned for the same S/N indipendontly
from fraquency. This criterion ronulte in
an incredse of the transmitlad RP powor
with tho operating frequency, pannlizing
the X-band choice. In tho following
referenco is made to a sigma-nought of -15
dD indegondent erom frequency and offnadir
angles.
4.2.2. Frequency choice, accoas anqle,
andresolution
In sizing tho SAR access nnglo, too low
oPf-nadie angle values should bo QVOided,
not to look at targets with n om vertical
incidonco. Too large v2luee ohould almo bo
avoidad to reduce shadowing aSUects and
oxceesivo image distorsions.
26-5
~ddem e'hooo Walitatitrcr considerations
nor6 graciee limtts are set by both
roqulatory and lightsate accommodation
rotatreinto. The upgar value impacte the
trcnnsvorpla antonna dlm~nsions, as shown in
Pig.4.2.1. showing the antenna width vs.
tho neximua off-madir angle, operating
iroquency and epacecraft altitude to
ewath ratio H/Sv.
, /
1 20 30 40 50
=IN. otf-nadlr angle. des.
Fig. 4.2.1. Antenna width vs. frequency
band, maximum oef-nadir angle and H/Sv
ratio.
This Figure shows that, at X-band, a 50'
off-nadir an910 is both feasible and
compatible with the assumed antenna
constraints even increasing the tl/Sw ratio
to 20 (e.g.: H-500' Km and Sw-25 Km).
At C-band, one ha0 to superiorly limit the
maximum oef-nadir angle to about 40',
loosing in coverage: alternatively a
maximum H/Sw ratio of about,12 could be
chosen implying however an altitudo of
only 300 Km for a 25 Km swath.
S-band antennas can hardly be accomodated
on lightsats at all off-nadir angles and
orbit heights.
JOO
10 '. . .
Pig. 4.2.2. Chiri, bandwidth vs. range
resolution and off-nadir angle.
Y I