CDF Format Guide

1. Introduction

The SWXSchema class provides an interface to examine the SWxSOC CDF Format Guide.

1.1 Purpose and Scope

This document is provided as a reference for construction of standard CDF files. It is intended to complement information available from the Space Physics Data Facility (listed in Sec. 1.2). It lays down REQUIREMENTS and RECOMMENDATIONS for Level 2 (and above) CDF files that are intended for public access, and should be taken as RECOMMENDATIONs for all other mission CDFs. This document is based on discussions with personnel at NASA’s Space Physics Data Facility (SPDF). It is intended to provide sufficient reference material to understand CDF files and the requirements for creating mission-specific CDF files.

1.2 References

Relevant documents that provide background material and support details provided in this guide are listed below:

• SPDF CDF User’s Guide

• SKTEditor

• ISTP Guidelines

• ISTP/IACG Global Attributes

2. Conventions

Scientific data products that will be shared between SWxSOC entities (e.g. missions, ITFs, IDS groups) or made available to the general research community, stored as CDF data files, are expected to be compatible with CDF version 3.5. Data that will not be shared beyond an individual team may be stored in any format that is convenient for that team.

2.1 Science Product Naming Conventions

Data products will be produced with the following filename format where the individual identifying components are described in Table 2-1. Additionally, to ensure software compatibility between disparate systems, filenames will consist of all lowercase characters. Filenames are used as a system identifier for a logical grouping of data and are also stored in the Logical_file_id global attribute field (see Section 3.1). It is expected that filenames will be created dynamically from the attributes identified in Section 3 of this document.

Filename Format

scId_instrumentId_mode_dataLevel_optionalDataProductDescriptor_startTime_vX.Y.Z.ext

Table 2-1: Filename Component Description

Short Name	Description	Valid Options
scID	Spacecraft ID	hermes, gdc, padre, …
instrumentId	Instrument or investigation identifier shortened to three letter acronym.	eea, mrt, nms, spn, meddea, …
mode	TBS	default, brst, …
dataLevel	The level to which the data product has been processed	l0, l1, ql, l2, l3, l4
optionalDataProductDescriptor	This is an optional field that may not be needed for all products. Where it is used, identifier should be short (e.q. 3-8 characters) descriptors that are helpful to end-users. If a descriptor contains multiple components, underscores are used to separate those components.	An optional time span may be specified as “2s” to represent a data file that spans two seconds. In this case, “10s” and “5m” are other expected values that correspond with ten seconds and 5 minutes respectively.
startTime	The start time of the contained data given in ISO 8601 format.	20230519T000003
vX.Y.Z	The 3-part version number of the data product. Full description of this identifier is provided in Section 3.1.1 of this document.	v0.0.0, v
.ext	The required file extension, where CDF is required.	cdf

2.1.1 Version Numbering Guidelines

The three-part version number contains the interface number, quality number, and bug fix/revision number. The initial release of CDF data that is suitable for scientific publication should begin with “v1.Y.Z”. Each component of the version number is incremented in integer steps, as needed, and Table 3-2 describes the instances in which the value should be incremented. Release “v0.Y.Z” may be used for early development purposes.

Table 2-2: Version Numbering Guidelines

Part	Name	Description
X	Interface Number	Increments in this number represent a significant change to the processing software and/or to the content/structure of the file. These changes may be incompatible with existing code. Increments in this number may require code changes to software.
Y	Quality Number	This number represents a change in the quality of the data in the file, such as change in calibration or increase in fidelity. Changes should not impact software but may require consideration when processing data.
Z	Bug Fix / Revision Number	This number changes to indicate minor changes to the contents of the file due to reprocessing of missing data. Any dependent data products should generally be reprocessed if this value changes.

3. Global Attributes

Global attributes are used to provide information about the data set as an entity. Together with variables and variable attributes, the global attributes make the data correctly and independently usable by someone not connected with the instrument team, and hence, a good archive product.

The required, recommended, and optional global attributes that have been identified for use with ISTP-compliant data products are listed below. Additional global attributes can be defined but they must start with a letter and can otherwise contain letters, numbers, and the underscore character (no other special characters allowed). Note that CDF attributes are case-sensitive and must exactly follow what is shown here.

Detailed descriptions of the attributes listed below are available at the ISTP/IACG Global Attributes Webpage.

