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University of Nebraska–Lincoln

Remote Sensing Glossary

Reference Information for Virtual Nebraska

Terms, Definitions and Concepts

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.

declination

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

Defense Meteorological Satellite Program (DMSP)

A U.S. Air Force 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. While some of the data is classified, most unclassified data is available to civilian users.

degree

A unit of angular measure represented by the symbol °. The circumference of a circle contains 360 degrees. When applied to the roughly spherical shape of the Earth for geographic and cartographic purposes, degrees are each divided into 60 minutes.

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.

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. See diagram, Keplerian elements.

Descending Node
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 gray.

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.

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 exp 16 (10 to the 16th power) 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 degree 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 degree 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 light shifting winds. See Wind.

doppler effect (aka Dopler 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).

Dopler 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.

drage (aka N1)

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-section 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 down-ward. 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.

dynamics

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