A measure of the ability of a surface or a medium to absorb incident energy, often at specific wavelengths.
The difference between a result obtained and the 'true' value.
The (footprint) direction perpendicular to the flight direction of the satellite.
The ratio of the radiation reflected from an object to the total amount incident upon it, for a particular portion of the spectrum.
A set of defined instructions for calculating a function.
The (footprint) direction parallel to the flight direction of the satellite.
Along track data
Data chronologically ordered, following the satellite "ground track". **Altitude: ** distance from the satellite to the reference ellipsoid.
The combination of image ancillary data and satellite ancillary data. A classical definition is 'All on-board data, other than observation and HKTM data, necessary for the product processing'. This would include in particular not only various parameters and settings but also satellite data such as OBT and time correlations if needed, navigation data, etc.
A data set containing data other than instrument measurements and ancillary data, for instance pixel quality data and ECMWF data derived from Level-1 processing.
The ascending node of an orbit is the intersection of that orbit, when the satellite goes from the southern to the northern hemisphere, with the x-y plane of the Earth fixed reference frame.
The Auxiliary Data describes all auxiliary information that is used by the ESA Copernicus Ground Segment for the processing of MSI data. The auxiliary data required in MultiSpectral Instrument (MSI) data generation are:
European Centre for Medium-Range Weather Forecasts (ECMWF): ozone, surface pressure and water vapour data required for Level-1C processing
Copernicus Atmosphere Monitoring Service (CAMS) auxiliary parameters
International Earth Rotation & Reference Systems service (IERS) data
Precise Orbit Determination (POD) data.
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).
Backscatter is the portion of the outgoing radar signal that the target redirects directly back towards the radar antenna. Backscattering is the process by which backscatter is formed. The scattering cross section in the direction toward the radar is called the backscattering cross section; the usual notation is the symbol sigma (?). It is a measure of the reflective strength of a radar target. The normalised measure of the radar return from a distributed target is called the backscatter coefficient, or sigma nought , and is defined as per unit area on the ground. If the signal formed by backscatter is undesired, it is called clutter. Other portions of the incident radar energy may be reflected and scattered away from the radar or absorbed.
A selection of wavelengths or range of radar frequencies. For example Sentinel-1 operates at C-band (central frequency of 5.404 GHz). Other spaceborne SAR's operate at L-band (1.3 GZ) or X-band (9.6 GHz).
A selection of wavelengths or range of radar frequencies. For example Sentinel-1 operates at C-band (central frequency of 5.404 GHz). Other spaceborne SAR's operate at L-band (1.3 GZ) or X-band (9.6 GHz).
The SAR operating configuration defined by the swath width and spatial resolution.
The area within the total possible swath that is actually illuminated while being governed by the characteristics of a specific beam mode.
ß° radar brightness coefficient. The reflectivity per unit area in slant range which is dimensionless.
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.
A nominal frequency range, from 8 to 4 GHz (3.75 to 7.5 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. Imaging radars equipped with C-band are generally not hindered by atmospheric effects and are capable of imaging through tropical clouds and rain showers. Its penetration capability with regard to vegetation canopies or soils is limited and is restricted to the top layers.
Calibration is the process of quantitatively defining the system response to known controlled signal inputs.
Validation is the process of assessing, by independent means, the quality of the data products derived from the system.
The calibration and validation (cal/val) of the MSI instrument and acquisition chain is the process of updating and validating the on-board and on-ground sensor and associated algorithmic parameters in order to meet predetermined product data quality requirements, and to mitigate drift in responses over the lifetime of the mission.
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.
Spectral channel (S1-S9 + F1-F2).
The time interval for which the data that are read out during the individual detector exposures are summed on board the instrument.
Sea surface extending from the coast up to 300 km offshore.
Coherence is the fixed relationship between waves in a beam of electromagnetic (EM) radiation. Two wave trains of EM radiation are coherent when they are in phase. That is, they vibrate in unison. In terms of the application to things like radar, the term coherence is also used to describe systems that preserve the phase of the received signal.
