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64-402: In September 1994, after nearly five years of directed observations, the gas supply for its attitude control was exhausted and the observatory was placed in a non-directed survey mode. Transmissions finally ceased on 27 November 1998. With seven different instruments on board, Granat was designed to observe the universe at energies ranging from X-ray to gamma ray . Its main instrument, SIGMA,

128-408: A Be entrance window. The side surfaces were protected by a 5 mm thick lead layer. The burst detection threshold was 500 to 50 microjoules per square meter (5 × 10 to 5 × 10 erg/cm), depending on the burst spectrum and rise time . Spectra were taken in two 31-channel pulse height analyzers (PHAs), of which the first eight were measured with 1/16 s time resolution and

192-413: A 6° by 6° field of view. The visible detectors had a field of view of 5° by 5°. The instrument was designed to look for optical counterparts of high-energy burst sources, as well as performing spectral analysis of the high-energy events. Over the initial four years of directed observations, Granat observed many galactic and extra-galactic X-ray sources with emphasis on the deep imaging and spectroscopy of

256-566: A burst or transient event, count rates were accumulated with a time resolution of 1 second per 36 energy channels. The KONUS-B instrument, designed by the Ioffe Physico-Technical Institute in St. Petersburg , consisted of seven detectors distributed around the spacecraft that responded to photons of 10 keV to 8 MeV energy. They consisted of NaI (Tl) scintillator crystals 200 mm in diameter by 50 mm thick behind

320-633: A continuous sweeping motion that is desirable for fields and particles instruments, as well as some optical scanning instruments, but they may require complicated systems to de-spin antennas or optical instruments that must be pointed at targets for science observations or communications with Earth. Three-axis controlled craft can point optical instruments and antennas without having to de-spin them, but they may have to carry out special rotating maneuvers to best utilize their fields and particle instruments. If thrusters are used for routine stabilization, optical observations such as imaging must be designed knowing that

384-402: A ground station. The attitude control algorithms are written and implemented based on requirement for a particular attitude maneuver. Asides the implementation of passive attitude control such as the gravity-gradient stabilization , most spacecraft make use of active control which exhibits a typical attitude control loop. The design of the control algorithm depends on the actuator to be used for

448-440: A long-duration mission by producing control moments without fuel expenditure. For example, Mariner 10 adjusted its attitude using its solar cells and antennas as small solar sails. In orbit, a spacecraft with one axis much longer than the other two will spontaneously orient so that its long axis points at the planet's center of mass. This system has the virtue of needing no active control system or expenditure of fuel. The effect

512-491: A minimum of three reaction wheels must be used, with additional units providing single failure protection. See Euler angles . These are rotors spun at constant speed, mounted on gimbals to provide attitude control. Although a CMG provides control about the two axes orthogonal to the gyro spin axis, triaxial control still requires two units. A CMG is a bit more expensive in terms of cost and mass, because gimbals and their drive motors must be provided. The maximum torque (but not

576-426: A phenomenon known as Gimbal lock . A rotation matrix, on the other hand, provides a full description of the attitude at the expense of requiring nine values instead of three. The use of a rotation matrix can lead to increased computational expense and they can be more difficult to work with. Quaternions offer a decent compromise in that they do not suffer from gimbal lock and only require four values to fully describe

640-609: A total span of 8.5 m across its solar arrays . The power made available to the scientific instruments was approximately 400  W . The spacecraft was launched on 1 December 1989 aboard a Proton-K from the Baikonur Cosmodrome in Kazakh SSR . It was placed in a highly eccentric 98-hour orbit with an initial apogee / perigee of 202,480 km/1,760 km respectively and an inclination of 51.9 degrees. This meant that solar and lunar perturbations would significantly increase

704-511: Is a device that senses the direction to the Sun . This can be as simple as some solar cells and shades, or as complex as a steerable telescope , depending on mission requirements. An Earth sensor is a device that senses the direction to Earth . It is usually an infrared camera ; nowadays the main method to detect attitude is the star tracker , but Earth sensors are still integrated in satellites for their low cost and reliability. A star tracker