3.1 Required Global Attributes

The following global attributes shown in Table 3-1 are required with ISTP-compliant data products. For each attribute the following information is provided:

• description: (str) A brief description of the attribute

• default: (str) The default value used if none is provided

• derived: (bool) Whether the attibute can be derived by the swxsoc SWXSchema class

• required: (bool) Whether the attribute is required by ISTP standards

• validate: (bool) Whether the attribute is included in the validate() checks (Note, not all attributes that are required are validated)

• overwrite: (bool) Whether the SWXSchema attribute derivations will overwrite an existing attribute value with an updated attribute value from the derivation process.

Note that this table is derived from swxsoc/data/swxsoc_default_global_cdf_attrs_schema.yaml

Table 3-1: Required Global Attributes

Attribute	description	default	derived	required	validate	overwrite
CDF_Lib_version	Version of the CDF Binaries library used to generate the CDF File		True	False	False	False
Data_level	This attribute is used in file name creation and records the level of processsing done on the dataset. Valid values: - l0>Level 0 - l1>Level 1 - l2>Level 2 - l3>Level 3 - l4>Level 4 - ql>Quicklook		False	True	True	True
Data_product_descriptor	This is an optional field that may not be needed for all products. Where it is used, identifier should be short (e.q. 3-8 characters) descriptors that are helpful to end- users. If a descriptor contains multiple components, underscores are used to separate those components.		False	False	False	True
Data_type	This attribute is used by CDF file writing software to create a filename. It is a combination of the following filename components: mode, data level, and optional data product descriptor.		True	True	True	True
Data_version	This attribute identifies the version (vX.Y.Z) of a particular CDF data file.		False	True	True	False
Descriptor	This attribute identifies the name of the instrument or sensor that collected the data. Both a long name and a short name are given. For any data file, only a single value is allowed. For example, from the HERMES mission, the following are valid values: - EEA>Electron Electrostatic Analyzer - MERIT>Miniaturized Electron pRoton Telescope - NEMISIS> Noise Eliminating Magnetometer In a Small Integrated System - SPAN-I>Solar Probe Analyzer for Ions		False	True	True	False
Discipline	This attribute describes both the science discipline and sub discipline. The list for space physics is: “Space Physics>Magnetospheric Science”, “Space Physics>Interplanetary Studies”, “Space Physics>Ionospheric Science”		False	True	True	False
Generation_date	Date stamps the creation of the file using the syntax yyyymmdd, e.g., “		True	False	False	True
SWxSOC_version	Version of swxsoc originally used to generate the given CDF File		True	False	False	False
Instrument_type	This attribute is used to facilitate making choices of instrument type. More than one entry is allowed. Acceptable values include: - Magnetic Fields (space) - Particles (space) - Plasma and Solar Wind - Ephemeris -> Ephemeris/Attitude/Ancillary		False	True	True	False
Logical_file_id	This attribute stores the name of the CDF file but without the file extension (e.g. “.cdf”). This attribute is required to avoid loss of the original source in the case of accidental (or intentional) renaming.		True	True	True	True
Logical_source	This attribute determines the file naming convention in the SKT Editor and is used by CDA Web. It is composed of the following values: - source_name - (e.g. spacecraft identifier) - descriptor - (e.g. instrument identifier - see Section Error! Reference source not found.) - data_type - (e.g. mode, data level, and optional data product descriptor - value come from ‘Data_type’ attribute)		True	True	True	True
Logical_source_description	This attribute writes out the full words associated with the encrypted Logical_source above, e.g., “Level 1 Dual Electron Spectrometer Survey Data”. Users on CDAWeb see this value on their website.		True	True	True	True
Mission_group	This attribute has a single value and is used to facilitate making choices of source through CDAWeb.		False	True	True	False
PI_affiliation	This attribute value should include a recognizable abbreviation.		False	True	True	False
PI_name	This attribute value should include first initial and last name.		False	True	True	False
Project	This attribute identifies the name of the project and indicates ownership. For ISTP missions and investigations, the value used is “ISTP>International Solar-Terrestrial Physics”. For the Cluster mission, the value is “STSP Cluster>Solar Terrestrial Science Programmes, Cluster”. Other acceptable values are “IACG>Inter-Agency Consultative Group”, “CDAWxx>Coordinated Data Analysis Workshop xx”, and “SPDS>Space Physics Data System”. Others may be defined in future. This attribute can be multi-valued if the data has been supplied to more than one project.		False	True	True	False
Source_name	This attribute identifies the mission or investigation that contains the sensors. For ISTP, this is the mission name for spacecraft missions or the investigation name for ground-based or theory investigations. Both a long name and a short name are provided. This attribute should be single valued. Examples:		False	True	True	False
TEXT	This attribute is an SPDF standard global attribute, which is a text description of the experiment whose data is included in the CDF. A reference to a journal article(s) or to a World Wide Web page describing the experiment is essential and constitutes the minimum requirement. A written description of the data set is also desirable. This attribute can have as many entries as necessary to contain the desired information. Typically, this attribute is about a paragraph in length and is not shown on CDAWeb. CDAWeb is the web portal for access to SPDF data, available at https://cdaweb.gsfc.nasa.gov.		False	True	True	False

3.2 Recommended Attributes

The following global attributes are recommended but not required with ISTP-compliant data products.

