Typical Onboard Subsystems--2
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SECTION II. SPACE FLIGHT PROJECTS (Cont'd)
Chapter 11. Typical Onboard Subsystems (Cont'd)
Attitude and Articulation Control Subsystems
A spacecraft's attitude, its orientation in space, must be stabilized and
controlled so that its high-gain antenna may be accurately pointed to Earth, so
that onboard experiments may accomplish precise pointing for accurate
collection and subsequent interpretation of data, so that the heating and
cooling effects of sunlight and shadow may be used intelligently for thermal
control, and so that propulsive maneuvers may be executed in the right
direction.
Stabilization can be accomplished by setting the vehicle spinning, as do the
Pioneers 10 and 11 spacecraft in the outer solar system, and the Galileo
spacecraft orbiting
Jupiter. The gyroscopic action of the rotating
spacecraft mass is the stabilizing mechanism. Propulsion-system thrusters are
fired to make desired changes in the spin-stabilized attitude.
Alternatively, the spacecraft may be designed for active three-axis
stabilization. One method is to use small propulsion-system thrusters to nudge
the spacecraft back and forth within a deadband of allowed attitude error.
Voyagers 1 and 2 have been doing that since 1977. Another method is to use
electrically-powered reaction wheels, also called momentum wheels. Massive
wheels are mounted in three orthogonal axes aboard the spacecraft. To rotate
the vehicle in one direction, you spin up the proper wheel in the opposite
direction. To rotate the vehicle back, you slow down the wheel. Excess
momentum which builds up in the system, due to internal friction and external
forces, must be occasionally removed from the system via propulsive
maneuvers.
There are advantages and disadvantages to either approach. Spin-stabilized
craft provide a continuous sweeping desirable for fields and particles
instruments, but they may require complicated systems to de-spin antennas or
optical instruments which must be pointed at targets. Three-axis controlled
craft can point optical instruments and antennas without having to de-spin
them, but they may have to carry out rotation maneuvers to best utilize their
fields and particle instruments.
The attitude and articulation control subsystem (AACS) computer manages the
tasks involved in stabilization via its interface equipment. For attitude
reference, star trackers, star scanners, solar trackers, sun sensors, and
planetary limb trackers come into use. Voyager's AACS uses a sun sensor for
yaw and pitch reference, and a star tracker trained continuously on a bright
star at right angles to sunpoint for roll reference. Galileo takes its
references from a star scanner which rotates with the spinning part of the
spacecraft, and a sun gate is available for use in maneuvers. Magellan used a
star scanner to take a fix on two bright stars during a special maneuver once
every orbit or two, and its solar panels each had a sun sensor.
Gyroscopes are carried for attitude reference for those periods when celestial
references are not being used. For some spacecraft, such as Magellan, this is
the case nearly continuously, since celestial references are used only during
star scan maneuvers once every orbit or two. Other spacecraft are designed to
use celestial reference nearly continuously, and they rely on gyroscopes for
their attitude reference only during relatively short maneuvers when celestial
reference is lost. In either case, gyro data must be taken with a grain of
salt; today's gyroscopes are mechanical, so they precess and drift due to
internal friction. Great pains are taken to calibrate their rates of drift, so
that the AACS may compensate for it when it computes its attitude
knowledge.
AACS also controls the articulation of a spacecraft's moveable appendages such
as solar panels, high-gain antennas, de-spun components, or optical instrument
scan platforms. The AACS is a likely candidate for doing this because it keeps
track of the spacecraft's attitude, the sun's and Earth's locations, and it
can compute the direction to point the appendages.
Telecommunications Subsystems
This section deals specifically with telecommunications equipment on board a
spacecraft. A broader view of the whole telecommunications system, including
Earth-based components may be found in Chapter 10.
Telecommunications subsystem components are chosen for a particular spacecraft
in response to the requirements of the mission profile. Anticipated maximum
distances, planned frequency bands, data rates and available on-board
transmitter power are all taken into account. Each of the components of this
subsystem is discussed below:
High-Gain Antennas
Dish-shaped high-gain antennas (HGAs) are the spacecraft antennas principally
used for communications with Earth. The amount of gain achieved by an antenna
(indicated in this workbook as high, low, or medium) refers to the amount of
incoming radio power it can collect and focus into the spacecraft's receiving
subsystems. In the frequency ranges used by spacecraft, this means that HGAs
incorporate large paraboloidal reflectors. The cassegrain arrangement,
described in Chapter 6, is the HGA configuration used most frequently aboard
interplanetary spacecraft. Ulysses, which uses a prime focus feed, is one
exception.
HGAs may be either steerable or fixed to the spacecraft bus. The Magellan HGA,
which also served as a radar antenna for mapping (and as a drogue for
aerobraking), was not articulated; the whole spacecraft had to be maneuvered to
point the HGA to Earth for communications. Magellan's HGA, by the way, also
served as a fine sunshade. Mission ops people routinely pointed it to the sun in
order to provide some needed shade for the rest of the spacecraft.