For radar systems, a complex number implies that the representation of a signal, or data file, includes both magnitude and phase values. In the digital SAR context, a complex number is often represented by an equivalent pair of numbers, the real in-phase component (I) and the imaginary quadrature component (Q).
The radiometric counterpart of the context window, obtained by extracting the OLCI channel radiometry corresponding to the context window. If C is a Context imagette W(C) represents the corresponding context window.
Small window (grid) moved around the search window (along shift vectors) in OLCI channel geometry. It is a set of coordinates and is used to extract a context imagette of OLCI channel for correlation with the search chip during the inter-instrument mis-registration estimation.
These grids are the main output of Level-1C processing, stored in the mis-registration data files in the Level-1C product. These are grids that link any OLCI pixel in the reference band to the corresponding sub-pixel location in the other OLCI and SLSTR bands such that if a detector were placed at the sub-pixel location it would have seen the same target on Earth as the reference pixel.
In sun-synchronous orbits, the ground track repeats precisely after a constant integer number of orbits and a constant duration. The duration in days of that period is called the repeat cycle, whereas the corresponding number of orbits is called the cycle length.
Within a given datatake, a portion of image downlinked during a pass to a given station is termed a "datastrip". If a particular orbit is acquired by more than one station, a datatake is composed of one or more datastrips. This happens when the instrument is switched to a new operational mode, such as over oceans. It is expected that the maximum length of a datastrip downlinked to a ground station is approximately 5,000 km.
The continuous acquisition of an image from one Sentinel-2 satellite in a given MSI imaging mode is called a "datatake". The maximum length of an imaging datatake is 15,000 km (continuous observation from northern Russia to southern Africa). However, this would require the potential for continuous acquisition by a single ground station and is currently unlikely. The future introduction of the European Data Relay Satellite (EDRS) geostationary system offers the potential for continuous acquisition of data-takes.
In Level-1C processing this term refers to the interpolation model applied on the (potentially) irregular grid of ground control points and representing the deformation field between OLCI and SLSTR in the OLCI geometry.
Since March 2021, the Digital Elevation Model (DEM) used for the orthorectification process of the L1C is the Copernicus DEM GLO-90m. The Copernicus DEM (COP-DEM) is a Digital Surface Model (DSM) which represents the surface of the Earth including buildings, infrastructure and vegetation and is based on the radar satellite data acquired during the TanDEM-X Mission.
Processing stage at which the strength of the signal is determined for each pixel value. Detection removes phase information from the data file. The preferred detection scheme uses a magnitude squared method, which is energy conserving, and has units of voltage squared per pixel.
Focal plane array, i.e. CCD/CMOS in spectrometer.
Pixel array at band N.
Detector module (DEM)
The sub-assembly including the detector and its read-out electronics (i.e., the Front End Electronics (FEE)).
(Direct) Geolocation function
Function that maps a point (k,j) (possibly non-integers) in an image to a point (x,y,z) on the ellipsoid surface. It is subtended by a model of the line of sight coming from point (k,j).
(Direct) Ortho-rectified geolocation function
Function that maps a point (k,j) (possibly non-integers) in an image to a point (x,y) on the Earth's surface, by taking into account a Digital Elevation Model (DEM) z = DEM(x,y). Theoretically (x,y,z) is the intersection of the line of sight coming from (k,j) with the Earth surface modelled as a DEM on top of a reference ellipsoid. The terrain point location is corrected from the relief effect, compared to the one computed with the direct geolocation function.
The Doppler frequency depends on the component of satellite velocity in the line-of-sight direction to the target. This direction changes with each satellite position along the flight path, so the Doppler frequency varies with azimuth time. For this reason, azimuth frequency is often referred to as Doppler frequency.
The Earth surface is modeled as a Digital Elevation Model (GETASSE30 provided as CFI) on top of the WGS84 ellipsoid model.
