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23-10
between the modulator output g md the thruster
activation 3 are stored in the on board computer
memory accordhg to t;iblc 4.2-2.
Forcc Gencration
According lo figures 4.1-1. there are two diffcrent
S/C orientations in orbit. Fur 50th cases, table 4.2-
3 summarizes the nominal and redundant thrustcr
sets. which are used for orbit corrections in normal
and tangential direction.
423 Momennrm and -reactioa whee,
The preferred wheel arrangement a. shown in fig.
4.2-2 ronsists of
Two flywhcels in a V-configuration in a plane
rotated about thc y-axis by an ;ingle q
Two rcilction whccls alonq thc x- and L - ~ C S
In the nominal contip~ration only ohc tl~u.hccl
(cither FMW I or FMW 2) and hoth rcaction
whccls ilrc in opcration. all running ;it bias speeds
such that a residual bias angular momcntum Gector
is nominallv perpcndiculx to the orbit pland.
In cse of a failure of one reaction whccl thc resi-
dual rcaction wheel and hoth flphcels udll bc
used.
If 00 the other hand the ‘nomind‘ flywheel fails.
the cold redundant futcd momentum whccl id acti-
vated.
43 AOCS HIW and SnV Impletneatation
Prcsentlv. based on a cooperation agrcerncni bet-
ween DASA. Acrospatialc. md Alenia Spado joint
ciforts arc bein8 undertakcn io devclop and qualify
an ‘Intclpatcd Control and Data Al;ina~cmcnt
System’ (ICDS) for nem gcncration communication
and application satellites. within the so-called ‘Spa-
ccbus Improvcmcnt Program‘ (SIP). This prayam
also incorporates devclopmcnt and qu;ilificaiion of
advanccd components like the precision sun sensor
(tablc 3.2. I), second generation liquid bipropellant
thrusters (table 3.2-3) and in particular aLw ;I po-
werful Onboard computer Unit (ORCU) and the
associated operational and application SW. The
ICDS in question (see ref. 10) is to the largest
cment directly suited or easily adaptable for the
case under discussion here. The main difference to
the concept favoured for communiwtion SIC ap-
plications consists in that for TACSAT missions a
‘two computer solution“ insicod of a nnc ctnval
compute approach is regarded absolutely necessiuy
in View of the flexibility- and computational requi-
rcments of thc different payloads. The amount of
payioad data to be transmitted to ground within
short ground cantact intervals will furthermore
necessitate direct priority link to the telemetry
system. Subsequently only the AOCS part of the
ICDS concept, which then can be regarded largely
indcpcndcnt from the payload data management
systcm will be shortly outlined.
Hardware
Fig. 4.3-1 shows the ICJIS hardware configuration
(without rcdundmcies) i.e. not only the AOCS-, but
also the DMC hardwarc units are represented.
Thc functional sharing of these units and comments
on thc notations within ligure 4.3-1 arc listed in
table 4.3- I.
Thc serial OBDH Data Bus is Ihc link within a
modular cxpanda1)lc t\OCS.
As Iar ;is newly devclopcd cquipmcnt and signal
conditioning clcctronics arc conccrncd (e.g. sun
sensor clcctronics. UPSE), t hc intcrfaces are de-
sipncd such as to directlv match the data bus IF
rcquircmrnts. Thc signal conditioning for classical.
off-thc-shelf equipment (earth sensors. gyros.
whcels. star scntors) to thc OBDH data bus format
is performed in the Platform ‘Interface Unit
(PRU).
The ccntral component of the ICDS hardware is
t’lc On Board Computer Unit (OBCU). It has
functional interfaces with the ground scpcnt and
all on-board S/C subsystems for the distribution of
commands and thc acqi:isition of data. Figure 4.3-2
shows the blockdiagram of the OBCU. The
functional units arc:
- processor Modulc (PM)
16 bit processor. MIL-STD 1750 A instruc-
tion set ruchitccrurc
processing power at least loo0 kips lor DAIS
MIX (Digital Aeronautics Instr. Set -
ZOhlHz)
RAM 123 kwords of 16 bits
ROM 96 kwords of 16 bits
- Telccommand Dccodcr bModulc (TC)
- Tckmcrry Modulc (TM)
- Reconlipation Modulc (RM)
supports autonomous satellite failure detcc-
tion. isolation and recovery (FDIR)
functional elements: Alarm Level Manage-
ment, OBCU Reconfiguration Commands.
0 B D H - bus R e c o n f ig u r H t ion CO m m a n d s,
Thruster On-Time Control
- Safeguard Memory (SGM)