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A B C D E F G H I J K L M N O P R S T U V W Y Z


Words begining with "D"

Data Collection System (DCS)

DCS units are flown on both GOES and NOAA polar orbiting spacecraft. They gather and relay data from both mobile and stationary platforms at various locations. DCS units on NOAA satellites can also determine the precise location of moving platforms at the time the data were acquired. See TIROS-N/NOAA satellites.

Data Rate

The amount of information transmitted per unit time.

Decay

See period decay

Decibel (dB)

A tenth of a bel. A unit used to measure the volume of a sound, equal to ten times the common logarithm of the ratio of the intensity of the sound to the intensity of an arbitrarily chosen standard sound. The decibel also is used to measure relative strengths of antenna and amplified signals and always refers to a ratio or difference between two values.

Declination

The angular distance from the equator to the satellite, measured as positive north and neqative south.

Defense Meteorological Satellite Program (DMSPI)

A U.S. Air Force-managed meteorological satellite program with satellites circling in sun-synchronous orbit. Imagery is collected in the visible - to near - infrared band (0.4 to 1.1 micrometers) and in the thermal-infrared band (about 8 to 13 micrometers) at a resolution of about three kilometers. DMSP data is available directly from the satellite for local use aboard ships and at military deployment locations, but is also usually available to civilian users.

Degree

A unit of angular measure. The circumference of a circle contains 360 degrees. When applied to the roughly spherical shape of the Earth for geo- graphic and cartographic purposes, degrees are each divided into 60 minutes, and each minute is divided into 60 seconds.

Delta

The fan-shaped area at the mouth or lower end of a river, formed by eroded material that has been carried downstream and dropped in quantities larger than can be carried off by tides or currents.

Demodulation

The process of retrieving information (data) from a modulated carrier wave, the reverse of modulation.

Department of the Interior (DOI)

Responsible for our nationally-owned public lands and natural resources, the DOI is chartered to foster the wisest use of our land and water resources, protect fish and wildlife, preserve the environmental and cultural values of national parks and historical places, and provide for the enjoyment of life through outdoor recreation. The department assesses energy and mineral resources and is responsible for assuring that their development is in the best interest of all citizens. The U.S. Geological Survey (USGS) is part of the DOI.

Descending Node

The point in a satellite's orbit at which it crosses the equatorial plane from north to south.

Desert

A land area so dry that little or no plant or animal life can survive.

Desertification

The man-made or natural formation of desert from usable land.

Detector

A device in a radiometer that senses the presence and intensity of radiation. The incoming radiation is usually modified by filters or other optical components that restrict the radiation to a specific spectral band. The information can either be transmitted immediately or recorded for transmittal at a later time.

Dew

Atmospheric moisture that condenses after a warm day and appears during the night on cool surfaces as small drops. The cool surfaces cause the water vapor in the air to cool to the point where the water vapor condenses.

Dew Point

The temperature to which air must be cooled for saturation to occur, exclusive of air pressure or moisture content change. At that temperature dew begins to form, and water vapor condenses into liquid.

Digital Image

An analog image converted to numerical form so that it can be stored and used in a computer The image is divided into a matrix of small regions called picture elements or pixels. At sub-satellite point each pixel represents a specific amount of area. For example, in APT each pixel represents 4.1 kilometers. Each pixel has a numerical value or data number value, quantifying the radiance of the image at that spot. The data number value of each pixel usually represents a value between black and white, i.e., shades of qray.

False color can be applied to the image by assigning a graduated color palette to represent the gray shades. The color is "false" because it represents an assigned, not actual, color.

Digital System

A system in which information is transmitted in a series of pulses. The source is periodically sampled, analyzed, and converted or coded into numerical values and transmitted. Digital transmissions typically use the binary coding used by computers so most data is in appropriate form, but verbal and visual communication must be converted. Many satellite transmissions use digital formats because noise will not interfere with the quality of the end product, producing clear and higher-resolution imagery.

Direct Readout

The capability to acquire data directly from environmental satellites via an Earth station. Data can be acquired from NOAA and other nations' environmental satellites, which offer weather information from geostationary and polar-orbiting satellites.

Director

Parasitic element(s) of a VHF antenna located forward of the driven element See antenna.

DIS

Data and Information System

Diurnal

Performed in twenty-four hours, such as the diurnal revolution of the Earth.

Diurnal Arc

The apparent arc described by heavenly bodies from their rising to their setting.

Dobson Unit (DU)

The standard way to express ozone amounts in the atmosphere. One DU is 2.7 X 10 raised to the 16th power (27,000,000,000,000,000) ozone molecules per square centimeter. One Dobson unit refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0 degrees C} and pressure (the average pressure at the surface of the Earth). For example, 300 Dobson units of ozone brought down to the surface of the Earth at 0 degrees C would occupy a layer only 0.3 cm thick in a column. Dobson was a researcher at Oxford University who, in the 1920s, built the first instrument (now called the Dobson meter) to measure total ozone from the ground.

Doldrums

Region near the equator characterized by low pressure and liqht shiftinq winds. See wind.

Doppler Effect (aka Doppler shift)

The apparent change in frequency of sound or light waves, varying with the relative velocity of the source and the observer. If the source and observer draw closer together, the frequency is increased. Named for Christian Doppler, Austrian mathematician and physicist (1803-1853).

Doppler Radar

The weather radar system that uses the Doppler shift of radio waves to detect air motion that can result in tornadoes and precipitation, as previously-developed weather radar systems do. It can also measure the speed and direction of rain and ice, as well as detect the formation of tornadoes sooner than older radars.

Downconverter

Any radio frequency circuit that converts a higher frequency to a lower frequency. This enables signal processing by a receiver. A typical downconverter will feature one or more states of RF preamplification, a mixer where the frequency conversion occurs, a local oscillator chain, and often one or more intermediate frequency preamplifiers to minimize the effect of line losses between the converter and the receiver.

Drag (aka Nl)

A retarding force caused by the Earth's atmosphere. Thus by definition, drag will act opposite to the vehicle's instantaneous velocity vector with respect to the atmosphere. The magnitude of the drag force is directly proportional to the product of the vehicle's cross-sectional area, its drag coefficient, its velocity, and the atmospheric density, and inversely proportional to its mass. The effect of drag is to cause the orbit to decay, or spiral downward. A satellite of very high mass and very low cross-sectional area, and in a very high orbit, may be very little affected by drag, whereas a large satellite of low mass, in a low altitude orbit may be affected very strongly by drag. Drag is the predominant force affecting satellite lifetime.

Driven Element

See antenna.

Dynamics

The study of the action of forces on bodies and the changes in motion they produce.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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