Astrionics is the science and technology of the development and application of electronic systems, sub-systems, and components used in
spacecraft. The electronic systems on board a spacecraft include attitude determination and control, communications, command and telemetry, and computer systems. Sensors refers to the electronic components on board a spacecraft.
For engineers one of the most important considerations that must be made in the design process is the environment in which the spacecraft systems and components must operate and endure. The challenges of designing systems and components for the
space environmentinclude more than the fact that space is a vacuum.
Attitude Determination and Control
One of the most vital roles electronics and sensors play in a mission and performance of a spacecraft is to determine and control its attitude, or how it is orientated in space. The orientation of a spacecraft varies depending on the mission. The spacecraft may need to be stationary and always pointed at Earth, which is the case for a weather or communications satellite. However, there may also be the need to fix the spacecraft about an axis and the have it spin. The attitude determination and control system, ACS, ensures the spacecraft is behaving correctly. Below are several ways in which ACS can obtain the necessary measurements to determine this.
This device measures the strength of the
Earth's magnetic fieldin one direction. For measurements on 3-axis the device would consist of three orthogonal magnetometers. Given the spacecraft's position, the magnetic field measurements can be compared to a known magnetic field which is given by the International Geomagnetic Reference Field model. Measurements made by magnetometers are affected by noise consisting of alignment error, scale factor errors, and spacecraft electrical activity. For near Earth orbits, the error in the modeled field direction may vary from 0.5 degrees near the Equator to 3 degrees near the magnetic poles, where erratic auroral currents play a large role [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p258] . The limitation of such a device is that in orbits far from Earth, the magnetic field is too weak and is actually dominated by the interplanetary field which is complicated and unpredictable.
This device works on the light entering a thin slit on top of a rectangular chamber that casts an image of a thin line on the bottom of the chamber, which is lined with a network of light-sensitive cells. These cells measure the distance of the image from a centerline and using the height of the chamber can determine the angle of refraction. The cells operate based on the
photoelectric effect. Incoming photons are converted to electrons and therefore voltages, which are in turn converted into a digital signal. By placing two sensors perpendicular to each other the complete direction of the sun with respect to the sensor axes can be measured.
Digital Solar Aspect Detectors
Also known as DSADs, these devices are purely digital Sun sensors. They determine the angles of the Sun by determining which of the light-sensitive cells in the sensor is the most strongly illuminated. By knowing the intensity of light striking neighboring pixels, the direction of the centroid of the sun can be calculated to within a few arcseconds [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p261] .
A light-sensitive surface detects incoming light from a star and determines the angles between the incoming light and the spacecraft axis. Star sensors have come a long way. They used to be made of Cathode Ray Tubes and photomultipliers to measure the point of impact. There were several disadvantages to this, which were the large amounts of power and the CRTs generated large magnetic fields with sometimes interfered with other onboard devices. Today, star sensors use charged-coupled devices or CCDs. These have much smaller power requirements which pull less current and therefore smaller magnetic disturbances. Up until the past 5 years, the attitude determination from star data was left to spacecraft onboard computer, or ground processing [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p264] . Since processing capabilities have increased, star trackers are now able to track as many as 30 stars at once and process this data using a star catalog and algorithm to calculate the attitude. These sensors are typically only used on three-axis stabilized spacecraft, because they do not function accurately at angular velocities above a few degrees per second.
Earth Horizon Sensor
Static Earth horizon sensors contain a number of sensors and sense
infrared radiationfrom the Earth’s surface with a field of view slightly larger than the Earth. The accuracy of determining the geocenter is 0.1 degrees in near-Earth orbit to 0.01 degrees at GEO. Their use is generally restricted to spacecraft with a circular orbit [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p262] .
Scanning Earth horizon sensors use a spinning mirror or prism and focus a narrow beam of light onto a sensing element usually called a
bolometer. The spinning causes the device to sweep out the area of a cone and electronics inside the sensor detect when the infrared signal from Earth is first received and then lost. The time between is used to determine Earth’s width. From this the roll angle can be determined. A factor that plays into the accuracy of such sensors is the fact the Earth is not perfectly circular. Another is that the sensor does not detect land or ocean, but infrared in the atmosphere which can reach certain intensities due to the season and latitude.