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.
The set of measurements acquired by the OLCI instrument at a given time.
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.
The shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth's gravitational field.
Specific definitions and conventions to geolocation and re-sampling are provided below:
The Ellipsoid is defined as the WGS84 Earth reference ellipsoid.
Satellite Actual Reference Frame : The reference frame is close to the Geodetic Pointing Frame on top of which actual attitude is accounted for. It corresponds to the satellite actual (measured) attitude frame. Regardless of attitude, Z axis is aligned with satellite nadir, X axis with velocity and Y completes the right hand system. Nominal attitude would align Z axis with satellite nadir (the local normal to the ellipsoid that passes through satellite's centre of mass) while X remains in the (Z, Velocity) plane. Actual attitude modifies this frame by three consecutive rotations. It is schematically represented on the figure below.
Satellite Actual Reference Frame and Theoretical OLCI Camera Alignment
Earth Fixed Reference Frame: is the IERS Terrestrial Reference Frame details. Z axis points toward North Pole, X axis toward longitude 0 in the Equator plane.
Topocentric Reference Frame: The topocentric is defined at any location at the surface of the Earth reference ellipsoid. Its Z axis corresponds to the outward normal to the ellipsoid; X and Y axes lie in the tangent plane and point toward North and East respectively.
location coordinates (geodetic latitude, longitude) of a point on the ellipsoid, expressed in the Earth fixed coordinates system.
AC pointing angle: the angle between the projection of the LOS on the (Y=0) plane in the satellite fixed coordinates system, and the -Z axis of that system. Bears the sign of y. Notation:
AL pointing angle: the angle between the projection of the LOS on the (X=0) plane in the satellite fixed coordinates system, and the Z axis of that system. Bears the sign of x. Notation:
Across Track and Along Track Pointing Angles
The radar pulse used to measure altimetry is subjected to a number of disturbances as it passes through the atmosphere and when it is reflected by the sea surface.
Global Reference Image (GRI)
The GRI is a set of mono-spectral (B4, red channel) L1B Sentinel-2 images acquired globally, which has been processed to provide accurate relative coherence and an improved geolocation. The GRI is used during the Level-1 processing as a source of reference Ground Control Points (GCPs). Homologous points found between the GRI and the L1B products within the processing chain, are used to refine the geometric model.
Consequently, the GRI have an absolute geolocation performance which allows respecting the following two specifications:
The geolocation of Level-1C products refined with the GRI is better than 12.5 m CE95
The multi-temporal registration performance of refined products is better than 0.5 pixels.
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.
Granules and Tiles
A granule is the minimum indivisible partition of a product (containing all possible spectral bands). The granule size is dependent on the product level.
For Level-0, Level-1A and Level-1B, granules are sub-images of a detector with a given number of lines along track. A granule covers approximately 25 km across-track and 23 km along-track.
For Level-1C, the granules, also called tiles, are 110 km2 ortho-images in UTM/WGS84 projection. Earth is subdivided on a predefined set of tiles, defined in UTM/WGS84 projection and using a 100 km step. However, each tile has a surface of 110x110km² in order to provide large overlap with the neighbouring.
Ground Control point or Correlation point
Landmark, visible and located on two images, where local residual mis-registration between these images is estimated by a matching process.
That part of the Earth's surface that corresponds to an image pixel.
All facilities on the Earth (i.e., not in space) that are used for operating the spacecraft and processing the received data.
The trace made by the sub-satellite point on the surface of the Earth's reference ellipsoid due to the motion of the satellite along its orbit.
Linear polarisation with the electric vector oriented in the horizontal direction in antenna co-ordinates.
A mode of radar polarisation where the microwaves of the electric field are oriented in the horizontal plane for signal transmission, and where the vertically polarised electric field of the backscattered energy is received by the radar antenna.
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.
The image grid contains the radiometric values and the precise latitude/longitude/altitude for each pixel at the channel resolution (0.5 km or 1 km).
