FIDUCEO Vocabulary

An uncertainty given in the same unit as the measured value. This is generally written as the standard uncertainty u(x_i).

The AEROCOM-project is an open international initiative of scientists interested in the advancement of the understanding of the global aerosol and its impact on climate

Atmospheric Infrared Sounder

Aerosol Optical Depth

An offset (additive) or scaling factor (multiplicative) that affects all measurements by a particular instrument. The bias may be estimated, in which case it can be corrected for (a correction), or may be an unknown error.

Calibration is the process of converting the raw signal recorded by the satellite to the measurand. Examples include converting raw AVHRR counts to a radiance or brightness temperature. The calibration process is normally defined by an algorithm and a set of calibration coefficients.

(ESA) Climate Change Initiative

Climate Data Store

Climate and Forecast (metadata convention)

European Centre for Medium Range Weather Forecasts

European Reanalysis of Global Climate Observations -2

Expanded uncertainty is the standard uncertainty multiplied by a coverage factor, k. The coverage factor is chosen to obtain a desired level of confidence. Most commonly a 95 % confidence interval is chosen. For a Gaussian distribution this is achieved with a coverage factor k = 2. (Note that strictly this provides a 95.45 % confidence interval).

This term refers to the process by which the geographical coordinates (e.g., latitude and longitude) of each satellite measurement are determined. The precise determination of geographical coordinates requires information on the time, satellite orbit, the satellite attitude parameters, and the Geoid. In the case of absence of this information, geographical coordinates are often determined by techniques that use landmarks and control points. The result of geo-location is a geo-referenced satellite measurement without a change in the original geometry of the measurement.

This term refers to satellite measurements that have been geo-located or navigated.

GCOS Reference Upper Air Network

A harmonised satellite series is one where all the calibrations of the sensors have been done consistently relative to reference datasets which can be traced back to known reference sources, in an ideal case back to SI. Each sensor is calibrated to the reference in a way that maintains the characteristics of that individual sensor such that the calibration radiances represent the unique nature of each sensor. This means that two sensors which have been harmonised may see different signals when looking at the same location at the same time where the difference is related to known differences in the responses of each sensor such as differences in the sensors spectral response functions etc.

Unlike harmonisation, homogenisation is where all satellites are forced to look the same such that when looking at the same location at the same time they would (in theory) give the same signal. In reality the signals from different sensors would be different and homogenisation is adding in corrective terms to each satellite to make them look the same. It is likely that these corrective terms will not be 100% effective and that the process of homogenisation will add in scene dependent errors to the uncertainty budget which may be difficult to assess.

Intercalibration is the process of cross comparing one satellite with another dataset used as a reference. Often the reference dataset is another satellite whose calibration is better characterised and/or updated relative to the satellite of interest and so can be used to recalibrate and/or provide information on problems/biases in the satellite of interest.

Local Area Coverage

(CEOS definition)
Reconstructed, unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters (e.g., platform ephemeris) computed and appended but not applied to Level 0 data.

Level 1B data that have been georeferenced in a standard georeferenced grid.

Variables mapped on uniform space-time grid scales, usually with some completeness and consistency.

A sensor “point” measurement that is matched with another sensor’s “point” measurement sufficiently close in space and time.

In a more practical context, a match up consists of two or more sensor pixels (with optional surrounding pixels) that have been acquired at almost the same time and cover almost the same location. Time and spatial difference allowed are defined by the scientific aim. The match up data consists of all data variables at the location and all metadata for each sensor.

The number and unit obtained from a measurement of a measurand.

The number, unit and uncertainty of a measurand that comes from measurement

Meteosat Visible Infra-Red Imager

NetCDF (network Common Data Form) is a set of software libraries and machine-independent data formats that support the creation, access, and sharing of array-oriented scientific data

Numerical Weather Prediction

This term refers to a systematic transformation of the latitudes and longitudes of locations on the surface of a sphere or an ellipsoid into locations on a plane. Different transformations have been developed and they vary in terms of the priorities they assign on the conservation of angles, area, or distance and on the region of the globe they are optimized for. Projection results in gridded satellite measurements in a specific type of projection.

