Standard Names For Satellite Observations

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The Climate and Forecast (CF) metadata convention maintains a list of standard names for data stored in variables of a netCDF file. Not many standard names in that list are relevant to satellite observation data so additional names are proposed here. The new names and accompanying information will follow the CF guidelines.

Contents

Instruments or Sensors?

There is some uncertainty about the hierarchy required to completely describe devices that are used to collect data in the field. This uncertainty boils down to whether this hierarchy requires two levels, i.e. platform and instrument, or three levels, i.e. platform, instrument, and sensor. The simple hierarchy works well in many cases, but there are also clear cases that require three levels, i.e. "platform" that hosts "instruments" that host multiple "sensors".

Various communities have adopted conventional approaches to this nomenclature:

  • GOES-R NcML examples use global attributes "platform_ID" and "instrument_ID" and metadata variables "gcmd_platform_keywords" and "gcmd_instrument_keywords".
  • The NESDIS STAR netCDF template uses global attributes "satellite_name" and "instrument_name"
  • The GHRSST data specification uses global attributes "platform" and "sensor"
  • NPP (HDF) uses global attributes "Platform" and "Instrument"

Of course agreement is more important than justification in this case. We propose to use sensor in these names because it works in the two level case and does not preclude the use of three levels when required.

Definitions from the SensorML Standard

SensorML is an XML dialect for describing processes and processing components associated with the measurement and post-measurement transformation of observations. It is one of the Open Geospatial Consortium standards. The following descriptions of the terms detector, sensor, and platform are taken verbatim from the SensorML's implementation specification document (ref: 07-000; version: 1.0.0, dated: 2007-07-17).

From Section 4, Terms and definitions:

detector
Atomic part of a composite Measurement System defining sampling and response characteristic of a simple detection device. A detector has only one input and one output, both being scalar quantities. More complex Sensors, such as a frame camera, which are composed of multiple detectors can be described as a detector group or array using a System or Sensor. In SensorML a detector is a particular type of Process Model.
sensor
An entity capable of observing a phenomenon and returning an observed value. In SensorML, modeled as a specific type of System representing a complete Sensor. This could be for example a complete airborne scanner which includes several Detectors (one for each band).
(sensor) platform
An entity to which can be attached sensors or other platforms. A platform has an associated local coordinate frame that can be referenced to an external coordinate reference frame and to which the frames of attached sensors and platforms can be referenced.

The SensorML document does not specifically define the term instrument.

Accepted Standard Names

Below are listed proposed standard names from this page that are now accepted in the official CF standard name table.

