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Appendix a Glossary Appendix A Glossary Only brief definitions are given to provide a quicklook reference for the reader with regard to terminology. Bold face type within a definition refers to a subject-related entry. 6788) 6789) 6790) Aberration. Geometrical errors in imagery whereby a perfect image is not formed. Typical aberrations include spherical aberration, astigmatism, coma, and chromatic aberrations. Lens bendings, locations, powers, materials, and increasing the number of lenses and aper­ ture stop positions are all used to minimize aberrations. The aberration theory of images generated by an optical system has been gradually develo­ ped from the seventeenth century on. Early analyses by e.g. Descartes, Roberval and Huy­ gens concern the spherical aberration which arises when imaging an object point through a single refracting or reflecting surface. Absorption band. A range of wavelengths, or frequencies, in the electromagnetic spectrum within which radiant energy is absorbed by a substance (gas, liquid or solid). In the gaseous phase, absorption lines are much narrower than in liquids or solids. In a polyatomic gas, an absorption band is actually composed of discrete absorption lines which appear to overlap. Each line is associated with a particular mode of vibration and rotation induced in a gas mol­ ecule by incident radiation. Examples: • Ozone (O3) has several absorption bands. They are: a) the Hartley bands (2000-3000 Å in UV with max absorption at 2550 Å); b) the Huggins bands (weak absorption between 3200-3600 Å); c) the Chappuis bands (weak and diffuse at 4500 Å and at 6500 Å in VIS); d) IR bands at 4.7, 9.6, and 14.1 mm. • Molecular oxygen (O2) also has several absorption bands. They are: a) Hopefield bands (between 670 and 1000 Å in UV); b) diffuse between 1019 and 1300 Å (UV); c) the Schumann-Runge continuum (between 1350-1760 Å); d) Schumann-Runge bands (bet­ ween 1760 - 1926 Å); e) the Herzberg bands (between 2400-2600 Å); f) the atmospheric bands (between 5380-7710 Å, VIS); g) IR at about 1 mm (=104 Å). Absorptivity. Ratio of the absorbed to the incident electromagnetic radiation on a surface. Accuracy. Refers to an estimate of how well a certain parameter or measurement is known; it is a measure of the absolute truth of a measurement requiring absolute (traceable) stan­ dards. Accuracy is a quality that characterizes the ability of a measuring instrument to give indications equivalent to the `true value' of the quantity measured. The quantitative expres­ sion of accuracy may also be given in terms of uncertainty. The actual or `true value' of a quantity cannot be determined; it can only be said to exist within tolerance limits of a mea­ sured value. The measurement error is the algebraic difference between the measured (or indicated) value and the true value. Hence, accuracy is by its very nature only an estimation of the true value, taking into account all aspects of measurement. Acid rain. Rain that is more acid than normal because the raindrops contain dissolved acid gases and/or dust particles (aerosols) from the atmosphere. The principal gases responsible for increased acidity are oxides of sulfur and nitrogen. Generally, rain with a pH below 4.5 is considered environmentally harmful. Actinometer. A generic term for any instrument used to measure the intensity of radiant energy, in particular that of the sun. _____________________ 6788) Portions of this glossary are taken from: “Glossary and list of Acronyms/Abbreviations,” Earth Observation System (EOS), July 1992, Courtesy of EOS Project Science Office (V. V. Salomonson), GSFC, Greenbelt, MD. 6789) Jeanne Hopkins, “Glossary of Astronomy and Astrophysics,” The University of Chicago Press, Second Edition, 1985 6790) R. J. Gurney, J. L. Foster, C. I. Parkinson (editors), “Atlas of satellite observations related to global change,” Cam­ bridge University Press, 1993 Survey of Spaceborne Missions and Sensors 2 Active sensor. A sensor having its own source of EMR (Electromagnetic Radiation); it transmits a series of signals to the target and detects the echo. Information is obtained by transmission and reception of radio waves (or light waves). A SAR instrument, a lidar, a radar altimeter, a scatterometer and cloud profile radars - are examples of active sensors. Actuators. Refer to a class of onboard devices or techniques (manipulators) that are being used in support of a great variety of functions. Some examples are: • Actuators in the field of attitude control: reaction wheels, momentum wheels, magne­ torquer coil/rods, permanent magnets, gravity-gradient boom, nutation damper, control moment gyros, cold gas thrusters, solid thrusters, ion thrusters, mono- or bi-propellant engine, etc. • Deployment and release mechanisms such as antenna and