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=--=--=START=--=--= NASA Spacelink File Name:6_2_2_46_3_8.TXT
ACTS Overall Description
ACTS is comprised of a spacecraft bus with basic housekeeping
functions and a payload, known as the multibeam communications
package (MCP).
At launch, ACTS weighs 6,108 pounds including the propellants
and the spacecraft adapter and clamp band which remain with the
Transfer Orbit Stage (TOS) upon separation. When in the stowed
configuration in the Shuttle payload bay, ACTS' overall height is
15.9 feet (5 m) from the spacecraft separation plane to the tip of
the highest antenna.
During the transfer orbit phase, the spacecraft is spin
stabilized, and the antenna reflectors and solar array panels are
retracted and stowed to provide better load support for these
appendages. During the on-orbit mission phase, the spacecraft is
three-axis stabilized with the large antenna reflectors facing the
Earth and the solar array panels rotating once per day to track the
Sun. On-orbit, ACTS measures 47.1 feet (14 m) from tip to tip of the
solar arrays and 29.9 feet (9 m) across the main receiving and
transmitting antenna reflectors.
Spacecraft Bus
The spacecraft bus structure is a rectangular box with a
cylindrical center structure that houses the apogee kick motor (AKM).
The multibeam antenna subsystem is mounted to the Earth facing panel
of the spacecraft bus. The North and South sides are each divided
into three panels. These panels are used to mount most of the
spacecraft bus and MCP electronics equipment. The spacecraft bus
provides support functions for the MCP such as electrical and
mechanical mounting surfaces, attitude control, electrical power,
thermal control, command reception, telemetry transmission and
ranging and propulsion for station keeping maneuvers.
Multibeam Communications Package
The multibeam communications package performs receiving,
switching, momentary storage, selectable coding and decoding,
amplifying and transmitting functions for Ka-band time division
multiple access (TDMA) communications signals. The multibeam antenna
(MBA) has fixed beams and hopping spot beams that can be used to
service traffic needs on a dynamic basis. (A hopping spot beam is an
antenna beam on the spacecraft that points at one location on the
ground for a fraction of a millisecond. It sends/receives voice or
data information and then the beam electronically "hops" to a second
location, then a third and so on. At the beginning of the second
millisecond the beam again points at the first location.)
In addition, the receiving antenna provides signals to the
autotrack receiver which generates input error signals to the
attitude control system for spacecraft pointing operations. Beam
forming networks (BFN) utilize hopping beams to provide independent
coverage of the East and West scan sectors, plus coverage for
isolated locations outside of either sector. The MBA also has three
fixed spot beams. A steerable beam antenna has been incorporated
into ACTS to provide antenna coverage of the entire disk of the Earth
as seen from l00 degrees west longitude and to any aircraft or low
Earth orbit spacecraft, including the Space Shuttle, within view of
the ACTS.
ACTS Deployment Sequence
ACTS will be deployed from Discovery's cargo bay approximately 8
hours after launch on orbit six. The TOS burn which will inject ACTS
into a geosynchronous transfer orbit. The spacecraft apogee kick
motor will inject ACTS into a drift orbit. Finally, ACTS will be
placed in a geostationary orbit at 100 degrees west longitude over
the equator, approximately in line with the center of the United
States. A geostationary orbit is one where a satellite takes 24
hours to complete one revolution, thus appearing to remain motionless
above a single place on the Earth.
About 2 hours before deployment from the orbiter, the astronauts
perform a sequence of events beginning with preliminary TOS checks,
unlatching the TOS cradle and elevating the ACTS/TOS flight element
to a 42 degree angle for deployment. The crew will fire the
"Super*Zip" separation system, and six springs on the TOS aft cradle
will push the flight element out of the cargo bay.
The TOS motor firing is controlled by an on-board timer and
occurs 45 minutes following deployment from the orbiter or about 8
hours and 45 minutes after STS-51 launch. The approximately two-
minute burn will place ACTS in a geotransfer orbit. The apogee kick
motor burn to inject ACTS into drift orbit will take place 42 1/2
hours after deployment, approximately 50 1/2 hours into the mission.
The 7-day drift will allow ACTS to move toward its final station
location of 100 degrees west longitude. Firing of the spacecraft's
thrusters will bring the perigee and apogee radii increasingly closer
to the geostationary orbit.
Upon reaching geostationary orbit, ACTS will transition from a
spinning to a three-axis stabilized spacecraft configuration and
deploy its solar arrays and antennas.
ACTS experiments will begin 12 weeks after launch following the
placement of the spacecraft on-station and spacecraft checkout. ACTS
is designed to have a minimum life of 2 years but will have enough
station keeping fuel for a 4-year-plus mission.
ACTS Ground Systems and Support
The facilities and support to be used for the ACTS mission
phases include the Guam and Carpentersville, N.J., C-band telemetry,
tracking and command stations and the ACTS ground segment.