Table 3-2: Recommended Attributes

Attribute	Description
Acknowledgement	This field indicates how the data should be cited.
Generated_by	This attribute indicates where users can get more information about this data and/or check for new versions.

3.3 Optional Attributes

Table 3-3: Optional Attributes

Attribute	Description
Parents	This attribute lists the parent data files for files of derived and merged data sets. The syntax for a CDF parent is: “CDF>logical_file_id”. Multiple entry values are used for multiple parents.
Skeleton_version	This is a text attribute containing the skeleton file version number.
Rules_of_use	Text containing information on citability and/or PI access restrictions. This may point to a World Wide Web page specifying the rules of use. Rules of Use are determined on both a mission and instrument basis, at the discretion of the PI.
Time_resolution	Specifies time resolution of the file, e.g., “3 seconds”.

4. Variables

There are three types of variables that should be included in CDF files: * data, * support data, * metadata.

Additionally, required attributes are listed with each variable type listed below.

To facilitate data exchange and software development, variable names should be consistent across instruments and spacecraft. Additionally, it is preferable that data types are consistent throughout all data products (e.g. all real variables are CDF_REAL4, all integer variables are CDF_INT2, and flag/status variables are UINT2). This is not to imply that only these data types are allowable within CDF files. All CDF supported data types are available for use by SWxSOC affiliated projects.

For detailed information and examples, please see the ISTP/IACG Webpage

4.1 Data

These are variables of primary importance (e.g., density, magnetic field, particle flux). Data is always time (record) varying but can be of any dimensionality or CDF supported data type. Real or Integer data are always defined as having one element.

4.1.1 Naming

SWxSOC affiliated data variables must adhere to the following naming convention * scId_instrumentId_paramName

An underscore is used to separate different fields in the variable name. It is strongly recommended that variable names employ further fields, qualifiers and information designed to identify unambiguously the nature of the variable, instrument mode and data processing level, with sufficient detail to lead the user to the unique source file which contains the variable. It is recommended that these follow the order shown below.

• scId_instrumentId_paramName[_coordSys][_paramQualifier][_subModeLevel][_mode][_dataLevel]

where the required fields are described in Table 4-1 and the optional fields are described in Table 4-2. An example data variable would be hermes_eea_n_gse_l2.

Table 4-1: Required Data Variable Fields

Required Field Name	Description
scId	Spacecraft identifier, see Table 2-1 for acceptable values
instrumentId	Instrument or investigation identifier, see Table 2-1 for acceptable values and note the caveats listed in Section 4.1.1.1.
paramName	Data parameter identifier, a short (a few letters) representation of the physical parameter held in the variable.

Table 4-2: Optional Data Variable Fields

Optional Field Name	Description
coordSys	An acronym for the coordinate system in which the parameter is cast.
paramQualifier	Parameter descriptor, which may include multiple components separated by a “_” as needed (e.g. “pa_0” indicates a pitch angle of 0).
subModeLevel	Qualifier(s) to include mode and data level information supplementary to the following two fields.
mode	See Table 2-1 for acceptable values.
dataLevel	See Table 2-1 for acceptable values.

4.1.1.1 Caveats

Note the following caveats in the variable naming conventions:

• CDF variable names must begin with a letter and can contain numbers and underscores, but no other special characters.

• In general, the instrumentId field follows the convention used for file names as defined in Section 2.1. However, since variable names cannot contain a hyphen, an underscore should be used instead of a hyphen when needing to separate instrument components. For instance, “eea-ion” is a valid instrumentId in a filename but when used in a variable name, “eea_ion” should be used instead.

• To ensure software compatibility between disparate systems, parameter names will consist of all lowercase characters.

4.1.2 Required Epoch Variable

All ISTP-compliant CDF data files must contain at least one variable of data type CDF_TIME_TT2000, typically named “Epoch”. This variable should normally be the first variable in each CDF data set. All time varying variables in the CDF data set will depend on either this “epoch” variable or on another variable of type CDF_TIME_TT2000 (e.g. hermes_eea_epoch). More than one CDF_TIME_TT2000 type variable is allowed in a data set to allow for more than one time resolution, using the required DEPEND_0 attribute (see Section 5.5) to associate a time variable to a given data variable. It is recommended that all such time variables use “epoch” within their variable name.

