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Tho fnasibllity of installinql SARs on
board liyhtsats for tactical and otrategic
oboervat Ion mlsslons 1s dllbcusrred,
enphaolzing hlqh resolution and short
rovlsit intervals as moln syoton drlvers.
Llqhtoat constollations deuign criteria
are prssentcd for both globnl on4 limited
latituda belt covoraqe. Pros and cons of
sunsynchronous versus ncdium 1ns:lination
non nunaynchronous orbjto are discussed.
Gross Bystem trJd0 Off3 for tho SAR sensor
are then arldreoaed, in tho spccific
context of a resourcc-llmitod liqhtnnt,
stressinq tho achicvcncnt of *@solutions
better than 5 n, swaths grestsr than 20
Km, access anqles of at least 30'. small
anton?ta dimensions and a reaeonnblo powwr
consumption. Roth X and C band SAR
solutlone are outlined. Ditn transmisslon
alternatives aro also discussed,* In the
contoxt of tactfcal and 1 stratoqlc
scenarion, outlininq thcir ' projectod
performance.
I
Conotellation orbits control and plattorn
attitudo are also addrt-ased lor tholr
Impact cn mission , SAR Inn70 Quality a.id
natellita design requircnents. Rsy aspects
of critical platform subsyetens are also
idsntified.
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1 L 1 !?IK~KG-roF?-
Liqhtsat constellations 'aro rrceiving
considorablo attention by industry.
qovernnanta 1 Aqonc I cs and commcrc la 1
carrlors. Thalr potontial for strateqic
connunications is well undarntood by
ailitary planncrs. which contnibutod to
the rant spinorf of major prnjncta boing
undertaken by Rome American sytaton houses.
Less well perceived are the 1 iqhtnots
capabilities for :emote senninq and
obscrvation tasks for both civil nnd
dofense uses. In 1990 , anticipstinq tho
need for new and unconvcntlonml oolutions
to observation sateltites, AlonIn Spazio
started an Intornal ntudy addraoning tho
feasibillty Of small SAR QatOlllteQ,
stressing tho achievcment oP ; high
resolutions and short revlsit Intbrvnls.
Tho '91 Gulf War provided furthor ntlmuli
in that diroction, reinforcod by tho
world-wide trend concernlnq npaco and
dofenco budget allocatlonn. Flree results
were publiehcd In [l]. Slnca thon, further
work on ayatem architectures and trade
offs hao boen performed, concontretlnq on
military applications and tho nosr-tom
foaslbIlity wlth available tochnoloqlras,
thus avoidinq long and contly now
developmente. This paper revlauo nom0 of
the most recent results achleved masfar.
,
SAN SENSORS ON TACSATS:
A FEksIllILIPY ASSEGSIENT
' C. Porrotta
Alanla Spario 9.p.A.
Via Saccomuro ?4, 00131 RODQ, Italy
I
2L SAR OBSERVATICIN MISSIONS
SAR sensors operational value comes from
their independence from time of the day
and clouds cover. Achievable ground
resolutions, swaths and access anqles are
comparabln to those of panchromatic
optical sonsors. Nevertheloss SARs respond
differently to the physical foatures of
the obsewr' 'cone: thio may be exploited
to enhancs certain target characteristics.
SAR and optical satellites can therefore
complement each other.
Howover, in a tactical scenario, where
weather and time-of-day independence of
the observations are a premium factor, SAR
satellites outperform the optical ones
from an opcrntlonal vlewpoint.
In addition, constellations of SAR
oatollitee can offer performance
unmatched by larqer spacecraft flown
individually, much as Inhorent redundancy,
mor0 frequent rovislts and an easier
dCCCGB to in-orbit rOEOUrCO8. If SAR
u~nscr~ can bo flown on liqhtsnts the
total syoton cost can be considerably
reducod, whilo achieving nom flexibility
in deployinq and managing them.
2.1. System confiqurntion alternatives
We will discuss rcpresentatlve missions
otreoninq tho hi-resolution ones for
dofonso applications. SAR llqhtsatn can bo
launched on-demand, for short duratlon
observation tasks over cpeciflc arcas, in
the event of qco-political or nilitary
ctlsee. Alternatively, lonq duration
mioolons with liqhtsats constellations can
ba envisaged to porform cont lnuous
obaorvations within a given latitude
bolt.
ThO cost-effcctiveness of short duration
aissione is ncvertheloss questionable,
considering the nced for a pormnnent
ground Infrastructure whlch has to bo
operated and mairbtainod anyway. and tho
satellite and launch costs bhich arc
poorly amortized. Permanent constellations
car., Instead, otfer, oven batter qlobal
porformanco at a higher initial cost which
ia howevor amortized over a much longer
aorvlco p0riod. Furttiermoro, a pernanent
constallation can provide shorter revisit
incorvals and, outside crises perlcds,
otratwic observation aervlcos as well as
remoto eensinq functionu for qovernment
and public une, with specific referenco to
pot-dioaeter damaqe aceesement. We will
thus concentrate, in the following, on
lightsat constellations.
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