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3-4
mggd and easily operated. This n~od for small
ground terminals drives the satelliPe design to the
use of high powered transmitters and high gain
antennas. Tho SHF satellite was configured as a
bent pipa in which little processlrrg 00 the signal is
&ne on bard the spacecraft. The oommuniciations
payload simply roceives the signal, shirts carrier
frctguency orad retransmits it towards the earths The
EHF sys?em, on the other hand, does much bre processing of the signal on the salellile. The signal
coF;..rtg inPo the satellite is taken off uf the carric?r and
shifted down to baseband where the bits the of
digital message are available. The digital bits are
routed lo Pheir destination, shilled up in frqudncy,
put on the carrier ana retransmitted to the grcund.
The SHF bent pipe system utilizes two
lransponders each having a nominal capacity of 80
MHz. A 61 element multi-beam antenna provides
aoti jam nulling on the uplink and a 19 beam multi-
beam antenna shapes the downlink coverage
pattern to the theater. In addition, a spot antenna
and an earth coverage horn are included in the
downlink. Throe solid state powered amplifiers are
incorporated in the design to provide redundancy.
The spare power amplifier can be switched into
either of the two active channels. The resultant
payload weighs approximately 84 kg and requires
225 watts 01 power to allow link closure with man
portable terminals having an antenna on the order of
0.6 meters in diameter. A capacity of approximately
2000, 2.4 kbps channels would be possible using
lhose man portable terminals.
A 36 channel EHF payload was also sized. This
payload was designed lo support EHF man portable
terminals. The payload consists 01 32 low data rate
communications channels and 4 channels for noise
characterization/acquisition. Thc sample payload
has a 61 beam multiple beam antenna with a nulling
processor on the uplink. The fully autonomous
operation of the processor represents the only
design area that may be pushing the state of the art.
The downlink includes a 19 element MEA, a spot
antenna and an earth coverage horn. The design
features fully redundant travelling wave tube
amplifiers. Assuming there is one user per terminal
and an average call duration of 4 minutes, the
nbmber of terminals that can be supporlod can be
calculated using message switching theory. With a
5% probability of call cancellation or a 20%
probability of call delay, approximately 400 to 500
user terminals can be supported by this 36 channel
system. The resulting payload weighs
approximately 1.20 kg and requires 245 watts of
power.
In the previous section, we sized 3 payloads; a
single payload that can accomplish two survelliance
missions, an SHF communications payload, and a
EHF communications payload. Payload weights
ranged from 84 kg 13 118 kg and the power
requirements were from 225 watts to 300 watts. If
we now try to design a common bus, one that
accommodates any of the three payloads, we find
that the payload governing the design is that of EHF
communications. It is the heavisst payload, has near
maximum power requirements. a 10 year life and
requires 45 kg of maneuver propellant (the reason
for which will be discussed later). The resultant
spacecraft weight, including payload, is 635 kg. In
comparison with a unique spacecraft designed for
each specific mission, the common bus spacecraft
represents approximately a 30% weight overdesign
for the theater surveillance mission and a 7 to 8%
overdesign for the theater missile tracking mission.
This basically comes about from the differences in
satellite design life which governs the amount of
propellant that must be carried for station keeping.
In addition, the theater siirveillance mission is
conducted from lower orbit and does not require tha
45kg of maneuver propellant. In comparison to a
satellite specifically designed for the SHF
communications mission, the common bus design
represents a 27% over design. This is mostly due to
the lighter SHF payload weight and reduced power
requirement. Penalties of this order of magnitude'
must be accepted to take advantage of a common
bus design. Not only does commonality achieve
cost reductions as a result of an increased
production buy and corresponding learning curve
leverage, but it promotes the use of standard test
procedures and test equipment. Payloads can be
handled as black boxes and thereby, integration arid
test times can be reduced. It is clear, however, that
the more the payload weights and mission
parameters diverge, the larger the penalty that must
be paid by using a common bus.
The surveillance and communications missions were
then used to define the more complete set of bus
design parameters shown in Figure 3. As expected,