A single pixel of an image; i.e. the result of binning and co-addition on pixel-level. Each pixel is a measure of radiance generally gridded, with coordinates (k,j) in an image. k indexes the rows, j indexes the columns.
A line of pixels in the SLST Level-1B product. Note that in the Level-1C product an image scan and an instrument scan should refer to the same thing.
The projection of the field of view on the Earth. Maximum distance on ground between the positions of two spatial samples belonging to the same row.
Most imaging radars produce two-dimensional images. The two dimensions are called range, and azimuth.
Impulse Response Function
Also known as the point spread function, the impulse response function (IRF) is the two-dimensional brightness pattern in an image (after processing) of a point target such as a corner reflector or transponder. The 3dB width of IRF gives the spatial resolution of the product while IRF sidelobes give an indication of the performance of the SAR instrument and the SAR processor.
Infra-red (IR) radiation
Electromagnetic radiation of wavelengths between about 0.75 µm and 15 µm. This is broken down into four wavelength regions:
Near-IR: 0.75-1.4 µm (OLCI Oa12 to Oa21, SLSTR S3 and S4 channels)
Short-Wave IR: 1.4-3 µm (SLSTR S5 and S6 channels)
Medium-Wave IR: 3.8 µm (SLSTR S7 and F1 cannels)
The parts of the satellite used to acquire the data that will generate the mission products. For TROPOMI, the instrument is the payload.
Pixels really acquired by an instrument, before any geometric transformation.
Instrument scan or scan trace
The trace of a single SLSTR detector element on the ground. Thus for example in the thermal channels each detector has two elements, and so a single scan will give two scan traces, displaced by 1 km in the along track direction. Adjacent scan traces represent adjacent 'rows' of the instrument grid.
Inter-channel spatial co-registration
Simply referred to here as co-registration or misregistration: The definition is: maximum equivalent ground distance between the positions of all pairs of spatial samples acquired in two spectral channels and related to the same target on Earth.
Inter-instrument spatial mis-registration
Mis-registration between all the channels within a same instrument.
Inverse Geolocation function
The inverse function of the direct geolocation function.
Inverse Ortho-rectified geolocation function
The inverse function of the direct ortho-rectified geolocation function.
A technique that uses the measured differences in the phase of the return signal between two satellite passes to detect slight changes of locations on the Earth's surface. Mapping height changes provides information on, for example, earthquake damage, volcanic activity, landslides and glacier movement.
The density of radiant flux incident on a surface per unit of wavelength.
Reconstructed unprocessed data at full space-time resolution with all available supplemental information to be used in subsequent processing (e.g. orbit ephemeris) appended. SAR Level-0 products may cover part of an acquisition segment (in case only part of a segment is down linked at a certain ground station in the same pass) or a full acquisition segment. Level-0 products are generated at the receiving station right after acquisition.
Reconstructed data at full resolution, time-referenced, and annotated with ancillary information, including radiometric and geometric calibration coefficients and geo-referencing parameters. Data may be radio-metrically corrected and calibrated at full spatial resolution, and re-sampled to a specified grid.
Derived geophysical parameters derived from Level-1 data.
The radar look direction defines the angle in the horizontal plane in which the radar antenna is pointing when transmitting a pulse and receiving the return signal from the ground or from an object. The look direction is an angular measurement (in degrees) and is usually made with respect to true North. In side-looking imaging radar, the look direction is often orthogonal (normal) to the flight trajectory (azimuth) of the platform carrying the radar and is thus synonymous with the range direction. The radar look direction is an important parameter when analysing features with a preferred orientation, for example fracture patterns in rock formations, regular street patterns, or ocean waves, as these may be enhanced through choice of appropriate radar illumination direction.