Tracking, Telemetry and Command
The ACTS mission telemetry, tracking and command (TT&C) control
and monitor functions are distributed between two geographically
separate locations: Lewis Research Center, Cleveland and the Martin
Marietta Satellite Operations Center (SOC), East Windsor, N.J. The
SOC is used to control the ACTS housekeeping functions during both
the transfer orbit and the on-station phases.
During the transfer orbit phases, the SOC controls the ACTS through
the C-band ground stations. During the on-station phase, command
parameters generated at the SOC are routed via landlines to Lewis to
be uplinked to the ACTS via Ka-band. Status information is displayed
at the Lewis ACTS master ground station for both the transfer orbit
and on-station phases.
ACTS Ground Segment
The ACTS ground segment is comprised of the ACTS master ground
station, the satellite operations center and the experimenter
terminals.
ACTS Master Ground Station
The ACTS master ground station is located at the NASA Lewis
Research Center. It includes:
* The NASA ground station (NGS), which consists of a Ka-band
radio frequency terminal, two traffic terminals and a reference
terminal. It up-converts signals for the baseband processor
mode of perations to 30 GHz for transmission to ACTS and
amplifies and down-converts the 20 GHz baseband processor
modulated signals received from ACTS. Modulation and
demodulation of the baseband communications signals are
performed in the NASA ground station. It also transmits and
receives signals in support of the command, ranging and
telemetry functions for ACTS.
* The master control station provides network control for the
spacecraft baseband processor and backup to the satellite
operations center for configuring the multibeam communications
package. The master control station also enables experiment
execution and telemetry collection.
* The microwave switch matrix-link evaluation terminal provides
the capability for the on-orbit testing of the microwave switch
matrix and the multibeam antenna. It also will conduct
wideband communications experiments.
* The command, ranging and telemetry equipment interfaces with
theNASA ground station at intermediate frequency and exchanges
command, ranging and telemetry information to and from the
master control station, the G.E. SOC and the microwave switch
matrix-link evaluation terminal.
The SOC has primary responsibility for generating flight system
commands and for analyzing, processing and displaying flight system
telemetry data. Orbital maneuver planning and execution also are
handled by the SOC. The primary housekeeping function is performed
at the SOC which is linked via land line to the Ka-band command,
ranging and telemetry equipment at the ACTS master control station.
The Ka-band experimenter network consists of a variety of ground
stations to be operated by industry, universities and government
organizations. These ground stations have varying communication
services ranging from High Data Rate (HDR) at 1 gigabit per second,
to Very Small Aperture Terminal (VSAT) at 1.5 megabits per second,
aeronautical and ground mobile voice and data at 500 kilabits per
second and Ultra Small Aperture Terminal (USAT) data at 4800 bits per
second.
Source:NASA Spacelink Modem:205-895-0028 Internet:192.149.89.61
=--=--=-END-=--=--=
=--=--=START=--=--= NASA Spacelink File Name:6_2_2_46_3_7.TXT
ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE (ACTS) HARDWARE
The Advanced Communications Technology Satellite (ACTS) provides
for the development and flight test of high-risk, advanced
communications satellite technology. Using advanced antenna beams
and advanced on-board switching and processing systems, ACTS will
pioneer new initiatives in communications satellite technology.
ACTS provides new communications satellite technology for:
* Operating in the Ka-band (30/20 GHz) where there is 2.5 GHz of
spectrum available (five times that available at lower frequency
bands)
* Very high-gain, multiple hopping beam antenna systems which
permit smaller aperture Earth stations
* On-board baseband switching which permits interconnectivity
between users at an individual circuit level
* A microwave switch matrix which enables gigabit per second
communication between users.
These technologies provide for up to three times the
communications capacity for the same weight as today's satellites
(more cost effective), much higher rate communications between users
(20 times that offered by conventional satellites), greater
networking flexibility and on-demand digital services not currently
available from communications systems today. The development and
flight validation of this advanced space communications technology by
NASA's ACTS will allow industry to adapt this technology to their
individual commercial requirements at minimal risk. It also will aid
the U.S. industry in competing with European and Asian companies
which have, in the last decade, developed significant capabilities
for producing communications satellites and associated ground
equipment.
ACTS technologies, which are applicable for a variety of frequency
bands, will potentially lower the cost or technical threshold so that
such new services as remote medical image diagnostics, global
personal communications, real-time TV transmissions to airliners,
direct transmission of reconnaissance image data to battlefield
commanders and interconnection of supercomputers will be feasible.
Technology spin-off is already occurring.
Motorola currently is adapting the ACTS Ka-band and on-board
switching technologies for their $3 billion Iridium satellite system,
which will provide global voice/data communications services. Norris
Communications also is proceeding with a Ka spot-beam communications
satellite.
Source:NASA Spacelink Modem:205-895-0028 Internet:192.149.89.61
=--=--=-END-=--=--=