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ACTS Solar Array and Battery Performance
Don R. Hilderman
ACTS Experiments Office
Space Communications Office
The Advanced Communications Technology Satellite (ACTS) is
a NASA experimental satellite communication system designed
to demonstrate on orbit Ka-band communications and switching
technologies that will be used by NASA and the commercial
sector in the 21st century.
To conduct the required experimental operations, six satellite
subsystems are used aboard the spacecraft to supply communications
and maintain a healthy and stable satellite platform in geosynchronous
orbit. The six subsystems are the thermal subsystem, attitude
control subsystem (ACS), multibeam communications package
(MCP), command, ranging and telemetry (CR&T), reaction
control subsystem, and the power subsystem.
The challenge to all satellite manufacturers is to design
and build satellites that will reliably operate in the harsh
space environment for many years. During the five year ACTS
experiments program, spacecraft engineers from Lockheed Martin
Astro Space have successfully maintained and operated all
ACTS subsystems. This article concentrates on the performance
of the solar arrays and batteries used in the ACTS power subsystem
during the past five years on orbit.
The ACTS solar array contains four planar solar panels, two
on each side of the spacecraft for a total area of 144 square
feet. High efficiency N-on-P, 2 3 4 cm solar cells are bonded
to a Kevlar facesheet which is bonded to 1 in. thick aluminum
honeycomb core panel. The solar cells are electrically connected
to each other with custom cut silver mesh and wire. Solar
array electrical performance data was collected after five
years of on orbit experiments and compared with performance
data after launch. Available power output of the solar array
was measured to be 1842 Watts beginning-of-life in September
1993. Spacecraft telemetry measurements as of December 1998
indicate that total solar array output power measured approximately
1650 watts. This represents a ten percent drop in available
solar power and is considered normal, acceptable, and within
design margins. Solar array power reduction
during the past five years is due to solar cell and solar
cell cover glass degradation caused by exposure to charged
particles and radiation in geosynchroneous orbit. ACTS uses
approximately 1100 Watts of electrical power to operate all
six electrical subsystems. Most of the excess 550 Watts (1650
W- 1100 W) that was generated by the solar arrays and not
used by the six electrical subsystems needs to be reduced
or shed as excess heat. One method used is through a bank
of shunt resistors in the thermal subsystem that serve to
dissipate and remove excess electrical power generated from
the spacecraft solar arrays.
The primary method used to reduce excess electrical power
generated by the solar arrays is to tilt the solar array panels
slightly away from the sun. This reduces the solar energy
collected and electrical energy generated. ACTS solar arrays
have been canted or turned approximately 40 degrees away from
the sun during the entire experiments program to help reduce
the excess power. Not all of the 550 Watts is dissipated in
the shunts or reduced by tilting the solar arrays. Approximately
50 Watts is held in reserve to provide margin for transient
loads within the six electrical subsystems while maintaining
35.5 Volts across the two batteries.
The primary purpose of the ACTS batteries is to provide electrical
power to the six electrical subsystems during solar eclipse
periods. ACTS contains two independently charged, 19 ampere-hour
nickel-cadmium batteries. Each battery contains 22 independent
cells connected in series for a nominal voltage of 33 Volts
per battery. The two batteries are connected in parallel for
a total battery capacity of 38 Ampere-hours. During 100% sunlight
periods, the batteries are trickle-charged at a constant C/60
rate (0.32 Ampere per battery). The letter "C" represents
nameplate capacity for one battery of 19 Ampere-hours. Immediately
after eclipse, each battery is charged at a constant rate
of C/20 (0.95 Amperes) for a period required to reach full
charge.
The batteries have been working so well over the past five
years that battery reconditioning is considered unnecessary
by spacecraft controllers. The purpose for reconditioning
each battery is to remove any "memory effect" on
the battery cells resulting from minimum use during 4.5 months
of continuous sunlight and charging between eclipse seasons.
Battery "memory effect" is the tendency for the
battery voltage not to provide the original power capacity
during charging. Each battery has a reconditioning circuit
to execute a deep discharge/rapid recharge on each cell in
a battery shortly before each eclipse season. Battery "memory
effect" has not been observed during the past five years
resulting in no reconditioning.
The excellent ACTS battery performance to date is due in
part to the spacecraft controllers at Lockheed Martin Astro
Space that manage all command and telemetry operations during
the past five years of experiments. Engineers have successfully
controlled the spacecraft temperatures, loads and discharge
rates prior to, during and after eclipse to reduce potential
battery stress. Spacecraft battery depth of discharge during
eclipse periods has not exceeded 38% with a battery voltage
no lower than 25.8 Volts and a temperature no greater than
23 °C. Overstressed batteries occur when depth of discharge
exceeds 50 percent, battery voltage falls below 24 Volts or
temperature rises above 25 °C. The voltage difference between
the highest battery cell and the lowest cell (cell divergence)
has never exceeded 0.039 Volts. Battery reconditioning is
recommended when cell divergence exceeds 0.05 Volts.
ACTS power subsystem battery and solar array performance
are forecasted to provide reliable and successful experiment
operations through the scheduled conclusion of ACTS in September
2000.
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