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.
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:
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 (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.
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.
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.
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.