Pre-Phase A Report - Lisa - Nasa

150 Chapter 7 Spacecraft Design
data rates for science and telemetry permits all data to be buffered and stored in DRAM
on the flight computer board, thus reducing the subsystem mass and power.
7.2.5 Tracking, telemetry and command
The TT&C functions are provided by an X-band telecommunications system, consisting of
transponders, a Radio Frequency Distribution Unit (RFDU) and antennas. The transponder
subsystem features two basic transponder units, each with its own solid-state power
amplifier. Each transponder operates with the receivers in hot redundancy. The transmitters
are configured for cold redundancy and can be switched on and off by telecommand.
The function of the RFDU is to control the routing of telecommand and telemetry data
between the two transponders and the antennas. The signal routing provides efficient
redundancy for both telecommand and telemetry functions.
During the operational phase, two steerable high-gain antennas configured on top of
the spacecraft are used. These have a diameter of 30 cm and a nominal boresight gain
of 25 dBi. The 3 dB two-sided beam width of the antenna is about 8◦ andanelevation
mechanism can be avoided. A mechanism providing 2π coverage in azimuth is required,
however.
To obtain the required omni-directional coverage for telecommand, two low-gain antennas
are mounted on opposite sides of the spacecraft. These, however, cannot provide for the
telemetry during the transfer phase, and medium gain antennas, accommodated according
to the spacecraft-Earth direction during the transfer, are required. The location of these
antennas is TBD.
With 5 W transmitted RF power, the high-gain antennas allow for a telemetry rate of
375 bps into the 15 m ESA stations. For a real-time science and housekeeping data rate
of 80 bps for each of the six spacecraft, a total daily contact time of 16 hours is required
(e.g. two stations 8 h each) and simultaneous telemetry from the two spacecraft at one
vertex.
7.2.6 Power subsystem and solar array
Each LISA composite consists of two modules. Apropulsion module jettisoned at the end
of cruise, and a sciencecraft module. The sciencecraft is a flat cylinder, 1.8 m in diameter
by 0.5 m thick. Anexternal sunshade is added to the outer sciencecraft edge on the sun
side. This shade combined with the nominal sciencecraft flat surface provides a total
sun-facing diameter of 2.2 m with a total surface area of 3.8m 2 .
The orbital configuration allows the sciencecraft to be in sunlight at all time, with a maximum
off sun angle of 30 ◦ (during science operations). Sun facing surfaces are expected
to reach 80 ◦ C.
GaAs solar cells with 19 percent efficiency are used for power generation for both the
sciencecraft and the SEP arrays. The batteries are of the Lithium-ion type, providing
80 Whr/kg specific energy density and 140 Wh/l volumetric density. Integrated Multichip
Module to VME board technology is used for power control, management and distribution,
and laser pyro drivers. This technology is expected to be demonstrated and qualified
3-3-1999 9:33 Corrected version 2.08