Means that the error in a measured value is considered to be a stochastic independent draw from an underlying probability distribution; “random” implies in this context both “unpredictable” and “uncorrelated across measurements”; random errors therefore tend to “average out” across many measured values; random effects may be operating at the same time as other types of effect, in which case only a component of the total error is random; an example of a random effect (an effect giving rise to random errors) is electronic noise in an amplifier circuit.

This term refers to the process of transforming the information represented in one grid into another grid.

A recalibrated dataset is one where the calibration coefficients and/or the calibration algorithm has been updated relative to the operational calibration used to create the original satellite Level 1 datasets. The operational calibration is normally derived from pre-launch measurements and there are many instances where the pre-launch data/algorithm is insufficient to calibrate the sensor in orbit either due to changes in the satellite response while in orbit or due to problems with the pre-launch data/algorithm itself or both.

Scene normalisation is a process which attempts to remove some of the variance seen in EO data to create values that are independent of view angle or atmospheric state or observing time etc. to give a uniform measure of a given variable across an image. It can be considered as a method to give what would have been observed by the same instrument under viewing identical conditions.

Some Level 1 correction schemes involve determining corrections to already calibrated radiances based on some defined reference. These sensor bias corrections can then be applied to correct for gross errors in the original calibration. One example of this are the corrections provided by the GSICS (Global Space-based Inter-Calibration System) consortium for a number of sensors. Note that the terms “Harmonisation” and “Homogenisation” can be applied to this form of correction.

International sytem of units

Sea and Land Surface Temperature Radiometer

Spectral Response Function

Special Sensor Microwave/Temperature-2

Standard uncertainty describes the standard deviation of the probability distribution describing the spread of possible values.

This term refers to the data that a satellite collects by scanning the area below its current location, i.e., the swath or the width of this area perpendicular to the satellite’s flight direction.

Effects for a particular measurement process that do not vary (or vary coherently) from (one set of) measurement(s) to (another set of) measurement(s) and therefore produce systematic errors that cannot be reduced by averaging.

Traceability is defined by the Committee of Earth Observation Satellites (CEOS) as:

Traceability involves both an unbroken chain to that reference – a clear link of “A was calibrated against B, which was calibrated against C and so on to the reference” and the documentary evidence that each step was performed in a reliable way, with clear uncertainty analysis in the form of an uncertainty budget for each step which includes the previous step as input as well as the uncertainties introduced by the current step. Ideally this documentation is reviewed through peer review or formal audit.

Note that there are other common uses of the term “traceability” including that it is possible to “trace” the origin of all the input data sets and that there are appropriate algorithmic documents (e.g. ATBDs) and that software is formally checked. These are all important aspects of a quality system. Metrological traceability includes all this, and also the unbroken chain of calibration and uncertainty analysis.

The GUM describes Type B methods for evaluating uncertainty as using “other methods”. This may include prior knowledge (e.g. from a calibration certificate or the behaviour of similar instruments), it may include performing theoretical modelling.

The GUM defines uncertainty as:

Uncertainty is a measure of the spread of the distribution of possible values.

A record of calibrated, geolocated, directly measured satellite observations in geophysical units (such as radiance) in which estimates of total uncertainty (or error covariance) and/or dominant components of uncertainty (or error covariance) are provided or characterised at pixel-level (and potentially larger) scales. The FCDR should be provided with all relevant auxiliary information for the data to be meaningful, including, e.g. time of acquisition, longitude and latitude, solar and viewing angles, sensor spectral response.

Vicarious calibration is a method that makes use of “invariant” natural targets of the Earth for the post-launch calibration of a sensor. This is most commonly used for reflectance channels where there is no on-board calibration source available to track changes in the instrument response.