Glossary

Glossary of Terms

Please choose a subject: Orbital Mechanics or Remote Sensing


Orbital Mechanics


 

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    A

    altimeter: active instrument used to measure altitude above a fixed level.

    altitude: height above the Earth's surface.

    anemometer: device for measuring wind speed.

    anomaly 2: angular distance between an Earth satellite and it's perigee as seen from center of the Earth. Also see anomaly 1 (Meteorology section).

    apogee: point at which satellite goes farthest from central body in an elliptical orbit.

    argument of perigee: Keplerian element that gives the rotation of the satellite on the orbit. The angle (measured from the center of the Earth) from the perigee to the ascending node. See perigee.

    ascending node: when a satellite crosses the equatorial plane from south to north in it's orbit.

    Astronomical Unit (AU): about 149,599,000 kilometers; the distance from the Earth to the Sun.

    azimuth: angular distance in degrees measured in a clockwise direction from true north.

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    D

    declination: angular distance between a satellite and the equator, where north is positive, while south is negative.

    degree: unit of measure for angular distances(). There are 360 in the circumference of a circle. A degree may be divided into 60 minutes for cartographic and geographic purposes.

    descending node: particular part of a satellite's orbit at which the equatorial plane is traversed from north to south.

    diurnal: every 24-hours. ex., diurnal revolution of the Earth.

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    E

    eccentricity: a Keplerian element describing the shape of the orbit; where: e=0-> circular orbit, e=between 0 and 1->elliptical orbit, e=1->parabolic orbit, e=greater than 1->hyperbolic orbit.

    elliptical orbits: generally bodies in space naturally have elliptical orbits as opposed to circular orbits due to gravity, drag, etc.,. In such an orbit the perigee is the point where the orbiting object comes closest to the body being orbited, and the apogee is like-wise the farthest point.

    ephemeris: an arrangement of a series of data points defining both the position and motion of a satellite.

    epoch: amount of time of a particular description of the satellite orbit.

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    G

    geostationary: an orbit in which a satellite remains over the same area of the Earth; appears stationary with respect to the spinning Earth.

    ground track: the imaginary line that connects the Earth's center to a satellite, created by the period of orbit, the orbital attitude and inclination of the satellite.

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    I

    inclination: a Keplerian element that indicates the angle from the orbital plane to the equatorial plane of the body being orbited. Both planes go through the center of the central body but are at a certain angle with respect to each other. This angle of inclination is measured counter-clockwise at the ascending node.

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    K

    Keplerian elements (satellite orbital elements): the six independent constants defining an orbit that were named after Johannes Kepler (1571-1630). These constants are:
    argument of perigee -- angle from ascending nodes to perigee point along orbit, measured in direction of satellite's motion
    eccentricity -- defines shape of orbit
    inclination angle -- gives angle of orbit plane to central body's equatorial plane
    right ascension of the ascending node -- gives the rotation of orbit plane from reference axis
    semi-major axis -- defines the size of orbit
    true anomaly -- defines satellite location on orbit.

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    L

    latitude (geodetic latitude): the distance, measured in degrees, directly south or directly north of the equator.

    line of apsides (major axis on ellipse): imaginary line connecting the apogee and perigee.

    longitude:the distance, measured in degrees, directly west or directly east of the equator

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    M

    mean motion: the average speed of satellites in non-circular orbits. While in circular orbits a satellite's speed is constant in contrast to elliptical orbits in which the speed decreases as the orbit takes the satellite farther from the Earth and vice-versa with it nearing the Earth.

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    N

    nadir: point directly beneath a satellite.

    nautical mile: a distance of 1.852 km or 6076.1 ft which is also approximately equal to 1/60 of a degree.

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    O

    orbital plane:an imaginary pane passing through the center of the Earth and containing the Earth satellite's orbit.

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    P

    perigee: point closest to central object in an elliptical orbit.

    period: time for one complete revolution of a satellite around its orbit.

    perturbations: small adjustments made to the Keplerian model of a satellite's orbit; due to Earth's gravity and drag, a satellite's orbit is not a perfect ellipse of constant shape and orientation.

    polar orbit: orbit with an approximately 90 degree inclination, in which the satellite's ground track passes by both poles (North and South) once in each orbit.

    prograde orbit: Earth orbits rotating in the same direction as the Earth.

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    R

    revolution:process of completing a full orbit. The Earth's revolution around the sun determines the seasons and length of an Earth year.

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    S

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    semi-major axis: Keplerian element indicating the size of orbit; one half the distance between the perigee and apogee in an orbital ellipse.



    Remote Sensing

     

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    A

    absorption: process in which radiant energy is absorbed by an element or substance, which itself may emit radiation if an energy conversion has occurred. The amount of the radiant energy that gets absorbed or re-emitted reveals characteristics about the absorbing medium.

    Acquisition of Signal(AOS): the time a signal from a spacecraft is received.

    active system (active sensor): remote sensing system that transmits its own signal (radiation) and detects an object or area by receiving the reflected transmitted radiation, ex., radar.

    A/D: referring to conversion of analog to digital data.

    Advanced Very High Resolution Radiometer (AVHRR): five channel scanning device that quantitatively measures electromagnetic radiation. Previous use includes determining cloud cover, and surface temperature, to observing lakes, shorelines, ice, snow, and vegetation.

    aerosol: liquid or solid particles diffused as a suspension in gas

    AI: Artificial Intelligence.

    AIR: Airborne Imaging Radar.

    albedo: ratio of reflected solar radiation to radiation incident upon that particular object.

    algorithm: in remote sensing, algorithms generally specify how to determine higher level data from lower level source data.

    ampere: standard unit of measurement of an electric current.

    attenuation: decrease in magnitude of power, current, or voltage signal being transmitted usually due to interference caused by clouds or rain.