Radar terminology refers to individual looks as groups of signal samples in a SAR processor that splits the full synthetic aperture into several sub-apertures, each representing an independent look of the identical scene. The resulting image formed by incoherent summing of these looks is characterised by reduced speckle and degraded spatial resolution. The SAR signal processor can use the full synthetic aperture and the complete signal data history in order to produce the highest possible spatial resolution, albeit very speckled, single-look complex (SLC) SAR image product. Multiple looks may be generated by averaging over range and/or azimuth bandwidths. For an improvement in radiometric resolution using multiple looks there is an associated degradation in spatial resolution.
Measurement Grid (or Instrument Reference Grid)
The instrument grid is the native grid on which the SLSTR collects data as it scans. A scan number identifies the number of the scan line, starting at the Ascending Crossing Node (ANX) – the point at which the satellite crosses the Equator from South to North), while a pixel number identifies the pixel count of any scan. A detector number then maps, for each pixel, the row and the column number (aligned with the Along Track (AL) and Across Track (AC) directions) onto a two-dimensional matrix of detectors. The image grid is aligned with the sub-satellite track
Mean dynamic topography
Oceanic relief corresponding to permanent ocean circulation.
Mean sea level
The sea surface height averaged across all the oceans of the globe. An increase in mean sea level is an indication of possible global warming.
Mean sea surface
A permanent component of sea surface height. The mean sea surface comprises a geoid contribution (approximately 100 m) and a permanent circulation contribution (approximately 1 m).
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.
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.
A form of atmospheric scatter that occurs when radiation interacts with atmospheric particles, the diameter of which is approximately equal to the wavelength of the radiation.
Residual error when all known corrections have been applied. The true value is given by adding the (unknown) (Mis-)Knowledge Error to the restituted value.
Probabilistic method allowing tracing of the path of each photon knowing the inherent (scattering and absorption) characteristics of the medium.
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.
Noise-Equivalent Temperature Difference. The sensitivity of a Thermal Infrared (TIR) sensor in resolving a temperature signal compared to the background noise.
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).
Noise Equivalent Sigma Nought
The backscatter or sigma0 of the thermal noise in SAR imagery.
All scientific data generated by the payload.
The logarithm to base 10 of the inverse of the transmittance.
The absolute orbit number considers the orbits elapsed since the first ascending node crossing after launch. The relative orbit number is a count of orbits from 1 to the number of orbits contained in a repeat cycle. Relative orbit number 1 corresponds to the orbit whose ascending node crossing is closest to the Greenwich Meridian (eastwards).
Orbital Revolution Number
This number identifies the Sentinel-3 orbit within the orbital cycle. There are 385 orbits per cycle, so the Orbital Revolution Number is between 0 and 384.
Orphan pixels or Removed pixels
These are pixels acquired by the instruments but not retained in the Level-1B gridded image, due to the Level-1B (nearest neighbour) projection on the product grid. For OLCI those pixels mainly come from overlapping areas between adjacent camera modules. For SLSTR they may come from a possible oversampled acquisition at nadir of the nadir-view, with respect to the Level-1B image gridding. In oblique view there are many orphans due to scan-to-scan along track overlap. To answer the Level-1C requirements, all those pixels are retained in Level-1B products but not gridded. Note: The expression 'orphan pixel' is used in SLSTR documents while 'removed pixel' is used in OLCI documents. The term 'ungridded pixels' is also used instead of orphan or removed pixels in this document.
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.
The degree to which a system or component accomplishes its designated functions within given constraints regarding processing time and throughput rate.
A photon is a particle of light (or other electromagnetic radiation).
Picture element: the set of measurements taken for a given location at a given time.
The process of confining the vibrations of the magnetic, or electric field, vector of light or other radiation to one plane.
The difference between one result and the mean of several results obtained by the same method, i.e. reproducibility (includes random errors only).
Any data produced by ground segment processing.
Pulse Repetition Frequency (PRF)
Rate of recurrence of the pulses transmitted by a radar.
A medium that accompanies the data and can be used to obtain information necessary to determine the suitability of a product for a certain use or application. Examples of quality indicators are: defective pixels mask, cloud masks, on-line quality control reports, etc.
