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23-2
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- missile wnrninq, e.g. against tactical h\Iistic
missilcs, to possibly improve thc cfficicncy of
dcfcnsivc actions
- nnviir.ntion_ to contributc to thc provision of
-
highly accuratc navigation data
ohscrvntion. survcillans and verificatinn, c.5. to
chcck disllrmamcnt trcaty tiolation or - in a
battlc ficld sccnario - to monitor [orcc dcploy-
mcnts and to provide target rccognition. tbcatcr
targctins and vcrification
As a conscqucncc of thc gcncral mission objcctivcs
oiillincd abovc, opcratiod rind performancc rc-
quircrncnts. cncironnrcntal conditions and son-
straints arc imposcd on thc Attitude- and Orbit
Control Systcm. which arc subscqucntlv outlincd to
establish thc gcncral framc for thc AOCS.
2.1 hllssinn rind Faylaad Kcquiremcnls
For varinus rcisons likc launch cost. in;m in orbit.
imasc rcsolution or powcr rixpircmcnls of obscrvntion
payloads thc spacecraft undcr discussion
will hc injcctcd into rclativciy low cnrth orbits
(about 300 to 1Mn) km). In gcncrd near pol;ir. sunsynchronous.
cirulw orbits arc prcfcrrcd, which arc
charactcrizcd by thc condition that lhc .product of
three orhit paramctcrs, i.e. orbit scmi-mnjor ;~l(is.
cxccntricity, and inclination has a spccific numcri.
cal vduc. cntrining thc orbit node to rotatc t\ith
onc rcvolution pcr ycar. For optical obscwation
pavlaods "dawn orbits' arc particularly suited. In
this casc the S/C will always view thc surfacc of thc
carth at any given latitud: at thc siunc local time
(sce e.g. rcf. 1, p. 68). A graphical rcprcscntation
of thc orbit pararnetcrs 'inclination" (in dcg.), rhc
"orbit pcriod" (in minutcs) ;ind thc "SIC velocity"
.(in rnfscc) as a function of altitudc (in km) for
circular sun synchronous orbi~s is givcn in ligs.
2.1-la to 2.1-lc of Anncx I. r,:spectivcly.
For such orbits thc revisit period of spccihc tcrrc.
strial locations in equatorial regions or ot law earth
latitude for ;I sin& S/C m;iv bc unaccctahlv long
for tactical earth obscrvation rcquircmcnts. whcrcas
the rcvisit pcriod in mostly unintcrcstiny polar
regions in principle equals thc orbit rcvoltltion
pcriod. This situation can tic improved as follows:
- For obscrvation of a particular re+n on earth
the SIC is launched into n dcdicatcd orbit. thc
revohtion period of which is an intcgcr fraction
of 24 hours. Thc revisit limc will then bc at
lcasl mcc or cven scvcral limes a clay. For in-
stance (ref. figs. 2.1-la and -1b) a satcllitc in
-
sun-synchronous orbit of (about) 570 km altitudc
and 97.6 dcg inclination will have an orbit pe-
riod of 96 min aid p~ over the same area
cvcry 15'" revolution. 16 orbits per 24 h (orbit
period ol M mh) would cvcn allow to revisit the
same targct cvcry 12 hours (in thc descending
N/S and ascending SM orbit crossing) but rcqui-
re an altitudc of 275 km (inclination 96.56 dcg)
and the S/C would experience sigificant air
drag as wil? bc discussed latcr (fig. 2.1-3, AMCX
1).
A numbcr of payloads can provide large ranges
of "sidc looking" capability, cnabliag observation
of a local arc3 undcr different aspect angles in
succcssivc orbits. Exccpt when phased am/
antcnnas arc crnploycd this generally rcquircs
nicch;inical slcwing of optics, mirrors ;rnd/or
rcflcctors and in turn inay givcn risc to sipifi-
cm intcrnnl torques and associated attitude
pcrturb;itiocs.
- Selection of low inclination orbits instead of
quasi-polar orbits with inclination angles covc-
ring the rqc of gcographical latitude of parti-
-
cular intcrcst.
Pkicing ii sufficicnt numbcr of satcuitcs into
appropriatcly inclined. mutually synchronizcd
orbit to cnsurc r,hc dcsircd covcragc probability.
Thc rchtionship lictwccn thc numbcr of satcllitcs
rccjuircd to cnsurc continuous coveragc from any
point on thc carrrh undcr a local elcvatim andc of
at Icast IO dcg as a function of circular orhit dtitu-
dc is shown in fig. 2.1-2 (rcf. 2 ) of Anncx 1. Pro-
per sclcction of orbit inclin:ition and phasing of thc
cirbitiny satcllitcs is rcquircd. Particular cxamplcs of
Low Earth Orbit (LEO) Communication Satcllitc
Systems c.g. MOTOROLNIRIDIUM or TRW-
ODYSSEY rcquiriny 77/19, satellites at 735/10350
km dtitudc arc schematically indicated in this hgu-
rc. Thc numbcr of satcllitcs Jccrcascs, of course if
poliir covcraqr is not rcquircd. In any casc orbit
inclination ;ind phasing of individual SIC forming
p;irt of a satellite syctcm hnvc to bc rclaincd or
respectively corrcctcd in c;rsc of dcvintions from
nominal during opcrational lirc timc.
For thc asscssmcnt of the amount (m,) of rnono-
propellant hydrazinc rcquircd to maintain a circular
orbit of givcn, rclativcly low altitudc, rcfcrcncc is
made to Eg. 2.1-3 (Anncx l), where thc drag factor
(9 pcr day (d) and unit cross-sclcctional iirca (0)
of thc S/C is plottcd. The propellant mass is thcn
givcn by