CF Standard Name Table Version 24, 27 June 2013

Standard name sensor_band_identifier
Canonical units N/A
Definition A variable with the standard name of sensor_band_identifier contains strings which give the alphanumeric identifier of a sensor band. These strings have not yet been standardised.
Standard name sensor_band_central_radiation_wavelength
Canonical units m
Definition sensor_band_central_radiation_wavelength is the central wavelength of a sensor's band, calculated as the first moment of the band's normalized spectral response function.
Standard name sensor_band_central_radiation_wavenumber
Canonical units m-1
Definition sensor_band_central_radiation_wavenumber is the central wavenumber of a sensor's band, calculated as the first moment of the band's normalized spectral response function.
Standard name sensor_band_central_radiation_frequency
Canonical units s-1
Definition sensor_band_central_radiation_frequency is the central frequency of a sensor's band, calculated as the first moment of the band's normalized spectral response function.
Standard name sensor_zenith_angle
Canonical units degree
Definition sensor_zenith_angle is the angle between the line of sight to the sensor and the local zenith at the observation target. This angle is measured starting from directly overhead and its range is from zero (directly overhead the observation target) to 180 degrees (directly below the observation target). Local zenith is a line perpendicular to the Earth's surface at a given location. "Observation target" means a location on the Earth defined by the sensor performing the observations. A standard name also exists for platform_zenith_angle, where "platform" refers to the vehicle from which observations are made e.g. aeroplane, ship, or satellite. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated zenith angle.
Standard name sensor_view_angle
Canonical units degree
Definition Sensor view angle is the angle between the line of sight from the sensor and the direction straight vertically down. Zero view angle means looking directly beneath the sensor. There is no standardized sign convention for sensor_view_angle. A standard name also exists for platform_view_angle, where "platform" refers to the vehicle from which observations are made e.g. aeroplane, ship, or satellite. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated view angle.
Standard name sensor_azimuth_angle
Canonical units degree
Definition sensor_azimuth_angle is the horizontal angle between the line of sight from the observation point to the sensor and a reference direction at the observation point, which is often due north. The angle is measured clockwise positive, starting from the reference direction. A comment attribute should be added to a data variable with this standard name to specify the reference direction. A standard name also exists for platform_azimuth_angle, where "platform" refers to the vehicle from which observations are made e.g. aeroplane, ship, or satellite. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated azimuth angle.
Standard name platform_view_angle
Canonical units degree
Definition Standard names for platform describe the motion and orientation of the vehicle from which observations are made e.g. aeroplane, ship or satellite. Platform view angle is the angle between the line of sight from the platform and the direction straight vertically down. Zero view angle means looking directly beneath the platform. There is no standardized sign convention for platform_view_angle. A standard name also exists for sensor_view_angle. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated view angle.
Standard name platform_azimuth_angle
Canonical units degree
Definition Standard names for platform describe the motion and orientation of the vehicle from which observations are made e.g. aeroplane, ship or satellite. Platform azimuth angle is the horizontal angle between the line of sight from the observation point to the platform and a reference direction at the observation point, which is often due north. The angle is measured clockwise positive, starting from the reference direction. A comment attribute should be added to a data variable with the standard name platform_azimuth_angle to specify the reference direction. A standard name also exists for sensor_azimuth_angle. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated azimuth angle.
Standard name relative_platform_azimuth_angle
Canonical units degree
Definition Standard names for platform describe the motion and orientation of the vehicle from which observations are made e.g. aeroplane, ship or satellite. relative_platform_azimuth_angle is the difference between the viewing geometries from two different platforms over the same observation target. It is the difference between the values of two quantities with standard name platform_azimuth_angle. There is no standardized sign convention for relative_platform_azimuth_angle. "Observation target" means a location on the Earth defined by the sensor performing the observations. A standard name also exists for relative_sensor_azimuth_angle. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated azimuth angle.
Standard name relative_sensor_azimuth_angle
Canonical units degree
Definition relative_sensor_azimuth_angle is the difference between the viewing geometries from two different sensors over the same observation target. It is the difference between the values of two quantities with standard name sensor_azimuth_angle. There is no standardized sign convention for relative_sensor_azimuth_angle. "Observation target" means a location on the Earth defined by the sensor performing the observations. A standard name also exists for relative_platform_azimuth_angle, where "platform" refers to the vehicle from which observations are made e.g. aeroplane, ship, or satellite. For some viewing geometries the sensor and the platform cannot be assumed to be close enough to neglect the difference in calculated azimuth angle.
Standard name toa_outgoing_radiance_per_unit_wavenumber
Canonical units W m-2 sr-1 (m-1)-1
Definition "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.
Standard name toa_outgoing_radiance_per_unit_wavelength
Canonical units W m-2 sr-1 m-1
Definition "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.
Standard name time_sample_difference_due_to_collocation
Canonical units s
Definition time_sample_difference_due_to_collocation is the difference in time between two events that are collocated. Two events are deemed to be collocated based on some set of spatial, temporal, and viewing geometry criteria.
Standard name toa_outgoing_radiance_per_unit_wavenumber_mean_within_collocation_target
Canonical units W m-2 sr-1 (m-1)-1
Definition toa_outgoing_radiance_per_unit_wavenumber_mean_within_collocation_target is an average of observations of the quantity with standard name toa_outgoing_radiance_per_unit_wavenumber from a sensor's adjacent field-of-views within a collocation target. "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The "collocation target" is an area on the Earth's surface at which observations from at least two sensors are collected. Its size is defined by the sensor with the largest field-of-view footprint. Two events are deemed to be collocated based on some set of spatial, temporal, and viewing geometry criteria.
Standard name toa_outgoing_radiance_per_unit_wavenumber_stdev_within_collocation_target
Canonical units W m-2 sr-1 (m-1)-1
Definition toa_outgoing_radiance_per_unit_wavenumber_stdev_within_collocation_target is the standard deviation of observations of the quantity with standard name toa_outgoing_radiance_per_unit_wavenumber from a sensor's adjacent field-of-views within a collocation target. "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The "collocation target" is an area on the Earth's surface at which observations from at least two sensors are collected. Its size is defined by the sensor with the largest field-of-view footprint. Two events are deemed to be collocated based on some set of spatial, temporal, and viewing geometry criteria.
Standard name toa_outgoing_radiance_per_unit_wavenumber_mean_within_collocation_scene
Canonical units W m-2 sr-1 (m-1)-1
Definition toa_outgoing_radiance_per_unit_wavenumber_mean_within_collocation_scene is an average of observations of the quantity with standard name toa_outgoing_radiance_per_unit_wavenumber from a sensor's adjacent field-of-views within a collocation scene. "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The "collocation scene" is a grouping of a sensor's adjacent field-of-views centered on a collocation target. The size of the collocation scene is typically about twice that of the collocation target. The "collocation target" is an area on the Earth's surface at which observations from at least two sensors are collected. Its size is defined by the sensor with the largest field-of-view footprint. Two events are deemed to be collocated based on some set of spatial, temporal, and viewing geometry criteria.
Standard name toa_outgoing_radiance_per_unit_wavenumber_stdev_within_collocation_scene
Canonical units W m-2 sr-1 (m-1)-1
Definition toa_outgoing_radiance_per_unit_wavenumber_stdev_within_collocation_scene is the standard deviation of observations of the quantity with standard name toa_outgoing_radiance_per_unit_wavenumber from a sensor's adjacent field-of-views within a collocation scene. "toa" means top of atmosphere. The TOA outgoing radiance is the upwelling radiance, i.e., toward outer space. Radiance is the radiative flux in a particular direction, per unit of solid angle. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The "collocation scene" is a grouping of a sensor's adjacent field-of-views centered on a collocation target. The size of the collocation scene is typically about twice that of the collocation target. The "collocation target" is an area on the Earth's surface at which observations from at least two sensors are collected. Its size is defined by the sensor with the largest field-of-view footprint. Two events are deemed to be collocated based on some set of spatial, temporal, and viewing geometry criteria.
Standard name toa_brightness_temperature_of_standard_scene
Canonical units K
Definition "toa" means top of atmosphere. The brightness temperature of a body is the temperature of a black body which radiates the same power per unit solid angle per unit area at a given wavelength, wavenumber, or frequency. The standard scene is a target area with typical Earth surface and atmospheric conditions that is accepted as a reference. The toa radiance of the standard scene is calculated using a radiative transfer model for a given viewing geometry. The resultant toa spectral radiance is then integrated with a sensor's spectral response function and converted to equivalent brightness temperature.
Standard name toa_brightness_temperature_bias_at_standard_scene_due_to_intercalibration
Canonical units K
Definition toa_brightness_temperature_bias_at_standard_scene_due_to_intercalibration is the difference between top-of-atmosphere (TOA) brightness temperature of the reference sensor and TOA brightness temperature of the monitored sensor. This TOA brightness temperature difference is a measure of the calibration difference between the monitored and reference sensors. The standard scene is a target area with typical Earth surface and atmospheric conditions that is accepted as a reference. Brightness temperature of a body is the temperature of a black body which radiates the same power per unit solid angle per unit area at a given wavelength, wavenumber, or frequency. TOA brightness temperature of the standard scene is calculated using a radiative transfer simulation for a given viewing geometry. The resultant top-of-atmosphere spectral radiance is then integrated with each sensor's spectral response function and converted to equivalent brightness temperature. The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.