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768-471: Is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10 or 1 ⁄ 1,000,000 ) of a second . Its symbol is μs , sometimes simplified to us when Unicode is not available. A microsecond is to one second, as one second is to approximately 11.57 days. A microsecond is equal to 1000 nanoseconds or 1 ⁄ 1,000 of a millisecond . Because

832-498: Is aerodynamic stabilization. This is achieved using a drag gradient, as demonstrated on the Get Away Special Passive Attitude Control Satellite (GASPACS) technology demonstration. In low Earth orbit, the force due to drag is many orders of magnitude more dominant than the force imparted due to gravity gradients. When a satellite is utilizing aerodynamic passive attitude control, air molecules from

896-426: Is an optical device that measures the position(s) of star (s) using photocell (s) or a camera. It uses magnitude of brightness and spectral type to identify and then calculate the relative position of stars around it. A magnetometer is a device that senses magnetic field strength and, when used in a three-axis triad, magnetic field direction. As a spacecraft navigational aid, sensed field strength and direction

960-502: Is based on the measurement of the rate of change of body-fixed magnetometer signals. where m {\displaystyle m} is the commanded magnetic dipole moment of the magnetic torquer and K {\displaystyle K} is the proportional gain and B ˙ {\displaystyle {\dot {B}}} is the rate of change of the Earth's magnetic field. Spacecraft attitude determination

1024-485: Is called guidance, navigation and control , which also involves non-attitude concepts, such as position determination and navigation . A spacecraft's attitude must typically be stabilized and controlled for a variety of reasons. It is often needed so that the spacecraft high-gain antenna may be accurately pointed to Earth for communications, so that onboard experiments may accomplish precise pointing for accurate collection and subsequent interpretation of data, so that

1088-440: Is caused by a tidal force . The upper end of the vehicle feels less gravitational pull than the lower end. This provides a restoring torque whenever the long axis is not co-linear with the direction of gravity. Unless some means of damping is provided, the spacecraft will oscillate about the local vertical. Sometimes tethers are used to connect two parts of a satellite, to increase the stabilizing torque. A problem with such tethers

1152-401: Is compared to a map of Earth's magnetic field stored in the memory of an on-board or ground-based guidance computer. If spacecraft position is known then attitude can be inferred. Attitude cannot be measured directly by any single measurement, and so must be calculated (or estimated ) from a set of measurements (often using different sensors). This can be done either statically (calculating

1216-406: Is most common reacts to an error signal (deviation) based on attitude as follows where T c {\displaystyle T_{c}} is the control torque, e {\displaystyle e} is the attitude deviation signal, and K p , K i , K d {\displaystyle K_{\text{p}},K_{\text{i}},K_{\text{d}}} are

1280-483: Is placed in space. (For some applications such as in robotics and computer vision, it is customary to combine position and attitude together into a single description known as Pose .) Attitude can be described using a variety of methods; however, the most common are Rotation matrices , Quaternions , and Euler angles . While Euler angles are oftentimes the most straightforward representation to visualize, they can cause problems for highly-maneuverable systems because of

1344-418: Is that meteoroids as small as a grain of sand can part them. Coils or (on very small satellites) permanent magnets exert a moment against the local magnetic field. This method works only where there is a magnetic field against which to react. One classic field "coil" is actually in the form of a conductive tether in a planetary magnetic field. Such a conductive tether can also generate electrical power, at

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1408-617: Is the process of determining the orientation of a spacecraft (vehicle or satellite). It is a pre-requisite for spacecraft attitude control. A variety of sensors are utilized for relative and absolute attitude determination. Many sensors generate outputs that reflect the rate of change in attitude. These require a known initial attitude, or external information to use them to determine attitude. Many of this class of sensor have some noise, leading to inaccuracies if not corrected by absolute attitude sensors. Gyroscopes are devices that sense rotation in three-dimensional space without reliance on