    Automatic Picture Transmission (APT): system that allows real-time transmission of satellite images between an APT equipped satellite and an environmental satellite ground station that is within range.

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    B

    band: radio: a narrow section of wavelenghts or frequencies in radio broadcasting , radiometry: the relatively slender section of the electromagnetic spectrum that a remote sensor can discern, spectroscopy: spectral section where absorption by atmospheric gases occurs.

    biomass: quantity of living material in a unit volume or area.

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    C

    calibration: process of comparing an instrument's accuracy to known standards

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    D

    digital image: image in numerical form which can be stored and manipulated by a computer. A numerical image is divided into a matrix of pixels (picture elements). Each pixel represents a particular amount of area (ex. 5 x 5km, 10 x 15km) and holds a numerical value indicating the radiance of the image at that point.

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    E

    Earth Observing System (EOS): a series of Earth observing spacecraft planned for launch beginning in 1998 by NASA, as part of the Mission to Planet Earth (MTPE). These spacecrafts will carry instruments that were designed specifically to study global climate change.

    electromagnetic radiation: ex. light, radar, radio waves are forms of electromagnetic radiation. The transportation of energy through a medium in the forms above mentioned.

    electromagnetic spectrum: the range of electromagnetic radiation wavelengths and frequencies. They are usually divided into seven categories: radio, microwave, infrared, visible, ultra-violet, x-ray, and gamma-ray radiation.

    elevation: optimal reception angle of an antenna from a spacecraft (measured above the horizon).

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    F

    far infrared: electromagnetic radiation with wave frequencies longer than that of thermal infrared (between 25 and 1000 micrometers).

    field of view: the range of angles, measured in degrees of arc, scanned by a remote sensing system.

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    G

    Geographic Information System (GIS): the manipulating of data stored and indexed according to geographic coordinates of it's elements.

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    H

    High-Resolution Picture Transmission (HRPT): real time, digital images containing all five spectral channels with telemetry data transmitted as high-speed digital transmissions. Images are gathered by the NOAA's environmental satellites.

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    I

    image: an illustrative representation acquired by satellite systems comprising of data arrays. The image can represent different parameters, exposing the information that is being sought.

    image resolution: depends on the amount of area that is represented by each individual pixel in an image; the smaller the area, the more accurate and thus detailed the image.

    Infrared Radiation (IR): electromagnetic radiation with wavelengths between .7 to 1000 micrometers (between visible[shorter] and microwave[longer] radiation). At the visible near-infrared sections of the spectrum, chemical composition, and a some biological properties of surface matter or vegetation can be measured. At the mid-infrared section of the spectrum, are detectable geological formations due to other absorption properties. And at the far-infrared section, emissions from the surface and atmosphere give information on temperatures.

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    L

    Landsat: Land Remote-Sensing Satellite, a series of satellites developed to gather data of Earth's resources in a systematic manner. Operated by US Earth Observation Satellite Company and used for: land use inventory, crop/forestry assessment, geological and mineralogical studies, and cartography. [Spatial Resolution: 30 m].

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    M

    microwave: electromagnetic radiation with wavelengths of 1000 micrometers to 1 meter.

    middle infrared: electromagnetic radiation with wavelengths between 2 and 5 micrometers; between near infrared and thermal infrared.

    multispectral Scanner (MSS): line scanning device used on the Landsat satellites, to continually scan the Earth. All five Landsat satellites have hade four channels in the visible and near infrared with only one, Landsat 3, having a fifth channel in the thermal infrared. [Spatial Resoulution: 80 m]

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    N

    near infrared: electromagnetic radiation with wavelengths between .7 to 2 micrometers; just longer than the visible.

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    P

    passive system: remote sensing system sensing radiation emmited only by object being observed or radiation emmited by source other than that system and reflected by object of interest.

    payload: all the instruments that that are placed on board a spacecraft.

    pixel: picture element; smallest element of an image that has been electronically coded in an array.

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    R

    radiation: energy in the form of electromagnetic waves or particles that transverses space matter and other media. See electromagnetic radiation.

    radiometer: an electromagnetic radiation measuring instrument.

    reflection: when light or sound waves bounce off a surface and may return to the source; if surface is a plane then the angle of reflection is also the angle of incidence.

    remote sensing: the gathering of information about the Earth from a distance, without actually coming in contact with it. See Remote Sensing page

    resolution: ability to seperate two discernable objects; remote sensing: description of the area depicted in one pixel of a digital image (where the smaller the area depicted the better the resolution).

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    S

    scanning radiometer: imaging system comprised of lenses and moving mirrors capturing the image on a solid-state image sensor; currently being used on all operational weather satellites.

    scattering: dispersion of electromagnetic radiation as a result of it's interaction with molecules in the atmosphere. The sky appears blue as a result of the blue region of the visual spectrum being scattered more than the red region.

    sensor: tool that is sensitive to stimulus such as electromagnetic radiation. On satellites sensors are used to identify emmited or reflected radiation and use this data to obtain other valuable information about the object or surface reflecting or emmiting it. Some examples of sensors are: imagers, specrometers, radiometers, and a combination of them. Each providing added resolution for certain aspects pursued.

    sounder:radiometer that measures atmoshperic temperature changes, and amounts of several chemical species in the atmosphere, as found at various levels.

    SPOT: System Pour l'Observation de la Terre. Polar-orbiting Earth observation satellite designed and operated by the French, having a ground resolution of 10m.

    thematic mapper:

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