The team's understanding of the Quality Assurance for Earth Observation (QA4EO) requirement is the need to assign to all data/information products a Quality Indicator (QI) that allows stakeholders to unequivocally evaluate the products' suitability for a particular application. It should be based on a quantitative assessment of its traceability to an agreed reference or measurement standard (ideally SI), but can be presented as numeric or a text descriptor, providing the quantitative linkage is defined.
A measure of the energy radiated by the object together with the frequency distribution of that radiation.
Radiative transfer equation
The equation which describes the radiation passage through a scattering and absorbing medium.
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.
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.
Raman scatter arises when the incident light excites molecules in the sample which subsequently scatters the light. While most of this scattered light is at the same wavelength as the incident light, some is scattered at different wavelengths. This inelastically scattered light is called Raman scatter. It results from the molecule changing its molecular motions.
Distance from the satellite to the Earth's surface.
A form of atmospheric scattering that is caused when radiation interacts with particles, the diameter of which is much smaller than its wavelength. It therefore affects shorter wavelengths.
An arbitrary reference surface that is a raw approximation of the Earth's shape, which is basically a sphere "flattened" at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level.
Repeated processing of data to improve the quality of the data by using better calibration data or improved data processing software.
Value retrieved when all known corrections have been applied.
A single measurement item, i.e. a single spectral value of a given pixel.
In a long synthetic aperture, SAR focusing involves the removal and compensation of path length differences from the antenna to the target on the ground. The main advantage of a focused synthetic aperture is that it increases its array length over those radar signals that can be processed, and thus increases potential SAR resolution at any range. SAR focusing is a necessary process when the length of a synthetic array is a significant fraction of the range to ground being imaged, as the lines-of-sight (range) from a particular point on the ground to each individual element of the array differ in distance. These range differences, or path length differences, of the radar signals can affect image quality. In a focused SAR image these phase errors can be compensated for by applying a phase correction to the return signal at each synthetic aperture element.
A technique for measuring height. Satellite altimetry measures the time taken by a radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver. Combined with precise satellite location data, altimetry measurements yield sea-surface heights.
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.
A scan is defined as a complete rotation of the SLSTR scan mirrors.
Comprises two consecutive scans during which a complete set of targets are sampled.
Scan trace, or scan locus
The trace of a single detector element on the ground. For example, in the thermal channels each detector has two elements, and so a single scan gives two scan traces, displaced by 1 km in the along track direction at the swath centre in the nadir. Adjacent scan traces represent adjacent 'rows' of the instrument grid.
The process in which a wave or beam of particles is diffused or deflected by collisions with particles of the medium which it traverses.
Scattering phase function
The angular function which describes the directional scattering probability as a photon interacts with a scattering particle.
MSI image data is packaged in a set of Instrument Source Packets (ISPs) on-board the satellite. This set of ISPs is called an "on-board scene" or "scene", and corresponds to a simultaneous observation of all bands and all detectors of approximately 3.6 seconds in duration. During this time, the satellite has moved along track by approximately 23 km measured at the Earth's surface.
Sea level anomaly
Variations in the sea surface height with respect to mean sea level. SLAs include seasonal variability.
Sea state bias
Bias due to the sea-surface state, consisting of two components (electromagnetic bias and tracker bias).
Sea surface height
Height of the sea surface with respect to a reference. In altimetry, usually the sea surface height is with respect to the reference ellipsoid.
The radiometric counterpart of the search window, obtained by resampling the SLSTR channel radiometry to the search window. If S is a Search imagette W(S) represents the corresponding search window.
Small window (grid) centered on a ground control point in OLCI geometry. It is a set of coordinates and is used to extract a search imagette of SLSTR channel for correlation with the context window during the inter-instrument mis-registration estimation.
