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280 65.0 63:6 40.1
360 84.6 81.8 56.7
a\ 60 108 104 , 72.5
560 127 123, 80.3
660 157 143 103.
-I
Table 4.a.a. AVerngQ trangnittad LIP power
vs. orbit sltitudo, nrjnth and
renolutim at 50' off-nadir.
This confirms the desire Tor flying, SARs
at rathor low altitudes, compatibly with
covorago and rsvislts goalo. Once the
swath is chosen it is kept conntant,
independantly from its rapooitioning
inside tho accom anglo, and 00 io the
peak powor for obvioun hardware
constrainto. HOWOVO~, from: 50' to 20. off-
nadir, tho antenna beam in tho alavation
plane must be broadened to cuv?r the
svath. An a result the pulsa duration must
be gradlaally increasod to r'ocovor the
beambroodoning lossea Thus, at 20' ofI-
nadir, 'tho avorage tra smitted RP power as
well as the absorbed DL1 power, Is 'about
1.5 times that at 50' off-nadir. 1
Other ShR modos can be inploriianted
according to mission needs, trading off
goomstric with radiomatric raoolutionn
(multi-look modee), or azimuth rooolution
with swe:h widths ( SCANSAR raod'~), or
rango roeolution with avornge, powor,
adaptively changing the chirp bandwidth
and peak pulse power and/or pulse
duration. The spacecraft design lo anyway
determinad by the most demanding hi-
resolution tasks.
C-band SAR trade-offs results
Parametric evaluations were also perform2d
at C-band, achieving similar roaults in
terms of swath widths, which aro anyway
superiorly limited by the antenna width.
Substantially lower RF average trnnenitted
powers, as well as DC pover requirements,
are needad consistently with tho lower
range reoolution feasible at C-band. A
SCANSrW mode would equalize tho azimuth
and rango resolutions while nearly
doubling the width of the imaged ground
strip v.r.t.. a hi-resolution X-band
implementation. In this way a eimple
mediun reaolutkon SAR could h m +mplm-ntcd
for wide area eurveillonce tasks.
4.2.4. SKantonna requirements
A phased array with electronic scanning in
the. elevation plane will poeltirn the
svath within tho access angle. Beeidos
providing beam steering in n +-15' w.r.t.
the antenna normal, phaoc control munt
also provide beambroadening in tho
elevation plane to match antenna beanwidth
to the swath position inside the access
angle.
26-7
[ha0 to thd o.p~rtuao ovorobzing factor of
Q ~ O U 2.l:l ~ at OihimuQ off-nadir anglo,
lbnitod boomo;sopimg in tho slovation plane
cm bo also inplanento8 to squalize the
mtanna gain through tho awath width. The
Righor boanolo@on outeido the swath will
inprovo ran90 ambiguity control, which is
particularly important at low incidence
snglan. Booidss, nadir echo suppression
roguires putting a beam null at nadir.
Achieving ouch Paatur0s with phose control
only ( conratant amplitude illumination
lboing proforrod to reducm antenna width at
maxfraum off-nadir angle) may be an
untx-ivhl task, But it is feasible. At X
bend a paaalv0 single polarization design,
using multiple panels of radiating
waveguide olots fed by power dividers
carrying embodded phase shifters, can be
realized starting from already proven but
simpler design 15). at a specific weight
of 10 to 12 #g/mA2 with less than 1.5 dB
losses. A probably lighter technology can
also be implemented at C-band.
5. DP.TA TRANSMISSION
Data transmission capabilities are very
imp.xtant to fulfill the observation
miasions herein considered. Two main
approacheo were considorod: direct data
transmission to ground and the use of ,data
rolay satellites. On board storage with
subsequent data dump was not considered
practical and affective, also due to
expected near-term technology limitations.
5.1. Direct transmisiion to ground
The simplicity of this approach is partly
offset by coverage limitations, which
render direct transmission unsuitable when
performing SAR observations over sites too
far apart from the data etation.
In a tactical scenario, however, opposing
forces are normally dep!?yed within a
circle a fow hundred miles wide. Data
receiving stations deployed within, or
close to, the theater can easily access
the satellites of the constellation during
overpasses and get real-time data for
inmediate, on-site, ground procossing.
Table 5.1.1, summarize8 the projected
characteristics of a direct trasmission
system at X-band.
-
** Satellite terminal
- Frequency: e CHZ
- Antenna: mechanical nteerinq', driven by
on-board navigation system:
- EIRP : 27 dBW (Ant. gain:21 dB:
Tx pwr: 10 W) :
- Data rate: up to 7"> ?fbit/sec:
- #odul./coding: QP. ./ > 4 dB coding gain;
-'Mass and DC power: 10 K9, 50 W
** Receiving Station
- Antonna: traneportablo, with tracking:
- C/T: 16 dB/K' (Ant. dl8rn.r 1.8 m)i
- Slant rango: up to 2400 Kmr
- syutem margin: > 3 dB
Table 5.1.1. Direet Data transmission to
grobnd: System performance