instrument deployment, po­ sitioners, aperture opening and closing devices • Actuators are being used for autonomous sample acquisition, autonomous instrument placement, robotic arms, rovers, etc. • Starting with the 1990s, miniature actuators (demonstrators) are being introduced. Pi­ ezoelectric actuators 6791) (ultrasonic rotary motors) offer special characteristics that are of interest for a number of applications, for instance: 6792) 6793) - Non-powered holding torque in the same range as the maximum driving torque (hard brake without backlash) - High positioning accuracy (micropositioning) in direct drive mode - Feasibility of non-magnetic motor designs The simplest actuators are direct piezoelectric actuators (DPA) which can produce dis­ placements of the order of ten to one hundred microns and exhibit high stiffness. These ac­ tuators are robust when highly pre-stressed and may be used in active satellite structures without the need for a locking mechanism during launch. - The number of applications of piezoelectric actuators in instruments flown in space is growing, in particular with regard to: - Precise pointing of optics (adaptive optics) - Active damping of vibrations • Small (mass, volume, power) CMGs (Control Moment Gyroscopes) are being introduced at the start of the 21st century providing the dual function of actuator/sensor in an attitude system of a spacecraft. Adaptive optics. A technique which tries to compensate for the atmospheric degradation (blurring) of the incoming signal of optical imaging systems. However, the compensations achieved, regardless of method, are never “perfect.” The following approaches are in use: 6794) 6795) • An on-line adaptive optics system (hardware solution). In this version, the adaptive optics system is capable of compensating for the distortion of electromagnetic radiation as it _____________________ 6791) Note: The piezoelectric effect was discovered in 1881 when Pierre and Jacques Curie (Pierre Curie: 1859-1906; Nobel prize in physics in 1903 together with Henri Becquerel) observed that quartz crystals generated an electric field when stressed along a primary axis. The term piezoelectric derives from the Greek word “piezein,” meaning to squeeze, and the electricity that results from pressure applied to the quartz crystal. 6792) R. Seiler, M.F. Six, M. Debarnot, R. Le Letty, F. Claeyssen, “The Ultrasonic Piezo Drive An Innovative Solution for High-Accuracy Positioning,” Proceedings of AIAA/USU Conference on Small Satellites, Logan, UT, Aug. 12-15, 2002, SSC02-VIII-4 6793) Y. Bar-Cohena, X. Baoa, W. Grandiab, “Rotary Ultrasonic Motors Actuated By Traveling Flexural Waves,” Pro­ ceedings of the Smart Structures and Materials Symposium, San Diego, CA, March 1-5, 1998, Paper 3329-82 6794) S. R. Restaino, D. M. Payne, “Adaptive Optics on a shoe string,” SPIE Vol. 3494, 1998, pp. 152-160 6795) D. Dayton, S. Browne, J. Gonglewski, “Control Loop Analysis for a Nematic Liquid Crystal Spatial Light Modula­ tor Used in an Adaptive Optics System,” SPIE Vol. 3494, 1998, pp. 161-160-167 3 Survey of Spaceborne Missions and Sensors passes through the turbulent atmosphere and the optical system. Elements of an adaptive optical system are a high-speed wavefront sensor (sensing the turbulence-induced aber­ rations), a flexible mirror system whose surface can be electronically controlled to correct for aberrations, and a computer controller that converts the wavefront measurements into deformable mirror commands. - More economic solutions are suggested by the use of LCPM (Liquid Crystal Phase Modulator) to deform the mirror, and by the use of the pupil masking technique. • A post facto approach (software solution), where the data are acquired by an imaging instrument and processed off-line with suitable iterative algorithms to increase the overall resolution obtained by the optics system. Speckle imaging is an example of such a technique. 6796) • A hybrid approach using an adaptive optics system in combination with post facto proc­ essing. Advection. Refers to a change in property of a moving air parcel from one region to another (say, from a warm region to a cool region, thereby changing its temperature). Commonly, advection is divided into horizontal and vertical components; it differs from convection only in scale. Convection is transport by random thermally induced currents, whereas advection is transport by steady vertical currents. Aerosol. The term is used to describe many types of small particles in the atmosphere that both absorb and reflect incoming sunlight. Aerosols are a stable suspension of fine (solid or liquid) particles such as dust, smoke particles, water droplets, etc. in the atmosphere. The smallest aerosols are the atoms of the various atmospheric gases. The range of sizes
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