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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
The amount of information transmitted per unit time.
See period decay
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.
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.
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.
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.
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.
The point in a satellite's orbit at which it crosses the
equatorial plane from north to south.
A land area so dry that little or no plant or animal life can
The man-made or natural formation of desert from usable land.
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.
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
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
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.
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.
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.
Parasitic element(s) of a VHF antenna located forward of the
driven element See antenna.
Data and Information System
Performed in twenty-four hours, such as the diurnal revolution of
The apparent arc described by heavenly bodies from their rising to
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.
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
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.
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
The study of the action of forces on bodies and the changes in
motion they produce.
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