Difference between revisions of "Standard Names For Satellite Observations"
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Revision as of 16:36, April 15, 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.
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:
|
Proposal #2
| Standard name | sensor_band_identifier |
| Canonical units | string |
| 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 | time_sample_difference_due_to_collocation |
| Canonical units | s |
| Definition | 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 | 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_radiance_per_unit_wavenumber |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | "toa" means top of atmosphere; "outgoing" means toward outer space. Radiance is the radiant power per unit area in a particular direction per unit of solid angle. |
| Standard name | toa_outgoing_radiance_per_unit_wavelength |
| Canonical units | mW m-2 sr-1 um-1 |
| Definition | "toa" means top of atmosphere; "outgoing" means toward outer space. Radiance is the radiant power per unit area in a particular direction per unit of solid angle. |
| Standard name | toa_outgoing_radiance_per_unit_wavenumber_mean_within_collocation_target |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | An average of toa_outgoing_radiance_per_unit_wavenumber 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_radiance_per_unit_wavenumber_stdev_within_collocation_target |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | Standard deviation of toa_outgoing_radiance_per_unit_wavenumber 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_radiance_per_unit_wavenumber_mean_within_collocation_scene |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | An average of toa_outgoing_radiance_per_unit_wavenumber 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_radiance_per_unit_wavenumber_stdev_within_collocation_scene |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | Standard deviation of toa_outgoing_radiance_per_unit_wavenumber 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_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. |
| Standard name | covariance_between_constant_and_linear_terms_of_radiance_per_unit_wavenumber_correction_due_to_intercalibration |
| Canonical units | mW m-2 sr-1 (cm-1)-1 |
| Definition | Covariance between constant_term_of_radiance_per_unit_wavenumber_correction_due_to_intercalibration and linear_term_of_radiance_per_unit_wavenumber_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 wavelength, frequency, or 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 wavelength, frequency, or 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. |
