Absorption: a measure of the ability of a surface or a medium to absorb incident energy, often at specific wavelengths.

Across-Track: The (footprint) direction perpendicular to the flight direction of the satellite.

Albedo: the ratio of the radiation reflected from an object to the total amount incident upon it, for a particular portion of the spectrum.

Algorithm: A set of defined instructions for calculating a function.

Along-Track: The (footprint) direction parallel to the flight direction of the satellite.

Ancillary data: The combination of image ancillary data and satellite ancillary data.

Auxiliary data: Supporting data sets provided outside the space segment data stream used to apply corrections to the space segment sensor data. Examples include previously derived calibration parameters and ground control data.

Background: Instrument response when not observing the wanted signal. A background measurement is done by switching or shuttering off the light source under investigation (calibration sources, radiance from the earth) or by moving the light source outside the instrument's field of view. The background signal may include contributions from internal and external sources such as dark current, electronic offset, EMC, PONU and signals from background illumination (e.g. by thermal radiation).

Binning: The addition of analogue signal of two or more pixels. This operation is performed on-chip, before the A/D converter, by appropriate clocking of the detector.

Calibration: In the context of TROPOMI, calibration relates the response of the instrument to a known absolute radiometric standard. This is achieved by a combination of dedicated calibration measurements and an instrument model as implemented in the Level-1B processor.

Calibration key data: The data that is required to convert Level-0 data into Level-1B data and higher-level data.

Co-addition period: The time interval for which the data that are read out during the individual detector exposures are summed on board the instrument.

Detector: Focal plane array, i.e. CCD/CMOS in spectrometer.

Detector module (DEM): The sub-assembly including the detector and its read-out electronics (i.e., the Front End Electronics (FEE)).

Exposure time: Duration of an exposure (including the frame transfer).

Field of view: The angular range subtended by the instrument at any given time. The projection of the FOV on Earth is the image swath.

Frame transfer CCD: A CCD, the area of which is divided in two equally sized parts: a storage section covered with an opaque mask, and an image section unmasked and available to collect photons during integration. After integration the charge collected is quickly transferred from the image section to the storage section, where it is slowly read out while the image section integrates the next exposure.

Granule: A period of time; typically used as a description of a set of data that is restricted to the time period. A granule can, for example be an orbit, a day or a year. The smallest size for a granule is a single measurement.

Ground pixel: That part of the Earth's surface that corresponds to an image pixel.

Ground segment: All facilities on the Earth (i.e., not in space) that are used for operating the spacecraft and processing the received data.

Image: The result of the binning and co-addition on data from a 2-dimensional detector array.

Image ancillary data: Data generated on-board by the payload in support of the observation data, needed to process the measurement data on ground.

Image pixel: A single pixel of an image; i.e. the result of binning and co-addition on pixel-level.

Image swath: The projection of the field of view on the Earth.

Instrument: The parts of the satellite used to acquire the data that will generate the mission products. For TROPOMI, the instrument is the payload.

Irradiance: The density of radiant flux incident on a surface per unit of wavelength.

Measurement: Data acquired during one co-addition period of the instrument. Several frames are co-added in the instrument during one co-addition period. The measurement is the smallest granule of data that is downlinked and available on-ground.

Metadata: Data about data. Metadata for data products describes at a high level the contents of these data products. For example, temporal or geospatial coverage, QA statistics and/or information on the software and input products that were used to generate the data product.

Nadir: Geocentric nadir is defined as the line from the centre of the satellite to the centre of the Earth. Geodetic nadir is defined as the line from the centre of the satellite to the perpendicular of the reference ellipsoid tangent. When not specifically identified, nadir will refer to the geocentric nadir.

Near Real-Time (NRT): Mission products that (compared to the standard products) need to be sent to the user within a reduced amount of time from sensing (typically 3 hours).

Observation data: All scientific data generated by the payload.

Payload: The equipment, carried on-board the spacecraft, that directly relates to the purpose of the flight; a facility of the spacecraft which directly performs the mission-unique science functions. For TROPOMI, the payload is the instrument.

Performance: The degree to which a system or component accomplishes its designated functions within given constraints regarding processing time and throughput rate.

Pixel: Picture element: the set of measurements taken for a given location at a given time.

Radiance: A measure of the energy radiated by the object together with the frequency distribution of that radiation.

Radiometer: An instrument for quantitatively measuring the intensity of electromagnetic radiation in some band of wavelengths in any part of the electromagnetic spectrum. Usually used with a modifier, such as an infra-red radiometer or a microwave radiometer.

Reflectance: Ratio of the intensity of reflected radiation to that of the incident radiation on a surface. The earth reflectance spectrum is the ratio between the Earth radiance spectrum and the solar irradiance spectrum.

Reprocessing: Repeated processing of data to improve the quality of the data by using better calibration data or improved data processing software.

Sample: A single measurement item, i.e. a single spectral value of a given pixel.

Satellite ancillary data: Data acquired on-board by the satellite in support of the observation data, such as orbit position, velocity and time, attitude (generated by the AOCS sensors) needed to process measurement data on the ground. Depending on timing constraints (NRT product or not), these data will be post-processed on-ground to improve the accuracy of orbit and attitude restitution.

Scattering: The process in which a wave or beam of particles is diffused or deflected by collisions with particles of the medium which it traverses.

Spectral band: A set of contiguous narrow spectral channels. The number of spectral channels in the spectral band is the spectral range of the band divided by the spectral sampling interval. In the context of TROPOMI, a spectral band also corresponds to one half-section of a detector and its associated electric read-out chain.

Spectral channel: Detector pixel targeted at measuring a specific wavelength (range) as described by the ISRF. Therefore, a spectral channel is the smallest spectral detection unit in the instrument.

Spectral pixel: Spectral channel.

Spectrometer: A device used to measure radiant intensity or to determine the wavelengths of various radiations. A spectrometer in TROPOMI includes the full electro-optical chain, from the common telescope to the detector and its read-out chain.

Sun zenith angle: Angle between the sun direction and the local zenith defined in the target reference frame.

Wavelength band: Spectral band.