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44
psi lion, attitudc control arid pointing agility. orbit
adjust control and actuation. prayloml torque anJ
mcmcntum compcnsation. safcmodc conlr~l and dcorb^,
;I q u i d . TRc subsystem hardwssrc consists of
x,xm. txtu;~~ors and propellant storage and &livery
systcm. All control law. safemode and oihcr software
rcsidc in thc ISE. Thrce faction whecls pvidc torquc
actuation, with agility rcquircmcnts and residual
payload toqucs aftcr momentum compensation driving
their six and magnctic torquers for desntlaration. Low
cost staf camcra scnsors providc thc kind of knowlcdgc
nceded. with a sun scrim for cwfsc pointing. 1
?he propulsion systcm consisls of 3 conventional
hydrazine mono-propcllant unk wiah proprllanl
Aanagcment systcm. lines. filler. valves,
ihstrumcnution and Lhrustcrs. Syslcm p~essuri?ation
could use cithcr a b!ow-down or a rc-pressurization
systcm. The propulsion systcm is uscd to correct
launch vehicle injection errors. perform orbit trim
haneuvers and dc-boost if rcquircd.
Major bus structurc components me: thc frame.
equipment pai!cls. propulsion module, and interfacc
ring. Thc modular design allows subsystem mounting
while mainiaining alignments during test,
mwcmion, and storage. Thc modulstr agproach also
reduces pm count and assembly spans. The interface
ring provide3 Ihe suuctural load path brKwam the bus
slructure and launch vehicle. Omghite-epoxy
composilc (Gr/E) was selected ova a meanllic malcrial.
for both primary and secondary structure. after
conducting Yadc studies. GrE providrs LI stslble (near
zero CTE! stru-ture to meet and maintain attitude
Figure 19. T} pical TacSat Block Diagram
31-9
control scnsor pointing accuracy. Thc Gr/E suucturc
saves 4.9 kilograms ovcr AVLi and 7.9 kilograms ovcr
Al. Thc primary structurc providcs continuity. strcngth
and rigidity between IRc payload and thc intcrfacc ring.
-
Spa-Ground Link Systcm (SGLS) and high data-ntc
anwnnm stow against thc vchiclc and dcploy on hingcs.
Spring driven mechanisms dcploy both antcnnas and
Ihc solar mays. Each solar array gimbal is drivcn by
small motors and rc8ucers. Rcx harness and slip rings
cany clccrrical power and lelcmctry across thc joints.
Temperature. exhfemcs allowed in olhcr subsystems
drivc the salcllite thcnnal subsyslcm. Assuming the
general case using a non-sun synchronous orbit.
TacSau would not have "hot" and "cold" sidcs, as the
thermal cnvironmcnt varics on thc satcllitc sidcs
depending cm inclination and timc of ycar. A TscSat is
subject to many cclipse cyclcs and must bc dcsigncd to
accommodate them. For example. componcnts that
operatc during sunlit portion of thc orbit would have
a significant thcrmal radiation arca to rcjcct both
intcmal and incident hat. Ilnfonunately, whcn this
surface is in cclipsc, heater power must now mainlain
the item with the larger radiator sizcd for the full-sun
operation case. Albedo and earthshine (planet IR
hearing) also influence LEO vehicle environments.
RE&
Fi$urc 17 shows thc two major communication system
hrdware groups. User support is provided by an
encryped two-way common dam link (CDL) at X-
Band. A small stecrablc downlink dish can provide
about 274 megsbit/scc in LEO orbits. The up-link