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effect they may have on iInQq0 gyslity.
6.3 Spacecraft platform rewironants
WO outlino some3 platform aubnyatona design
aspecto ROB~ relevant to SAR performance
and operation.
Low orbit altitudes will raqludra oloctric
propulaion, which must be cosmaidlalrod an
enabling technology [J] for thoso SAR
lightnats missions, as alroady discussed.
tJevertho1ese. flying satelhitan at lou
altitudo has also positive offmts, in
pdrticular concerning the more benign
environment (5).
The stiucture design, departihg from
conventional, should adopt a highly
integrated aQprOxh where a t avmt~ frame
utilizoa important SAR payload copponents,
like tho antenna or big boxm, ab part of
the structure itself, in ordar to save
mass. Since the bulk of tho heat is
produced by the SAR HPA, heat rejection
shc*ilci be by direct radiation Fo spaca as
far as practical: this is certainly easier
on down-dusk orbits. I
The solar array design ia ntrongly
dependant upon the SAR payload operating
duty. Infact, since the SAR will normally
operata for short intervals totalling 5 to
10 8 of tho orbit poriod, payload supply
can normally be from battgry, the solar
array merving mainly for battery
recharging. The orbit plane cholce, and
the percentage of time spont In Enrthshadow
p ~r orbit period, will alno impact
the OOlkr array siting t0 provide the
required energy. Bosidos, electric
propulsion DC power requiresnnts, will
also increase the solar army siting.
Accordingly, the solar array hype may
range Prom a fixed wings' configuration,
suitablo for satellites in' sunaynchronoue
down-dusk orbits, to a sun-tracking
configuration for Spacecraft :n low to
medium inclination orbits.
On the above grounds, long lifetime
operation of mass-efficient battories,
subjected to rathor deep and pariodic (up
to 30000) discharge-recharge cycles is an
oustanding issue for. such L.E.O.
obsorvation lightsats equipped with SAR
sensors.
Spacecraft tolocommand must bo oacuro, and
possibly jam-proof, to avoid unauthorizod
entries to the satellite nyntam. Data
encryption must be also implenontrd, in
SpacecraPt telemetry and SAR data
transmission to ground, to prevent
eavesdropping by unauthorized uners.
!
Lightreto in tho 500 to 800 Kg range can
carry CUI nQnaora for high resolution (2
to 3 m), short rovisit interval, tactical
ObQQFJatiOn missions. Medium resolution
(order of 5 m) missions, tor strategic
and, in genaral, international law-
onforcement applications, are also
Poaeiblo. Such spacecraft can form
priaan@nt constellations of, typically, 6
to 8 satallite8 per orbit plane to provide
global cbverage. Multiple orbit planes
constellations can offer enhanced
perfomanbe, allowing also a gradual
system build-up. Smaller constellations
of 4 to 6 spacecraft -can be alno
implernentgd to cover a narrower latitude
belt akound the equator, while
eignif icahtly Improving the average
revisit intervale at sites close to the
orbits inclinations.
In summary, SAR lightsats constellations
can offer certain performance unmatched by
existing, or plarlned, single and larger
observation satellites and can provide a
valuable answer to specific operational
military needs in both tactical and
strategic scenarios. The required
technologies exist, with few exceptions
needing an early in-orbit demonstration of
d@vices being now developed. Nevertheless,
advances in few fields wili certainly
benrfit future generation lightsats.
REFERENCES
(1) C. Perrotta " SAR sensors on board
small S8tQllitCfJ: problems and
prospectives"; Proceedings of 1991
International Conference on Radar; October
1991, Beijinq, china:
[2] F. Borrini " The Italian approach to
tactical use of space": this Conference: I
[3) G. Perrotta, G. Cirri, G. Matticari "
Eloctric propulsion for liqhtsats: a
review oP applications and advantages":
this Conference:
[4) D. Caeey, J. Way Orbit selection for
the EOS mission and its synergism
implications": IEEE Tranoacticns on
Geoscience $and Remote Sensing: Vol 29 n'
6, NOV. '91.*i
' I
[5] H.J.Schodol,T.Kutscheid I' The X-band
CFRP WaVeqUidQ antenna for the Spaco Radar
Lab.81: ESA Workshop on Antenna Technoloqy,
Nov. 1830. ,,
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