For ISTP, but not necessarily for all mission’s data, the time value of a record refers to the center of the accumulation period for the record if the measurement is not an instantaneous one. Time variables used as DEPEND_0 are strongly recommended to have DELTA_PLUS_VAR and DELTA_MINUS_VAR attributes which delineate the time interval over which the data was sampled, integrated, or otherwise representative of. This also locates the timetag within that interval.

The epoch datatype, CDF_TIME_TT2000, is defined as an 8-byte signed integer with the characteristics shown in Table 5-3.

Table 4-3: Characteristics of CDF_TIME_TT2000

Name	Example
time_base	J2000 (Julian date 2451545.0 TT or 2000 January 1, 12h TT)
resolution	nanoseconds
time_scale	Terrestrial Time (TT)
units	nanoseconds
reference_position	rotating Earth Geoid

Given a current list of leap seconds, conversion between TT and UTC is straightforward (TT = TAI + 32.184s; TT = UTC + deltaAT + 32.184s, where deltaAT is the sum of the leap seconds since 1960; for example, for 2009, deltaAT = 34s). Pad values of - 9223372036854775808 (0x8000000000000000) which corresponds to 1707-09- 22T12:13:15.145224192; recommended FILLVAL is same.

It is proposed that the required data variables VALIDMIN and VALIDMAX are given values corresponding to the dates 1990-01-01T00:00:00 and 2100-01-01T00:00:00 as these are well outside any expected valid times.

4.1.3 Required Attributes: Data Variables

Data variables require the following attributes:

• CATDESC

• DEPEND_0

• DEPEND_i [for dimensional data variables]

• DISPLAY_TYPE

• FIELDNAM

• FILLVAL

• FORMAT or FORM_PTR

• LABLAXIS or LABL_PTR_i

• SI_CONVERSION

• UNITS or UNIT_PTR

• VALIDMIN and VALIDMAX

• VAR_TYPE

In addition, the following attributes are strongly recommended for vectors, tensors and quaternions which are held in or relate to a particular coordinate system:

• COORDINATE_SYSTEM

• TENSOR_ORDER

• REPRESENTATION_i

• OPERATOR_TYPE [for quaternions]

4.1.4 Attributes for DEPEND_i Variables

Variables appearing in a data variable’s DEPEND_i attribute require a minimal set of their own attributes to fulfill their role in supporting the data variable. The standard SUPPORT_DATA variable attributes are listed in Section 4.2.2. Other standard variable attributes are optional.

4.2 Support Data

These are variables of secondary importance employed as DEPEND_i variables as described in section 4.1.3 (e.g., time, energy_bands associated with particle flux), but they may also be used for housekeeping or other information not normally used for scientific analysis.

4.2.1 Naming

Support data variable names must begin with a letter and can contain numbers and underscores, but no other special characters. Support data variable names need not follow the same naming convention as Data Variables (4.1.1) but may be shortened for convenience.

4.2.2 Required Attributes: Support Variables

• CATDESC

• DEPEND_0 (if time varying)

• FIELDNAM

• FILLVAL (if time varying)

• FORMAT/FORM_PTR

• LABLAXIS or LABL_PTR_i

• SI_CONVERSION

• UNITS/UNIT_PTR

• VALIDMIN (if time varying)

• VALIDMAX (if time varying)

• VAR_TYPE = “support_data”

Other attributes may also be present.

4.3 Metadata

These are variables of secondary importance (e.g. a variable holding “Bx”, “By”, “Bz” to label magnetic field). Metadata are usually text strings as opposed to the numerical values held in DEPEND_i support data.

4.3.1 Naming

Metadata variable names must begin with a letter and can contain numbers and underscores, but no other special characters. Metadata variable names need not follow the same naming convention as Data Variables (4.1.1) but may be shortened for convenience.

4.3.2 Required Attributes: Metadata Variables

• CATDESC

• DEPEND_0 (if time varying, this value must be “Epoch”)

• FIELDNAM

• FILLVAL (if time varying)

• FORMAT/FORM_PTR

• VAR_TYPE = metadata

4.4 Variable Attribute Schema

The following variable attributes shown in Table 5-4 are required with ISTP-compliant data products. For each attribute the following information is provided:

• description: (str) A brief description of the attribute

• derived: (bool) Whether the attibute can be derived by the swxsoc SWXSchema class

• required: (bool) Whether the attribute is required by ISTP standards

• overwrite: (bool) Whether the SWXSchema attribute derivations will overwrite an existing attribute value with an updated attribute value from the derivation process.