The conventional measure of the strength of a radar signal reflected from a geometric object (natural or manufactured) such as a corner reflector. Sigma specifies the strength of reflection in terms of the geometric cross section of a conducting sphere that would give rise to the same level of reflectivity. (Units of area, such as metres squared).
Sigma Nought (or Sigma 0)
Scattering coefficient, or the conventional measure of the strength of radar signals reflected by a distributed scatterer, usually expressed in dB. It is a normalised dimensionless number, comparing the strength observed to that expected from an area of one square meter. Sigma nought is defined with respect to the nominally horizontal plane, and in general has a significant variation with incidence angle, wavelength, and polarisation, as well as with properties of the scattering surface itself.
Significant wave height
Average wave height (from trough to crest) of the highest third (33.33%) of the waves in a given sample period.
Gives the quantitative change of direction of a ray of light in passing from one medium to another. The product n sin z is the same on both sides of a plane interface between two media, where n is the local refractive index, and z is the local angle the ray makes with the normal to the interface.
Channels with centre wavelength lower than 3.0 µm (SLSTR S1 to S6 and all OLCI channels)
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.
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.
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.
The normal projection of the position of the satellite in orbit on to the surface of the Earth´s reference ellipsoid. It is also referred to as nadir.
Sun zenith angle
Angle between the sun direction and the local zenith defined in the target reference frame.
The width of an imaged scene in the range dimension, measured in either ground range or slant range.
Synthetic Aperture Radar (SAR)
A synthetic aperture radar, or SAR, is a coherent radar system that generates high-resolution remote sensing imagery. Signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture to create an image. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance). High range resolution is achieved through pulse compression techniques.
Either the Earth view or one of the internal calibration targets (the VISCAL and the two black bodies); in the context of the telemetry data it refers to a section of the scan during which valid data is obtained, when the detectors are viewing the earth view or one of the calibration targets.
Channels with centre wavelength larger than 3.0 µm (SLSTR S7 to S9 and F1, F2)
For Level-1C , the granules, also called tiles, are 100 km squared orthoimages in UTM/WGS84 projection.
There are three categories of timeliness defined for Sentinel-2 operation:
Nominal; corresponding to the on-line availability of product being between 3 and 24 hours after sensing.
Near Real-Time (NRT); corresponding to the on-line availability of product being between 100 minutes and 3 hours after data sensing.
Real-Time (RT); corresponding to the on-line availability of product no later than 100 minutes after data sensing.
True Colour Images (TCI)
Following the introduction of the Compact Naming Convention on the 6th of December 2016 (see the Sentinel-2 User Guide Product Naming Convention page for details), a TCI (True Colour Image) in JPEG2000 format is included within the Tile folder of Level-1C products in this format(1). The TCI is an RGB image built from the B02 (Blue), B03 (Green), and B04 (Red) Bands. The reflectances are coded between 1 and 255, 0 being reserved for 'No Data'. The saturation level of 255 digital counts correspond to a level of 3558 for L1C products or 2000 for L2A products (0.3558 and 0.2 in reflectance value respectively).
(1) Products acquired before the 6th of December 2016, and products reprocessed within the Systematic Processing and Reprocessing Services still use the old format Naming Convention.
Linear polarisation with the electric vector oriented in the vertical direction in antenna co-ordinates.
A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for signal transmission, and where the horizontally polarised electric field of the backscattered energy is received by the radar antenna.
A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for both signal transmission and reception by means of a radar antenna.
Electromagnetic radiation detectable by the human eye with a wavelength between approximately 400 nm and 700 nm (OLCI Oa1 to Oa11 and SLSTR S1 and S2 channels)
The magnitude and shape of the radar altimetry return echoes.
Zero Doppler Time
Zero Doppler time is the along track (azimuth) time at which a target on the ground would have a Doppler shift of zero with respect to the satellite (i.e. when the target was perpendicular to the flight path). Also called the closest approach azimuth time. The SAR processor locates targets in the image at the zero-Doppler azimuth time.