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Absorption Bands

Regions of the electromagnetic spectrum where atmospheric gases, such as carbon dioxide, absorb radiation.


A suspension of liquid or solid particles in the air.


A measure of the overall reflectivity of a given surface or region. Specifically, the albedo is the ratio of the radiance of the light reflected off of a surface to the total light that impacts the same surface.


Bad pixel

Bad pixels are those deemed unusable for the sums that generate the OCO spectral samples. Removal of bad pixels from the spectral sample improves the signal to noise ratio (SNR).


Irregularly shaped envelope of the earth's air, water, and land encompassing the heights and depths at which living things exist.


Carbon Cycle

The exchange of carbon between living organisms and the nonliving environment.

Carbon Dioxide (CO2) Bands

Wavelength ranges of electromagnetic radiation that are absorbed by carbon dioxide. OCO will measure radiation in two of these bands to detect the presence of carbon dioxide in the atmosphere. One range spans in wavelength from 1.594 microns to 1.619 microns. The second range spans in wavelength from 2.042 microns to 2.082 microns.

Chromatic Aberrations

The failure of a lens to bring light of different wavelengths into a common focus. Chromatic aberrations generate a blurry or distorted image.

Climate Change

Climate change records the long-term fluctuations in temperature, precipitation, wind, and all other aspects of the Earth's climate. The United Nations Convention on Climate Change defines this phenomenon as "change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods."

CO2 Sinks

Physical processes that absorb CO2 from the atmosphere.

CO2 Sources

Physical processes that release CO2 into the atmosphere.


To diverge light or some other form of electromagnetic radiation from a point source into a parallel beam. Instruments often must collimate light in order to make specific measurements.


A device designed to convert incoming light into a beam of parallel rays. The device employs a convex lens at one end and a small opening or slit at the other. The opening or slit is located at the principal focus of the lens.


A specific frequency or wavelength of light within the electromagnetic spectrum. Each of OCO's three spectrometers separates light from a very narrow spectral region into over 1000 distinct colors.

Column Abundance

The fraction of carbon dioxide in the atmosphere in a column projected over a fixed area on the Earth. The column extends from ground level to space.

This is an image of earth as a globe with a column extending from the ground level to space.

This is an image of earth as a globe with a column extending from the ground level to space

Column averaged dry air mole fraction (XCO2)

The fraction of moles of a particular component of air divided by the total number of moles of gas in the same sample. The calculation does not include water vapor in the air sample.


Dichroic Beam Splitters

A device that either separates or combines the light of two different wavelengths. These beam splitters use multi-layer thin-film coatings to transmit specific wavelengths of light and reflect the remainder of the spectrum. Thermal evaporation and high vacuum deposition techniques are used to deposit these multi-layered coatings onto a glass substrate. Since dichroic beam splitters absorb almost no energy, they transmit light far more efficiently than other types of filters.



Pixel or spectral sample datum.


Focal Length

In an optical system, the distance (in mm.) from either the lens or the primary mirror to the focal point, where the telescope is in focus. In general, the longer the focal length of the telescope, the more power it has. Longer focal lengths typically generate a larger telescopic image and a smaller telescopic field of view.

Focal Ratio

The ratio of the focal length of the telescope to its aperture. For example, a telescope with a 2032 mm focal length and an aperture of 8" (203.2mm), has a focal ratio of 10 (2032/203.2 = 10). The standard means to specify a focal ratio of 10 is f/10.


A two half-shell structure constructed of a graphite/epoxy composite. This structure protects the spacecraft from the time of encapsulation through the atmospheric ascent. The fairing also provides thermal and acoustic enclosures for the OCO instrument and the launch vehicle's electronic compartments during pre-launch and ascent.

Fold Mirror

A flat mirror that is used to change the direction of a light beam. If a fold mirror is placed at a 45° angle with a light beam, the fold mirror will change the path of the beam by 90°.


A set of soundings collected across the instrument slit that are nearly simultaneous, but at different viewing angles.


The area on the Earth's surface that is in view during a single spectrometer measurement.


Greenhouse Gas

An atmospheric gas that is known to trap heat near the surface of the Earth. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide and ozone.


A gigaton is 109 tons or 1012 kg or 2.2 X 1012 pounds or 1015 grams (also called petagram).


High Resolution Color Slice

The OCO telemetry returns sets of single pixel data that vary over space but represent the same wavelength. These sets of high resolution data are known as color slices.


Honeycomb panels are used to form lightweight, high strength sandwich units that are extremely rigid and light. Sophisticated computer-controlled equipment can create a honeycomb with the appropriate size, depth of material, thickness of cell walls and grade of aluminum to suit specific requirements for each unit. The manufacturing equipment stretches the aluminum core material wafer block into a sheet of hexagonal structure with consistent cell dimensions. The walls of the cells can have micro perforations to allow movement of air between adjacent cells. This design equalizes the pressure in environments with rapid temperature variation.


Fuel for the thrusters that are used to adjust and control the position of the spacecraft. The chemical formula for hydrazine is NH2NH2.


Molecular Oxygen (O2) Band

A range of wavelengths of the electromagnetic radiation that are absorbed by molecular oxygen. The OCO project will measure radiation in the O2 band with wavelengths that range from 0.758 microns to 0.772 microns.


Near Infrared (NIR)

A range of the electromagnetic spectrum with slightly higher wavelength than that of visible light.



Does not allow light to pass through. In spectroscopy, a medium is opaque in a particular range of wavelength if the medium absorbs all of the energy at that wavelength.



The photon is one of the elementary particles. In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing the camera optics. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances. Normally, light is formed from a large number of photons, with the intensity or brightness related to the number of them.


A single detector or the data from a single detector on the OCO instrument focal plane.



A radio frequency in the 2000 MHz to 4000 MHz range.


For the OCO mission, a sounding is a set of the three collocated spectra acquired nearly contemporaneously at the same viewing angle. Each of the three spectra represents one of the three spectral bands. The three spectral bands include the O2 A-band with wavelengths near 0.76 µm, the weak CO2 band with wavelengths near 1.61 µm and the strong CO2 band with wavelengths near 2.06 µm.

Spectral Radiances

The incident photon rate normalized to view angle, surface area and unit wavelength. The OCO Level 1B Product reports a spectral radiance for each spectral sample, where each sample represents a discrete wavelength of electromagnetic radiation.

Spectral Sample

The outcome of on-board summing of the OCO focal plane pixels. All of the summed pixels represent the same spectral wavelength at slightly different spatial locations. Thus, spectral samples have a degraded spatial resolution relative to single detector measurements.


The distribution of energy emitted by a radiant source, arranged in order of wavelengths. The plural is spectra.

Specular Reflection

Reflection where the angle of incidence is the equal to the angle of reflection.

Star Tracker

An instrument that tracks the position of stars. The instrument compares the location of a specific set of stars with an on-board star catalog. The instrument uses the detected locations to determine precisely how the spacecraft is situated relative to a specified reference position.


The steradian is the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere that has an area r2. Since the surface area of this sphere is 4π2, a sphere measures 4π steradians.



For OCO, the terminator is the great circle that contains all of the points on the Earth's surface that are 90 degrees from the instantaneous sub-solar point. A terminator crossing occurs when the Observatory nadir crosses this great circle.


Allows light to pass through. In spectroscopy, a medium is transparent in a particular range of wavelength if the medium absorbs none of the energy at that wavelength.



A radio frequency in the 8000 MHz to 12,000 MHz range.