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264
giving tho nunbor of oaeollitnn wrt Plana0
Vn. em txbit inclination CIM eltltwdla,
with aiditional constraint oinioun
ana aaximllm oft-nadir en9100 of 20' and
50'
c,:-nd, ai latitudes cloeo to tho orbit
inclinatior the ground track0 Blro rfc?nnGr
and DOVO onst-west instead of northaouth.
This feature may be very uaefull in
certain regional sceiiar loa.
itowever, if all satellites are in thO setae
plane thoro vi11 bo a cluntouing ol
revielea every 24 hours. To achiave an
even upreading of the revisita trough the
day, it is essential to redintribute the
satelliten in t4 orbital planoo, with a
proper phasing (riqht ascmnion and
satellites' true acomaly). An oxample of
application way presented in (11 and is
also mentioned in a companion paper (21.
Third, ono m y combine orbital planes
having Biglorent inclination0 and same
fundaaental. Coverage continuity n mds a
propoe phaelng of satelliton orbital
parPmoeorm, achiovinq also B roduction of
tho rovioit intewaln at irrtitudas
corrooponding to each orb': planQ
inclination. This approach rducoa the
oproad in tho averaqo rovlolt intorvalo,
typical of constellations with one orbit
plane inclination only.
.;ne orbit plane oriontntlon playn a
fundsnQnts1 rolo in liqhtn@t deoign.
sin910 plana down-dusk nunoynchronous
orbit0 aro vory convrnicnt for SAR
satellitas: infect fixed solar nro-ayn
be- uod since thoy aro always LIIuminated
by tho Dun. This fncilitntcn continuouu
SAR opaetion, unrestrained by battcrics
capacity. Solar array wlnqo arc) parallel
to tho orbit planc ninlnitinq tho
drag allact. One nido of the opacocralt 16
alwaym oxponad to cold npaco pravidlnq an
ideal haet aink for thermal control.
Mult Iplo plnnes sunsynchronous oabitn can
be oriented nynmetricnlly w.r.t. PRe 6 A#-
6 1% plnno implying nu!)-tracklnq nolar
nrrayo. Earth shadowinq IJr nhout 501 of
the orbit wlll, anyway, rcfiult rquirinq
to augport the EAR operottop Pron on-board
battorion. Aooldes, the ' vanimblo nun
vector incidence on tho sntelllta, during
the orbit, complicates tho thamal
contrcl. Novrrtheloss the eunnynchrcnlclty
and eyotoa oytnmetry , tho lattar only in
cand oP an even number of orbit plenea,
will holp in controlllnq tho growth in
system complexity.
Thia w111 not bo so in case iP non-
ounoynchronoua inclincd orbits. Tho dayly
nodal oAiPt wlll cause a slow potion OP
the orbit plane w.r.t. tho clin vactor. Tho
syntoa daalqn muat, than, cow with
orbital lporiod timo-varylng phoncrnona as
well a8 with Slowly ct.angin9 omn.
Thie may considerably affect tho aatolllee
d-i9n, SAR operation and minaion
planning. With medium Inclination orbits,
-r-------- .
-liiui7:
tho nolor onnny doaig? bocomee even more
critical, rotpiring a I-DOF sun-tracking
nachanilzn. Tha nsceasity for continuous
oolcpr wing13 roorientation will cause a
tlma-changing satellite cross-secti In ,
adding #NIO?.hQr variable to the problem of
drag compensation. Basides, variable
cxtornel torques may Impact the natellito
0ttitudo control, speclal ly at, low
altitudes. The thermal control also
becomes mor0 critical due to the full
variability ol onvironmental conditlons.
In summary the complexity and ccst of SAR
lightseta increases qoing from down-dusk
to nultiple planes sunsynchronous orbits
and, eventually, to sinqle or multiple
inclined non sunsynchronous orbits. The
increased tlntcllite complevity necesarily
reducos ; the payload nccomodation
capability Por the samo launch mics. These
considerations must be borne in mind when
evaluatinq tho mission benefits 01' various
orbit alternatives.
41SAR-CakRYrNcL!_C1(TSATS
4,l, CApabilities and ljmitotlons OL
1 iqhtAats
The doetqn of R SAR-cnrryinq liqhtsat must
follow a bottom-up approach starting from
a cot of constrainta and defining which
(wrformance cnn bo reaasnnbly achieved. A
liqhtsat cnrrylnq a SAR scnsor for
pruleeetonai uses cannot be; too small: a
500 to 800 Kq launch macs ran90 Wa6
c3onun. bolnq ulthln tho injection
capnbllltIti?s, in 1.EO. of scveral planned
anal) launch vehicles. For a 5 years
1 IleLime, draq conpcnaat Ion is the
dcmlnnnt factor In sizinq thc propulnlon
systom. A conpanfrn pnpcr [I1 shows that,
for long mlnnion duratlonn, electric
PrOpUlGiCIC tc mandatory to keep t.he
proprJllnnt mass within 100 Kg at very low
altitudoe. Tho ansociated IC power
consumption must bo considerad in
satelllto power plnnt oizinq.
Tho SAR antcnnn cannot be too lnrgc: when
Poldcd and ntoucd it must fit tho limfted
volumc ineide thc shroud cnvclopo: its
mnoa and area muot bo compatihlc vith thu
attitude control cnpabllitlcs and should
not contributa niqnificnntly to drno. All
antenna lcnqht of 6 n and n width of 1.5 m
were definod as upper bounds.
Tho SAR will normally operate lor a
frzction ol the orbit period, so that two
paramotere aro of concern: tho nverncp
onorgy por orbit and tho required pcnk
power. [loth Increase with orhit hoiqht,
thoreforo SAR-carrying lightsats must
preporably fly rather low.
Tha avornqo enerqy col lcctod by tho
li$htoat dopcnde from tho chosen orblt
plana oriontation w.r.t tho sun.
Typically, it can be in the 0.7 to 1.8
C(UR/orbit ranqo. Considorinq the cnnrgy
conaunad by ossmntial platform Iunctlocs,
that available to payload io between 0.5
and 1.3 KWh/orbit.