Difference between revisions of "Standard Names For Satellite Observations"

Revision as of 20:34, March 19, 2013

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.

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.

Proposed Names

Standard name	sensor_band_identifier
Canonical units	N/A
Definition	Alphanumeric identifier of a sensor band.

Standard name	sensor_band_central_wavelength
Canonical units	m
Definition	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_wavenumber
Canonical units	m-1
Definition	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_frequency
Canonical units	Hz
Definition	The central frequency of a sensor's band, calculated as the first moment of the band's normalized spectral response function.

Standard name	collocation_time_interval
Canonical units	s
Definition	The temporal difference between two measurement events being collocated. Collocation is a process in remote sensing of grouping two independent measurements based on a set of spatial, temporal, and viewing geometry criteria.

Standard name	datetime_iso8601
Canonical units	N/A
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".

Standard name	sensor_zenith_angle
Canonical units	degree
Definition	The angle between the line of sight to the sensor and the local zenith; a value of zero is directly overhead.

Standard name	platform_look_angle
Canonical units	degree
Definition	"platform" refers to the vehicle from which observations are made e.g. airplane, ship, or satellite. Platform look angle is the angle between the line of sight from the platform and the direction straight vertically down. Zero look angle means looking directly beneath the platform.

Standard name	sensor_look_angle
Canonical units	degree
Definition	The angle between the line of sight from the sensor and the direction straight vertically down. Zero look angle means looking directly beneath the sensor.

Standard name	platform_azimuth_angle
Canonical units	degree
Definition	"platform" refers to the vehicle on which the sensor making observations is mounted on, e.g. airplane, ship, or satellite. Platform azimuth angle is the horizontal angle where the observation target is at the vertex, one side of the angle points to the reference direction (typically due north) and the other side points to the platform. The angle is measured clockwise starting from the reference direction. The observation target is a location on the Earth defined by the sensor performing the observations.

Standard name	sensor_azimuth_angle
Canonical units	degree
Definition	The horizontal angle with the observation target at its vertex, one side of the angle points to the reference direction (typically due north) and the other side points to the sensor. The angle is measured clockwise starting from the reference direction. The observation target is a location on the Earth defined by the sensor performing the observations.

Standard name	relative_platform_azimuth_angle
Canonical units	degree
Definition	Difference between two platform_azimuth_angle values.

Standard name	relative_sensor_azimuth_angle
Canonical units	degree
Definition	Difference between two sensor_azimuth_angle values.

Standard name	toa_outgoing_spectral_radiance
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	"toa" means top of atmosphere; "outgoing" means emitted toward outer space; "spectral" means per unit wavenumber or as a function of wavenumber. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.

Standard name	toa_outgoing_spectral_radiance_mean_within_collocation_target
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	An average of toa_outgoing_spectral_radiance observations from sensor's adjacent field of views within a collocation target. 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.

Standard name	toa_outgoing_spectral_radiance_stdev_within_collocation_target
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	Standard deviation of toa_outgoing_spectral_radiance observations from sensor's adjacent field of views within a collocation target. 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.

Standard name	toa_outgoing_spectral_radiance_mean_within_collocation_scene
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	An average of toa_outgoing_spectral_radiance observations within a collocation scene. Collocation scene is a grouping of sensor's adjacent field of views (FOVs) centered on a collocation target. 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 FOV footprint. Collocation scene's size is typically about twice the size of its collocation target.

Standard name	toa_outgoing_spectral_radiance_stdev_within_collocation_scene
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	Standard deviation of toa_outgoing_spectral_radiance observations within a collocation scene. Collocation scene is a grouping of sensor's adjacent field of views (FOVs) centered on a collocation target. 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 FOV footprint. Collocation scene's size is typically about twice the size of its collocation target.

Standard name	constant_term_of_spectral_radiance_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. 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. "Spectral" means per unit wavenumber or as a function of wavenumber. 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 wavenumber or as a function of wavenumber. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.

Standard name	quadratic_term_of_spectral_radiance_correction_due_to_intercalibration
Canonical units	m2 sr cm-1 mW-1
Definition	Quadratic term 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 wavenumber or as a function of wavenumber. Radiance is the radiant power per unit area in a particular direction per unit of solid angle.

Standard name	covariance_between_constant_and_linear_terms_of_spectral_radiance_correction
Canonical units	mW m-2 sr-1 (cm-1)-1
Definition	Covariance between constant_term_of_spectral_radiance_correction_due_to_intercalibration and linear_term_of_spectral_radiance_correction_due_to_intercalibration values.

Standard name	toa_brightness_temperature_of_standard_scene
Canonical units	K
Definition	"toa" means top of atmosphere. 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 wavenumber. Standard scene is a target area with typical Earth surface and atmospheric conditions that is accepted as a reference. The top-of-atmosphere radiance of the standard scene is calculated using a radiative transfer model for a given viewing geometry. The resultant top-of-atmosphere 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_wrt_intercalibration
Canonical units	K
Definition	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. 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 wavenumber. 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.

Table Template for Standard Name Proposals

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

@@ Line 5: / Line 5: @@
 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".
+*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"
@@ Line 94: / Line 94: @@
 |-
 |'''Definition'''
-|String representing date-time information according to the ISO
+|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:
-: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:
 <pre>