1472-516: The Crab nebula source (= 1 "mCrab") in an eight-hour exposure. The maximum time resolution was 4 ms. The ART-S X-ray spectrometer, also built by the IKI, covered the energy range 3 to 100 keV. Its field of view was 2° by 2°. The instrument consisted of four detectors based on spectroscopic MWPCs, making an effective area of 2,400 cm at 10 keV and 800 cm at 100 keV. The time resolution

1536-525: The Galactic Center , broad-band observations of black hole candidates, and X-ray novae . After 1994, the observatory was switched to survey mode and carried out a sensitive all-sky survey in the 40 to 200 keV energy band. Some of the highlights included: After the end of the Soviet Union , two problems arose for the project. The first was geopolitical in nature: the main spacecraft control center

1600-481: The National Aeronautics and Space Administration . Attitude dynamics and control Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere , certain fields, and nearby objects, etc. Controlling vehicle attitude requires actuators to apply

1664-416: The 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator . Taken together, the instruments' three fields of view covered approximately 75% of the sky. The energy resolution was 30% FWHM at 60 keV. During quiet periods, count rates in two energy bands (6 to 15 and 15 to 180 keV) were accumulated for 4, 8, or 16 seconds, depending on onboard computer memory availability. During

1728-467: The ART-P telescope, each consisting of a position sensitive multi-wire proportional counter (MWPC) together with a URA coded mask. Each module had an effective area of approximately 600 cm, producing a field of view of 1.8° by 1.8°. The angular resolution was 5 arcmin ; temporal and energy resolutions were 3.9  ms and 22% at 6 keV, respectively. The instrument achieved a sensitivity of 0.001 of

1792-437: The Earth's upper atmosphere strike the satellite in such a way that the center of pressure remains behind the center of mass, similar to how the feathers on an arrow stabilize the arrow. GASPACS utilized a 1 m inflatable 'AeroBoom', which extended behind the satellite, creating a stabilizing torque along the satellite's velocity vector. Control algorithms are computer programs that receive data from vehicle sensors and derive

1856-458: The PID controller parameters. A simple implementation of this can be the application of the proportional control for nadir pointing making use of either momentum or reaction wheels as actuators. Based on the change in momentum of the wheels, the control law can be defined in 3-axes x, y, z as This control algorithm also affects momentum dumping. Another important and common control algorithm involves

1920-472: The Sun so they can provide electrical power to the spacecraft. Cassini ' s main engine nozzles were steerable. Knowing where to point a solar panel, or scan platform, or a nozzle — that is, how to articulate it — requires knowledge of the spacecraft's attitude. Because a single subsystem keeps track of the spacecraft's attitude, the Sun's location, and Earth's location, it can compute the proper direction to point

1984-833: The X-ray energy range while the KONUS-B and TOURNESOL experiments covered both the X-ray and gamma ray spectrum. Granat was a three-axis-stabilized spacecraft and the last of the 4MV Bus produced by the Lavochkin Scientific Production Association . It was similar to the Astron observatory which was functional from 1983 to 1989; for this reason, the spacecraft was originally known as the Astron 2. It weighed 4.4 metric tons and carried almost 2.3 metric tons of international scientific instrumentation. Granat stood 6.5 m tall and had

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2048-634: The aeronautical field, such as: This class of sensors sense the position or orientation of fields, objects or other phenomena outside the spacecraft. A horizon sensor is an optical instrument that detects light from the 'limb' of Earth's atmosphere, i.e., at the horizon. Thermal infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder cosmic background . This sensor provides orientation with respect to Earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth sensor. Similar to

2112-448: The angular rate is not estimated directly, but rather the measured angular rate from the gyro is used directly to propagate the rotational dynamics forward in time. This is valid for most applications as gyros are typically far more precise than one's knowledge of disturbance torques acting on the system (which is required for precise estimation of the angular rate). For some sensors and applications (such as spacecraft using magnetometers)