This sensor is simple in that using one signal many characteristics can be determined. A signal carries satellite identification, position, the duration of the propagated signal and clock information [Abid, Mohamed M. Spacecraft Sensors. West Sussex, John Wiley and Sons Ltd., 2005, p301] . Using a constellation of 36 GPS satellites, of which only 4 are needed, navigation, positioning, precise time, orbit, and attitude can be determined. One advantage of GPS is all orbits from
Low Earth orbitto Geosynchronous orbitcan use GPSfor ACS.
Command and Telemetry
Another system which is vital to a spacecraft is the command and telemetry system, so much in fact, that it is the first system to be redundant. The communication from the ground to the spacecraft is the responsibility of the command system. The telemetry system handles communications from the spacecraft to the ground. Signals from ground stations are sent to command the spacecraft what to do, while telemetry reports back on the status of those commands including spacecraft vitals and mission specific data.
The purpose of a command system is to give the spacecraft a set of instructions to perform. Commands for a spacecraft are executed based on priority. Some commands require immediate execution; other may specify particular delay times that must elapse prior to their execution, an absolute time at which the command must be executed, or an event or combination of events that must occur before the command is executed [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p600] . Spacecraft perform a range of functions based on the command they receive. These include: power to be applied to or removed from a spacecraft subsystem or experiment, alter operating modes of the subsystem, and control various functions of the spacecraft guidance and ACS. Commands also control booms, antennas, solar cell arrays, and protective covers. A command system may also be used to upload entire programs into the RAM of programmable, micro-processor based, onboard subsystems [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p601] .
The radio-frequency signal that is transmitted from the ground is received by the command receiver and is amplified and demodulated. Amplification is necessary because the signal is very weak after traveling the long distance. Next in the command system is the command decoder. This device examines the subcarrier signal and detects the command message that it is carrying. The output for the decoder is normally non-return-to-zero data. The command decoder also provides clock information to the command logic and this tells the command logic when a bit is valid on the serial data line. The command bit stream that is sent to the command processor has a unique feature for spacecraft. Among the different types of bits sent, the first are spacecraft address bits. These carry a specific identification code for a particular spacecraft and prevent the intended command from being performed by another spacecraft. This is necessary due to the fact there are many satellites using the same frequency and modulation type [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p606] .
The microprocessor receives inputs from the command decoder, operates on these inputs in accordance with a program that is stored in ROM or RAM, and then outputs the results to the interface circuitry. Because there is such a wide variety of command types and messages, most command systems are implemented using programmable micro-processors. The type of interface circuitry needed is based on the command sent by the processor. These commands include relay, pulse, level, and data commands. Relay commands activate the coils of electromagnetic relays in the central power switching unit. Pulse commands are short pulses of voltage or current that is sent by the command logic to the appropriate subsystem. A level command is exactly like a logic pulse command except that a logic level is delivered instead of a logic pulse. Data commands transfer data words to the destination subsystem [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p612-615] .
Commands to a spacecraft would be useless if ground control did not know what the spacecraft was doing. Types of information telemetry includes is: The status data concerning spacecraft resources, health, attitude and mode of operation. Scientific data gathered by onboard sensors: telescopes, spectrometers, magnetometers, accelerometers, electrometers, thermometers, etc.. Specific spacecraft orbit and timing data that may be sued for guidance and navigation by ground, sea, or air vehicles. Images captured by onboard cameras (visible or infrared). Locations of other objects, either on the Earth or in space, that are being tracked by the spacecraft. Telemetry data that have been relayed from the ground or from another spacecraft in a satellite constellation [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p617] . The telemetry system is responsible for acquisition from the sensors, conditioners, selectors, and/or converters, processing, including compression, format, and storage, and finally transmission, which includes encoding, modulating, transmitting and the antenna.
There are several unique features of the telemetry system design for spacecraft. One of these is the approach to the fact that for any given satellite in LEO, because it is traveling so fast, it may only be in contact with a particular station for ten to twenty minutes. This would require hundreds of ground stations to stay in constant communication which is not at all practical. One solution to this is onboard
data storage. Data storage can accumulate data slowly throughout the orbit and dump it quickly when over a ground station. In deep space missions the recorder is often used the opposite way, to capture high-rate data and play it back slowly over data-rate-limited links [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p567] . Another solution is data relay satellites. NASAhas satellites in GEO called TDRS, Tracking and Data Relay Satellites, which relay commands and telemetry from LEO satellites. Prior to TDRS, astronauts could communicate with the Earth for only about 15% of the orbit, using 14 NASA ground stations around the world. With TDRS, coverage of low-altitude satellites is global, from a single ground station at White Sands, New Mexico[Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p569] .
