Page 182 - Intro to Space Sciences Spacecraft Applications

Payload Spacecraft Systems 169

The payload mass and mass dynamics (e.g., scanning antennas and
spinning mirrors), pointing accuracy, dynamic envelope dimensions, data
and power operating profiles, and other characteristics propagate into
many complex aspects of the satellite design.
For example, the mass of the payload sets the pace for the calculation of
the on-orbit mass of the entire satellite, thus setting the goal for the propul-
sion subsystem Av. The ARC subsystem keys on the pointing accuracy
required by the most demanding payload element and uses this value to
determine the candidate control architectures capable of achieving this accu-
racy, then goes on to compute the control system’s mass required to stabilize
the satellite in its orbit. The on-off duty cycle and power demands of the pay-
load create the profile for the power subsystem that must collect and store
energy as the satellite goes in and out of (eclipses) a view of the sun. The
thermal subsystem considers eclipse and sun periods and satellite orientation
along with the mass and power requirements of the payload to arrive at an
estimate of the thermal subsystem mass. The orbital maintenance subsystem
uses the mass of the payload to drive the total on-orbit mass and converts this
into a satellite design envelope yielding a ballistic cross section. This esti-
mation is necessary to compute drag (which varies with the environment pre-
dicted during the expected mission lifetime), which in turn determines the
fuel necessary to maintain the orbit over the expected mission lifetime. The
TT&C and data handling subsystems key on the payload data rate and duty
cycle to determine the need for downlink bandwidth and the type and capac-
ity of onboard data recorders. The structure subsystem evaluates the needs of
the satellite using both static and dynamic mass estimates from the payload
and other subsystems to compute the total structural mass.
Payload requirements influence and can also be limited by satellite per-
formance attributes such as pointing accuracy, structural rigidity, power,
and thermal requirements. These and many other important considerations
are cross-linked as the designer proceeds from mission and payload spec-
ification to a review of each of the satellite bus subsystems.

Launch Vehicle and Site Selection

The launch vehicle may be selected prior to satellite bus subsystem
design or after the design has been completed. If the former option is cho-
sen, the design proceeds down a path of launch vehicle optimization, and a