2176-458: The appendages. It logically falls to the same subsystem – the Attitude and Articulation Control Subsystem (AACS), then, to manage both attitude and articulation. The name AACS may even be carried over to a spacecraft even if it has no appendages to articulate. Attitude is part of the description of how an object is placed in the space it occupies. Attitude and position fully describe how an object

2240-448: The appropriate commands to the actuators to rotate the vehicle to the desired attitude. The algorithms range from very simple, e.g. proportional control , to complex nonlinear estimators or many in-between types, depending on mission requirements. Typically, the attitude control algorithms are part of the software running on the computer hardware, which receives commands from the ground and formats vehicle data telemetry for transmission to

2304-651: The attitude using only the measurements currently available), or through the use of a statistical filter (most commonly, the Kalman filter ) that statistically combine previous attitude estimates with current sensor measurements to obtain an optimal estimate of the current attitude. Static attitude estimation methods are solutions to Wahba's problem . Many solutions have been proposed, notably Davenport's q-method, QUEST, TRIAD, and singular value decomposition . Crassidis, John L., and John L. Junkins.. Chapman and Hall/CRC, 2004. Kalman filtering can be used to sequentially estimate

2368-540: The attitude, as well as the angular rate. Because attitude dynamics (combination of rigid body dynamics and attitude kinematics) are non-linear, a linear Kalman filter is not sufficient. Because attitude dynamics is not very non-linear, the Extended Kalman filter is usually sufficient (however Crassidis and Markely demonstrated that the Unscented Kalman filter could be used, and can provide benefits in cases where

2432-413: The attitude. Attitude control can be obtained by several mechanisms, including: Vernier thrusters are the most common actuators, as they may be used for station keeping as well. Thrusters must be organized as a system to provide stabilization about all three axes, and at least two thrusters are generally used in each axis to provide torque as a couple in order to prevent imparting a translation to

2496-574: The concept of detumbling, which is attenuating the angular momentum of the spacecraft. The need to detumble the spacecraft arises from the uncontrollable state after release from the launch vehicle. Most spacecraft in low Earth orbit (LEO) makes use of magnetic detumbling concept which utilizes the effect of the Earth's magnetic field . The control algorithm is called the B-Dot controller and relies on magnetic coils or torque rods as control actuators. The control law

2560-428: The direction opposite to that required to re-orient the vehicle. Because momentum wheels make up a small fraction of the spacecraft's mass and are computer controlled, they give precise control. Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. Spacecraft Reaction wheels often use mechanical ball bearings. To maintain orientation in three dimensional space

2624-561: The expense of orbital decay . Conversely, by inducing a counter-current, using solar cell power, the orbit may be raised. Due to massive variability in Earth's magnetic field from an ideal radial field, control laws based on torques coupling to this field will be highly non-linear. Moreover, only two-axis control is available at any given time meaning that a vehicle reorient may be necessary to null all rates. Three main types of passive attitude control exist for satellites. The first one uses gravity gradient, and it leads to four stable states with

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2688-406: The heating and cooling effects of sunlight and shadow may be used intelligently for thermal control, and also for guidance: short propulsive maneuvers must be executed in the right direction. Attitude control of spacecraft is maintained using one of two principal approaches: There are advantages and disadvantages to both spin stabilization and three-axis stabilization. Spin-stabilized craft provide

2752-481: The initial estimate is poor). Multiple methods have been proposed, however the Multiplicative Extended Kalman Filter (MEKF) is by far the most common approach. This approach utilizes the multiplicative formulation of the error quaternion, which allows for the unity constraint on the quaternion to be better handled. It is also common to use a technique known as dynamic model replacement, where

2816-453: The long axis (axis with smallest moment of inertia) pointing towards Earth. As this system has four stable states, if the satellite has a preferred orientation, e.g. a camera pointed at the planet, some way to flip the satellite and its tether end-for-end is needed. The second passive system orients the satellite along Earth's magnetic field thanks to a magnet. These purely passive attitude control systems have limited pointing accuracy, because