Another unique feature of telemetry systems is autonomy. Spacecraft require the ability to monitor their internal functions and act on information without ground control interaction. The need for autonomy originates from problems such as insufficient ground coverage, communication geometry, too near the Earth-Sun line, where solar noise interferes with radio frequencies, or simply for security purposes. The reason autonomy is important when talking about command and telemetry systems is that the telemetry system already has the capability to monitor the spacecraft functions and the command systems has the ability to give the necessary commands to reconfigure based on the needs of the action to be taken. There are three steps to this process:
1. The telemetry system must be able to recognize when one of the functions it’s monitoring deviates beyond the normal ranges.
2. The command system must know how to interpret abnormal functions, so that it can generate a proper command response.
3. The command and telemetry systems must be capable of communicating with each other. [Pisacane, Vincent L. "Fundamentals of Space Systems". New York, Oxford University Press, 2005, p623] .
Sensors can be classified under two categories: health sensors and payload sensors. Health sensors monitor the spacecraft or payload functionality and can include temperature sensors, strain gauges, gyros and accelerometers. Payload sensors would include radar imaging systems and IR cameras. While payload sensors represent some of the reason the mission exists, it is the health sensors that measure and control systems to ensure optimum operation.
Wikimedia Foundation. 2010.
Look at other dictionaries:
astrionics — /as tree on iks/, n. (used with a sing. v.) Rocketry. the science dealing with the application of electronics to astronautics. [ASTR(O) + I + (ELECTR)ONICS] * * * … Universalium
astrionics — /as tree on iks/, n. (used with a sing. v.) Rocketry. the science dealing with the application of electronics to astronautics. [ASTR(O) + I + (ELECTR)ONICS] … Useful english dictionary
Saturn V Instrument Unit — The Saturn V Instrument Unit is a ring shaped structure fitted to the top of the Saturn V rocket s third stage (S IVB) and the Saturn IB s second stage (S IVB). It was immediately below the SLA (Spacecraft/Lunar Module Adapter) panels that… … Wikipedia
Spacecraft — Spaceship redirects here. For other uses, see Spaceship (disambiguation). More than 100 Russian Soyuz manned spacecraft (TMA version shown) have flown since 1967, originally for a Soviet manned lunar program, but currently supporting the… … Wikipedia
Saturn Launch Vehicle Digital Computer — The Saturn Launch Vehicle Digital Computer (LVDC) was one of the major components of the Instrument Unit fitted to the S IVB stage of the Saturn V and Saturn IB rockets. Its primary role was to provide an autopilot for the Saturn from launch to… … Wikipedia
Attitude control system — In spaceflight, the attitude control system (ACS) or attitude determination and control (ADAC) system of a spacecraft consists of equipment to measure, report and change the orientation of the vehicle. [cite web… … Wikipedia
List of Sherman Fairchild companies — Overlooked by historians, the press and the public were the vast holdings of Sherman Mills Fairchild. His vast stock, real estate and other private holding may never be fully known. Fortunately a former Fairchild employee, Theron Rinehart,… … Wikipedia
AZUSA — refers to a ground based radar tracking system installed at Cape Canaveral, Florida and the NASA Kennedy Space Center. AZUSA dated back to the early 1950s and was named after the southern California town (Azusa, California) where the system was… … Wikipedia
ST-124-M3 inertial platform — The ST 124 M3 is a device for measuring acceleration and attitude of the Saturn V launch vehicle. It was carried by the Saturn V Instrument Unit, a 3 ft high by 22 foot diameter section of the Saturn V that fit between the third stage (S IVB) and … Wikipedia
ODOP — Doppler radar tracking system functional block diagram. The ODOP (Offset DOPpler) radar tracking system is essentially the same as the UDOP system used for many years at the Atlantic Missile Range, but ODOP operates at different frequencies. It… … Wikipedia