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i3-6
yuratiori. Whcel desaturatioo is nominally pcr-
formed usiog magnetic torquers in quarter orbit
cycling.
- ROSAT, dcvcloped within a German, UY US
scientific satcute program operates in a 5% km
circular orbit, inclined by 53' w.r.t thc equdtori-
a1 plane. Attitude reference is established by
high prccision star Sensors and a set of 4 inte-
grating gyros (one skcwed). A coarsc SUO sen-
sor asscmbly with 4-w FOV providing two axis
attitudc rcfcrencc is used for initial and einer-
gcncv sun acquisition. For thc scicntific migsion
two operational mcdcs are forcsecn:
In scanmodc the S/C rotiitcs about tllc sun
linc and the telcscopc mountcd pcrpenUicu-
I;ir to thc axis of rotation performs witbin o
months a completc sky suncv for dctcction
cif ncw X-ray sources
In pointing mode thc tclcscopc is rotatsd to
dircctions sclccicd by thc groundstatioh ior
dcdicafcd sourcc observation
In both n;odcs of opcration ,inertial attitude
refcrcncc is dcrivcd from star scnsor rneasurc-
mcnts and star identifications with i\n ooboard
storcd stnr cataloguc. A sct of 4 rcnction wheels
in skcwcd arrangement gcncratcs tlic ncccssary
control torques. A thrcc axis magnctomcrcr 2nd
3 mqnctic torqucrs arc urd rcspcctivcly for
-
-
dctcrmination of the carth mnbmetic ticld vcctor
and whcel uiloading. Duc to'cquipmcnt f7.1 ,i urcs
emergency stratcgcs for attitudc gcncration
from mapctomcter mcasurcmcnts and an
carth mapctic field modcl onboard of the S.'C
had to be dcvclopcd and implcmcntcd.
EURECA. the EUropcan P.€tricv;lblc CArricr.
dcvelopctl by DASA under ESA contract. shall
be the platform for 5 diffcrcnt scicntific missions.
whcrc thc first mission (launch and rcturn
by shuttic) performs c,xpcrimcnts under ,U-y
conditions. Thc AOCS (a coopcration bctwcen
DASA. MATRA. GALILEO. LABEN) is able
to pcrfoim orbit chmyc mancuvcrs (boost-up.
boost-down, inclination corrcction) suo and
earth acquisitions with a. hydrazine thrustcr
systcm and is suo oricntcd in normal opcration
with a low level cold gas system supported by
mapnetic torquers.
ASTRO-SPAS is a reusablc satellite platform
built by DASA udcr DAM contract. which
shall fly once per ycar, beginning in 1993.11 w i l l
be launched with the US Space Shuttle. sct frec
in orbit ind also berthed from the snnie shuttle.
Its ACS is equipped with a rate integrating gyro
packagc, d high precision star sensor, a GPS
rcccivcr and a sct of cold gas thrustcrs. Similiar
to the ROSAT mission two principle modes of
operation can be performed
In pointing mode the telescope is oriented
into ground selected orientations
lo scanmode earth atmospherc observations
arc performed whcrc the earth refcrencc
information frame is obtained from the GPS
receker.
As io ROSAT, in both mides inertial attitude is
obtaincd from star sensor measurcmeots and
star catalogucs.
32 AOCS Alternatives, Trade-offs and Trends
Whcn discussing AOCS altcrnativcs for TACSAI'
;ipplications. the ;ispccts associatcd with attitudc
rcl'crcncc gcneration. forcc- and torque generation
and attitilde control conccpts including thc relatcd
cquipmcnt and software havc IO bc adrcssed.
32.1 ..lrti!ude rcference generation equipment
The gcncral rcqucst for low cost solutions irnplics
th;tt attitudc mcasurcmcnt equipment should bc
uscd. which just satisfics the subsystcm performan-
cc necds. In vicw of thc large vnricty of mission
objcctivcs and associatcd pcrformancc requirements
(sec tablc 2.3-I), however. equipment of cquivalcnt
mcasurcmcnt quality standard\ (and costs) has to
bc sclcctcd. Conscqucotly the AOCS concept niust
providc thc ncccssary flexibility to tie-in different
kinds of mcasurcrncnt dcvices as rcquested by the
application case. Thc typcs of cquipment io quc-
stion arc of coursc Infra-Red earth Sensors (IRS),
Precision Sun Scnsors (PSS), STar-Sensors (STS)
and Ratc Integrating Qwos (RIG). Apart from
convcntional dcsign performancc chxractcristics.
ncw dcsiys arc of particular interest. For the typi-
cal performance data. rcfcrmce is made to table
3.2- 1.
322 Force and torque generation rechnolom
Spacccraft dircctly injcctcd into targct orhi:s with
Limited orbital accuracy deinands and/or or rclati-
vely short operational lifc time may not rcquirc
orbit corrections anti conscqiicotly also no propul-
sion subsystem. In gcocral, however, orbit COITCC-
lions will be nccesmy to ilchicve and maintain the
desired operational orbit. In view of aspccts likc
inherently different propellat utilivtioo efficiency,
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