2880-635: The maximum angular momentum change) exerted by a CMG is greater than for a momentum wheel, making it better suited to large spacecraft. A major drawback is the additional complexity, which increases the number of failure points. For this reason, the International Space Station uses a set of four CMGs to provide dual failure tolerance. Small solar sails (devices that produce thrust as a reaction force induced by reflecting incident light) may be used to make small attitude control and velocity adjustments. This application can save large amounts of fuel on

2944-408: The observation of external objects. Classically, a gyroscope consists of a spinning mass, but there are also " ring laser gyros " utilizing coherent light reflected around a closed path. Another type of "gyro" is a hemispherical resonator gyro where a crystal cup shaped like a wine glass can be driven into oscillation just as a wine glass "sings" as a finger is rubbed around its rim. The orientation of

3008-411: The observatory finally reentered the Earth's atmosphere on May 25, 1999. The hard X-ray and low-energy gamma-ray SIGMA telescope was a collaboration between CESR (Toulouse) and CEA (Saclay). It covered the energy range 35–1300 keV, with an effective area of 800 cm and a maximum sensitivity field of view of ~5°×5°. The maximum angular resolution was 15 arcmin. The energy resolution

3072-602: The opposing direction if a new orientation is to be held. Thruster systems have been used on most crewed space vehicles, including Vostok , Mercury , Gemini , Apollo , Soyuz , and the Space Shuttle . To minimize the fuel limitation on mission duration, auxiliary attitude control systems may be used to reduce vehicle rotation to lower levels, such as small ion thrusters that accelerate ionized gases electrically to extreme velocities, using power from solar cells. Momentum wheels are electric motor driven rotors made to spin in

3136-502: The orbits inclination while reducing its eccentricity, such that the orbit had become near-circular by the time Granat completed its directed observations in September 1994. (By 1991, the perigee had increased to 20,000 km; by September 1994, the apogee/perigee was 59,025 km / 144,550 km at an inclination of 86.7 degrees.) Three days out of the four-day orbit were devoted to observations. After over nine years in orbit,

3200-450: The oscillation is fixed in inertial space, so measuring the orientation of the oscillation relative to the spacecraft can be used to sense the motion of the spacecraft with respect to inertial space. Motion reference units are a kind of inertial measurement unit with single- or multi-axis motion sensors. They utilize MEMS gyroscopes . Some multi-axis MRUs are capable of measuring roll, pitch, yaw and heave . They have applications outside

3264-685: The precise location must also be known. While pose estimation can be employed, for spacecraft it is usually sufficient to estimate the position (via Orbit determination ) separate from the attitude estimation. For terrestrial vehicles and spacecraft operating near the Earth, the advent of Satellite navigation systems allows for precise position knowledge to be obtained easily. This problem becomes more complicated for deep space vehicles, or terrestrial vehicles operating in Global Navigation Satellite System (GNSS) denied environments (see Navigation ). Microsecond A microsecond

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3328-519: The remaining with variable time resolutions depending on the count rate. The range of resolutions covered 0.25 to 8 s. The KONUS-B instrument operated from 11 December 1989 until 20 February 1990. Over that period, the "on" time for the experiment was 27 days. Some 60 solar flares and 19 cosmic gamma-ray bursts were detected. The French TOURNESOL instrument consisted of four proportional counters and two optical detectors . The proportional counters detected photons between 2 keV and 20 MeV in

3392-719: The spacecraft is always slowly rocking back and forth, and not always exactly predictably. Reaction wheels provide a much steadier spacecraft from which to make observations, but they add mass to the spacecraft, they have a limited mechanical lifetime, and they require frequent momentum desaturation maneuvers, which can perturb navigation solutions because of accelerations imparted by the use of thrusters. Many spacecraft have components that require articulation. Voyager and Galileo , for example, were designed with scan platforms for pointing optical instruments at their targets largely independently of spacecraft orientation. Many spacecraft, such as Mars orbiters, have solar panels that must track