Template for Standard Name Proposals

Standard name ...standard name...
Canonical units ...units...
Definition ...text...

Proposed Names

Proposal #1

Standard name datetime_iso8601
Canonical units string
Definition String representing date-time information according to the ISO 8601:2004(E) standard. Variables with this standard name cannot serve as coordinate variables. Date-time information is in the Gregorian calendar. For dates preceding the Gregorian calendar the date-time information is in the proleptic Gregorian calendar. Possible date-time string forms are:
<datetime> = <date> "T" <time> <timezone> ;

<date> = YYYY "-" MM "-" DD | YYYY "-" DDD ;

<time> = hh | hh ":" mm | hh ":" mm ":" ss | hh ":" mm ":" ss "." S | hh ":" mm ":" ss "," S ;

<timezone> = "" | "Z" | "+" hh | "+" hh ":" mm | "-" hh | "-" hh ":" mm

Where:

  • "YYYY" is a four-digit year (0000-9999).
  • "MM" is a two-digit month of the year (01-12).
  • "DD" is a two-digit day of the month (01-31).
  • "DDD" is a three-digit ordinal day of the year (001-366).
  • "hh" is a two-digit hour (00-23).
  • "mm" is a two-digit minute (00-59)
  • "ss" is a two-digit second (00-59).
  • "S" is one or more digits representing a decimal fraction of the second.
  • The value of any designator when not specified is zero.
  • If <timezone> is ommitted the default value is "Z".

Proposal #2

Standard name constant_term_of_radiance_per_unit_wavenumber_correction_due_to_intercalibration
Canonical units mW m-2 sr-1 (cm-1)-1
Definition Constant term (offset) of the formula for correcting measured spectral radiance expressed as a function of wavenumber. The correction is derived from intercalibration between the monitored and the reference sensor. The resulting corrected spectral radiance of the monitored sensor becomes comparable with measured spectral radiance of the reference sensor. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.
Standard name linear_term_of_spectral_radiance_correction_due_to_intercalibration
Canonical units 1
Definition Linear term (slope) of the formula for correcting measured spectral radiance. The correction is derived from intercalibration between the monitored and the reference sensor. The resulting corrected spectral radiance of the monitored sensor becomes comparable to measured spectral radiance of the reference sensor. "Spectral" means per unit of either wavelength, frequency, or wavenumber. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.
Standard name quadratic_term_of_radiance_per_unit_wavenumber_correction_due_to_intercalibration
Canonical units m2 sr cm-1 mW-1
Definition Quadratic term of the formula for correcting measured spectral radiance expressed as a function of wavennumber. The correction is derived from intercalibration between the monitored and the reference sensor. The resulting corrected spectral radiance of the monitored sensor becomes comparable to measured spectral radiance of the reference sensor. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.
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