Space Navigation, Guidance and Control
The navigation system of a spacecraft determines both the position and the velocity of the vehicle. It may make these determinations by means of an internal inertial guidance system, radio signals transmitted from the earth or from the vehicle itself, or optical observations of the earth, moon, or stars.
When these data have been obtained, the guidance system analyzes them and determines what changes, if any, are necessary to bring the vehicle back to the desired (correct) course. It then transmits signals to the control system to make the necessary corrections. The corrections are usually made by means of rockets that, in accordance with the guidance system's commands, are fired in the appropriate direction for the correct length of time.
During an interplanetary space flight it is necessary to use different types of navigation references and methods of flight correction. During the early part, called the launch phase, of such a flight the craft's position may be measured relative to the earth. In this phase the forces needed to correct the vehicle's course are usually great, requiring large rocket engines. During the mid-course phase, the position of the spacecraft may be measured relative to a star or planet. Only a few minor correction maneuvers should be necessary, for which small rocket engines are adequate. In the terminal, or final phase, the vehicle's position is measured relative to the planet on which it will land. From this summary it can be seen that the necessary navigation and guidance equipment and control devices are different for each flight phase. The various items of equipment, moreover, are often mounted on different stages of the vehicle.
For a simple flight trajectory, such as that of a high-altitude sounding rocket (for measuring atmospheric density or temperature), only a simple guidance system is necessary. It is usually based on a timing device that closes and opens circuits at preset intervals, thus providing commands to turn devices on or off, to fire rocket engines, and so on. In more complex guidance systems, there is a central computer that contains stored information on factors influencing the vehicle's trajectory, and on correction maneuvers. It also contains logic circuits and complicated built-in mathematical flight programs and event sequences. The computer may be entirely in the vehicle, entirely on the ground and linked by radio to the vehicle, or partly on the ground and partly in the vehicle. The guidance system and its computer are designed to keep the vehicle on its intended flight trajectory and to maintain the correct attitude, or rotational position. Precise maintenance of attitude is important for many reasons. It makes possible accurate pointing of a built-in telescope or directional antenna, and ensures that solar cell panels face the sun so that the maximum possible solar radiation may be captured. It also minimizes heating of cryogenic liquid propellants by keeping the tanks from being exposed to sunlight, and ensures that the vehicle is accurately aimed before any maneuver with a principal rocket engine is performed.
The heart of an inertial guidance system is a small plate, called an inertial platform. This is mounted in gimbal rings and maintained in a constant, level position by gyroscopes, so that when the spacecraft changes direction or attitude, the platform remains stable. Mounted on the platform are devices called accelerometers, which measure changes in the spacecraft's velocity and position 'with respect to a reference point, such as the place from which the craft was launched or a tracking control center. Adding up the changes permits charting the course of the vehicle from within the vehicle, using no other external reference than the initial point of departure.
Radio guidance systems depend on radio signals from the spacecraft to the earth or to another spacecraft. The spacecraft's velocity and location may be determined from the direction of the returning reflected radio signal and from the time that elapses between transmission and reception.
Stability and Control
As noted above, there are many forces against which controls must act to stabilize the spacecraft. Such forces include motion within the vehicle itself, such as the sloshing of liquid propellants in tanks or movements of the crew. The controls also act against external forces, such as winds near the earth's surface and solar pressure in space. When an astronaut moves or turns around in a freely floating spacecraft, the spacecraft itself is caused to move or turn slightly in the opposite direction. For this reason, at the time of precise turning maneuvers, such as the accurate pointing of a telescope to a star, crew members must sit still and mechanisms such as fuel-transfer pumps must be turned off. External motions such as the rotation of an antenna or solar-cell panel also affect the spacecraft's stability and must be avoided during precise maneuvers.
Attitude-control rockets are used to counteract such effects and to turn the vehicle and orient it in the right direction. They are usually a series of small rockets of a few pounds' thrust; they are fixed to the spacecraft frame and operate in pairs to give a true torque to the vehicle. Control signals are also given to the larger main propulsion rockets during their operation. In this way the attitude of the spacecraft can be changed slightly.
Communication between the earth and a spacecraft in flight is obviously essential. Radio signals from unmanned vehicles inform ground observers of the craft's position and also transmit the results of any scientific measurements or observations being made aboard the vehicle. Radio signals from the ground to an unmanned vehicle are used to control the craft and any measuring devices or cameras it carries. Communication with both manned and unmanned spacecraft is usually carried out over one or more channels, or radio links.
The automatic transmission of data from spacecraft and launch vehicles is usually called telemetering. By means of a special code a telemetry system can simultaneously transmit information from several different measurements, such as altitude, pressure, temperature, stresses, and propellant flow rates.
Communication of data can be on either a real-time or a delay basis; that is, data on events can be either transmitted as the events take place, or recorded and transmitted at a later, more convenient, time. For example, delayed transmissions are used in cases where data can be communicated only while the vehicle is in range of a certain ground receiver antenna.
The power necessary for radio transmission increases with the distance between transmitter and receiver and with the rate at which data are relayed. A few watts is sufficient for a low-altitude earth-orbiting satellite with a teletype channel and a low rate of information flow. However, several hundred kilowatts may be required for clear, real-time television transmission from Mars.
During a spacecraft's reentry into the atmosphere a hot sheath of plasma, or ionized gases, is formed around its surface. Because this layer is electrically active, it interferes with the transmission of radio waves, and thereby some telemetering and other communications equipment is rendered useless. This reentry blackout, as it is called, is responsible for the loss of communication, lasting for less than a minute, during a spacecraft's reentry.
The use of precise equipment on the ground to locate a spacecraft and measure its trajectory in space is called tracking. Tracking may be performed by optical, radio, or radar methods. In optical tracking, the vehicle is observed by means of telescopes with rotary mountings, and in radar and radio tracking, by means of similarly mounted antennas. In each case the craft's position and course are determined from measurements of the azimuth and elevation angles of the rotating mount as the tracking instrument is following the vehicle. Optical tracking, using cameras and fast film, can provide a highly accurate fix, or determination of position, but this method is not as rapid as a radio or radar system. For optical tracking to be practicable, the vehicle must be visible, and thus must either be equipped with a powerful light or be designed to reflect sunlight. Radio tracking requires the vehicle to carry a continuously broadcasting radio beacon by means of which the ground antenna can determine direction. Radar tracking equipment must be very powerful, to ensure the reception of a reflected signal from the spacecraft. Radar has the advantage of being able to measure directly the distance between the vehicle and the radar antenna. From the measured angle-positions of the tracking instrument and from the time, known position of the tracking station, and speed of the earth's rotation, it is possible to calculate precisely the flight path of a space vehicle.
For deep-space tracking, very sensitive high-amplification, low-noise, movable radio antennas are necessary. Because the earth rotates, several tracking stations are required for continuous tracking of an interplanetary vehicle. The U.S. deep-space network has three tracking stations: one in Madrid, Spain; another near Canberra, Australia; and the third at Goldstone, California Because they can send and receive precise, narrow-angle radio signals, these stations are also used for communication with space vehicles. In addition, the United States has a manned-spacecraft network consisting of about 12 different stations for tracking, for vehicle control, and for communications with astronauts.
A central mission control center is used to control manned space nights on a worldwide basis. The center communicates with astronauts and with tracking and communication stations and monitors the functioning of all vehicle and ground equipment.