3456-614: The spacecraft so as to observe 4 π steradians . The burst mode was triggered when the count rate in the 0.1 to 1.5 MeV energy range exceeded the background level by 8 sigma in either 0.25 or 1.0 seconds. There were 116 energy channels. Starting in January 1990, four WATCH instruments, designed by the Danish Space Research Institute , were in operation on the Granat observatory. The instruments could localize bright sources in

3520-450: The spacecraft will oscillate around energy minima. This drawback is overcome by adding damper, which can be hysteretic materials or a viscous damper. The viscous damper is a small can or tank of fluid mounted in the spacecraft, possibly with internal baffles to increase internal friction. Friction within the damper will gradually convert oscillation energy into heat dissipated within the viscous damper. A third form of passive attitude control

3584-508: The specific attitude maneuver although using a simple proportional–integral–derivative controller ( PID controller ) satisfies most control needs. The appropriate commands to the actuators are obtained based on error signals described as the difference between the measured and desired attitude. The error signals are commonly measured as euler angles (Φ, θ, Ψ), however an alternative to this could be described in terms of direction cosine matrix or error quaternions . The PID controller which

3648-447: The torques needed to orient the vehicle to a desired attitude, and algorithms to command the actuators based on the current attitude and specification of a desired attitude. Before and during attitude control can be performed, spacecraft attitude determination must be performed, which requires sensors for absolute or relative measurement. The broader integrated field that studies the combination of sensors, actuators and algorithms

3712-419: The vehicle. Their limitations are fuel usage, engine wear, and cycles of the control valves. The fuel efficiency of an attitude control system is determined by its specific impulse (proportional to exhaust velocity) and the smallest torque impulse it can provide (which determines how often the thrusters must fire to provide precise control). Thrusters must be fired in one direction to start rotation, and again in

3776-460: The way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with Earth's spin vector, and therefore point north, an orbital gyrocompass uses a horizon sensor to sense the direction to Earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw. See Tait-Bryan angles . A Sun sensor

3840-435: Was 200 microseconds . The PHEBUS experiment was designed by CESR (Toulouse) to record high energy transient events in the range 100 keV to 100 MeV. It consisted of two independent detectors and their associated electronics . Each detector consisted of a bismuth germanate (BGO) crystal 78 mm in diameter by 120 mm thick, surrounded by a plastic anti-coincidence jacket. The two detectors were arranged on

3904-544: Was 8% at 511 keV. Its imaging capabilities were derived from the association of a coded mask and a position sensitive detector based on the Anger camera principle. The ART-P X-ray telescope was the responsibility of the IKI in Moscow . The instrument covered the energy range 4 to 60 keV for imaging and 4 to 100 keV for spectroscopy and timing. There were four identical modules of

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3968-431: Was capable of imaging both hard X-ray and soft gamma-ray sources. The PHEBUS instrument was meant to study gamma-ray bursts and other transient X-Ray sources. Other experiments such as ART-P were intended to image X-Ray sources in the 35 to 100  keV range. One instrument, WATCH, was designed to monitor the sky continuously and alert the other instruments to new or interesting X-Ray sources. The ART-S spectrometer covered

4032-409: Was in finding funds to support the continued operation of the spacecraft amid the spending crunch in post-Soviet Russia. The French space agency , having already contributed significantly to the project (both scientifically and financially), took upon itself to fund the continuing operations directly. [REDACTED]  This article incorporates public domain material from websites or documents of

4096-699: Was located at the Yevpatoria facility in the Crimea region. This control center was significant in the Soviet space program, being one of only two in the country equipped with a 70 m RT-70 dish antenna . With the breakup of the Union, the Crimea region found itself part of the newly independent Ukraine and the center was put under Ukrainian national control, prompting new political hurdles. The main and most urgent problem, however,

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