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Satellites circling the Earth demand
precision pointing of sensors, antennas, cameras, and other equipment.
Without highly accurate angular positioning of spacecraft, a
range of dutiful services would not be possible.
An innovative approach to Earth sensor design has been taken
by Servo Corporation of America in Westbury, New York. Through
a Small Business Innovation Research (SBIR) award from Goddard
Space Flight Center, the company built its Mini-Dual Earth Sensor
(MiDES). NASA's support for the work stemmed from interest in
evaluating a satellite autonavigation system.
The MiDES is an Earth horizon sensor that provides higher
accuracy than other types of units, making use of pyroelectric
arrays and a digital output. Servo's lightweight, low-cost, dual
array horizon sensor was flown in space, confirming its abilities.
MiDES is now being eyed for use on a number of commercial telecommunications
satellite constellations and other spacecraft to be built both
in the U.S. and abroad.
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Servo Corporation of America's
MiDES is a dual array Earth horizon sensor that provides high
accuracy for low-Earth orbiting satellites.
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A state-of-the-art Earth horizon sensor, MiDES combines the
versatility of a conical scanner with the accuracy and reliability
of a staring sensor. The resulting instrument is a lightweight,
low-cost, low-power alternative for low-Earth orbiting satellites.
Designed specifically to provide horizon attitude position for
Earth circling satellites, the MiDES derives its information
through the use of two pairs of pyroelectric arrays positioned
90 degrees apart in the imaging plane. Each 16-element array
is spatially separated into two 8-element staggered columns that
reject and compensate for light emitted by the Sun and reflected
by the Moon. The device detects temperature differences between
the Earth and space. An algorithm calculates the position of
the horizon based on the voltages obtained from the pixels, which
subtend the horizon gradient and the pixels that look at space
and Earth.
The heart of the MiDES is the hybridized lithium tantalate
pyroelectric array. Created by Servo, these lithium tantalate
detectors are specially processed, highly sensitive, uncooled
detectors that provide very stable outputs over a variety of
temperatures and operating conditions. The need for moving parts
has been dramatically reduced to near zero. A flex pivot-mounted
chopper design eliminates the need for scanning motors, bearings,
and lubricants.
MiDES was used on the STS-85 flight of the Space Shuttle Discovery,
carried by the Cryogenic Infrared Spectrometers and Telescopes
for the Atmosphere (CRISTA)-Shuttle Pallet Satellite II (SPAS).
The CRISTA-SPAS was a joint satellite project between NASA and
Germany. Once the satellite was deployed from the Shuttle, it
flew for 11 days. Several gigabytes of flight data on how MiDES
operated were obtained. These data showed that the MiDES accuracy
was significantly better than expected, keeping attitude errors
well under 0.1 degrees.
Sales of MiDES have begun, with delivery of a unit for flight
on a minisatellite program sponsored by the United Kingdom. MiDES
is a candidate for several constellates of low-Earth orbiting
telecommunications satellites. A variation of the basic MiDES
has also been proposed for use aboard several new geosynchronous
satellites being designed.
It seems that MiDES has proven itself capable of locking onto
new business, as well as providing a solid lock on the Earth's
horizon.
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