The Mars Global Surveyor HGA is on an articulated arm to allow the antenna to
maintain Earth-point independent of the spacecraft's attitude while it maps the
surface of Mars. Galileo's HGA was designed to unfold like an umbrella after
launch. This enabled the use of a larger diameter antenna than would have fit in
the Space Shuttle cargo bay if a fixed antenna had been chosen. However, the project has been unable to fully deploy the antenna, thus
severely limiting communications with the spacecraft. Efforts to overcome this
problem have not met with success, and the project is planning to carry out the
mission using Galileo's low-gain antennas constrained to low data rates.
Now on-board software and improvements in the DSN will permit recovery of 70% of the originally
planned science data.
The larger the collecting area of an HGA, the higher the gain, and the higher
the data rate it will support. The higher the gain, the more highly
directional it is. When using an HGA, it must be pointed to within a fraction
of a degree of Earth for communications to be feasible. Once this is achieved,
communications may take place at a high rate over the highly focused radio
signal. This is analogous to using a telescope, which provides magnification
(gain) of a weak light source, but it requires accurate pointing. No
magnification is achieved with the naked eye, but it covers a very wide field
of view, and need not be pointed with great accuracy to detect a source of
light, as long as it is bright enough. In case AACS fails to be able to point
a spacecraft's HGA with high accuracy for one reason or another, there must be
some other means of communicating with the spacecraft.
Low-gain Antennas
Low-gain antennas (LGAs) provide wide-angle coverage (the "naked-eye," to
continue the analogy) at the expense of gain. Coverage is nearly
omnidirectional, except for areas which may be shadowed by the spacecraft body.
LGAs are designed to be useable for relatively low data rates, as long as the
spacecraft is within relatively close range, several AU for example, and the
DSN transmitter is powerful enough. Magellan could use its LGA at Venus's
distance, but Voyager must depend on its HGA since it is over 40 AU away.
Some
LGAs are mounted atop the HGA's subreflector, as in the following diagram.
This is the case with Voyager, Magellan, and Galileo. A second
LGA, designated LGA-2, was added to the Galileo spacecraft in the redesign
which included an inner-solar system gravity assist. LGA-2 faces aft,
providing Galileo with fully omnidirectional coverage by accommodating LGA-
1's
blind spots.
Medium-gain Antennas
MGAs are a compromise, providing more gain than an LGA, with wider angles
of
pointing coverage than an HGA, on the order of 20 or 30 degrees. Magellan
carried an MGA consisting of a large cone-shaped feed horn, which was used
during some maneuvers when the HGA was off Earth-point.
Spacecraft Transmitters
A transmitter is an electronic device which generates a tone at a single
designated radio frequency, typically in the S-band (~2 GHz) or X-band (~5 GHz)
range. This tone is called the carrier. The carrier can be sent from the
spacecraft to Earth as it is, or it can be modulated with a data-carrying
subcarrier within the transmitter. The signal generated by the spacecraft
transmitter is passed to a power amplifier, where its power is boosted to the
neighborhood of tens of watts. This microwave-band power amplifier may be a
solid state amplifier (SSA) or a traveling wave tube (TWT, also TWTA,
pronounced "tweeta," for TWT Amplifier). A TWTA uses the interaction
between
the field of a wave propagated along a waveguide, and a beam of electrons
traveling along with the wave. The electrons tend to travel slightly faster
than the wave, and on the average are slowed slightly by the wave. The effect
amplifies the wave's total energy.
The output of the power amplifier is ducted through waveguides and
commandable
waveguide switches to the antenna of choice: HGA, MGA, or LGA.
Spacecraft Receivers
Commandable waveguide switches are also used to connect the antenna of choice
to a receiver. The receiver is an electronic device which is sensitive to a
narrow band of frequency, generally a width of plus and minus a few kHz of a
single frequency selected during mission design. Once an uplink is detected
within its bandwidth, the receiver's phase-lock-loop circuitry (PLL) will
follow any changes in the uplink's frequency within its bandwidth. JPL
invented PLL technology in the early 1960s, which has since become standard in
the telecommunications industry. The receiver can provide the transmitter with
a frequency reference keyed to the received uplink. The received uplink, once
detected, locked onto, and stepped down in frequency, is stripped of its
command-data-carrying subcarrier, which is passed to circuitry called a
command
detector unit (CDU). This unit converts the analog phase-shifts which were
modulated onto the uplink's subcarrier into binary 1s and 0s, which are then
typically passed to the spacecraft's CDS.
Frequently, transmitters and receivers are combined into one electronic device
which is called a transponder.
Recap
- When using an ___ ___ ___ , it must be pointed to within a fraction of a
degree of Earth.
- LGA coverage is nearly _____________________________ , except for
areas
shadowed by the spacecraft body.
- The output of the ___________________ ___________________ is ducted
through waveguides and commandable waveguide switches to the antenna.
- The receiver's _______________ - _____________ - ________________
circuitry
will follow any changes in the uplink's frequency.
- HGA
- omnidirectional
- ower amplifier
- phase-lock-loop