• valid_values: (list) List of allowed values the attribute can take for ISTP-compliant products, if applicable

• alternate: (str) An additional attribute name that can be treated as an alternative of the given attribute. Not all attributes have an alternative and only one of a given attribute or its alternate are required.

• var_types: (str) A list of the variable types that require the given attribute to be present.

Note that this table is derived from swxsoc/data/swxsoc_default_variable_cdf_attrs_schema.yaml

Table 4-4 SWxSOC Variable Attribute Schema

Attribute	alternate	derived	description	overwrite	required	valid_values	var_types
TIME_BASE		True	fixed (0AD, 1900, 1970 (POSIX), J2000 (used by CDF_TIME_TT2000), 4714 BC (Julian)) or flexible (provider-defined)	False	False
RESOLUTION		True	Using ISO8601 relative time format, for example: “1s” = 1 second. Resolution provides the smallest change in time that is measured.	False	False
TIME_SCALE		True	TT (same as TDT, used by CDF_TIME_TT2000), TAI (same as IAT, TT-32.184s), UTC (includes leap seconds), TDB (same as SPICE ET), EME1950 [default: UTC]	False	False
REFERENCE_POSITION		True	Topocenter (local), Geocenter , rotating Earth geoid (used by CDF_TIME_TT2000). Reference_Position is optional metadata to account for time variance with position in the gravity wells and with relative velocity. While we could use a combined TimeSystem attribute that defines mission-specific time scales where needed, such as UTC-at-STEREO-B, it’s cleaner to keep them separate as Time_Scale=UTC and Reference_Position=STEREO-B.	False	False
LEAP_SECONDS_INCLUDED		False	comma-delimited list (within brackets) of leap seconds included in the form of a lists of ISO8601 times when each leap second was added, appended with the size of the leap second in ISO8601 relative time (+/- time, most commonly: “+1s”) [default: standard list of leap seconds up to time of data]. Leap_Seconds_Included is needed to account for time scales that don’t have all 34 (in 2009) leap seconds and for the clocks in various countries that started using leap seconds at different times. The full list is required to handle the equally or more common case where a time scale starts at a pecific UTC but continues on without leap seconds in TAI mode; this is basically what missions that don’t add leap seconds are doing. $ cat tai-utc.dat | awk ‘ORS=”,” { val = $7 - prev } {prev = $7} { print $1$2”01+” val “s” }’	False	False
ABSOLUTE_ERROR		False	Absolute or systematic error, in same units as Units attribute.	False	False
RELATIVE_ERROR		False	Relative or random error, in same units as Units attribute - to specify the accuracy of the time stamps relative to each other. This is usually much smaller than Absolute_Error.	False	False
BIN_LOCATION		False	relative position of time stamp to the data measurement bin, with 0.0 at the beginning of time bin and 1.0 at the end. Default is 0.5 for the time at the center of the data measurement. Since clock readings are usually truncated, the real value may be closer to 0.0.	False	False
CATDESC		False	This is a human readable description of the data variable. Generally, this is an 80- character string which describes the variable and what it depends on.	False	True		data support_data metadata
DEPEND_0		True	Explicitly ties a data variable to the time variable on which it depends. All variables which change with time must have a DEPEND_0 attribute defined.	False	True		data
DEPEND_i		False	Ties a dimensional data variable to a SUPPORT_DATA variable on which the i-th dimension of the data variable depends. The number of DEPEND attributes must match the dimensionality of the variable, i.e., a one-dimensional variable must have a DEPEND_1, a two-dimensional variable must have a DEPEND_1 and a DEPEND_2 attribute, etc. The value of the attribute must be a variable in the same CDF data set. It is strongly recommended that DEPEND_i variables hold values in physical units.	False	False
DISPLAY_TYPE		True	This tells automated software, such as CDAWeb, how the data should be displayed.	False	True	time_series time_series>noerrorbars spectrogram stack_plot image	data
FIELDNAM		True	A shortened version of CATDESC which can be used to label a plot axis or as a data listing heading. This is a string, up to ~30 characters in length.	False	True		data support_data metadata
FILLVAL		True	Identifies the fill value used where data values are known to be bad or missing. FILLVAL is required for time-varying variables. Fill data are always non-valid data.	False	True		data support_data metadata
FORMAT	FORM_PTR	True	This field allows software to properly format the associated data when displayed on a screen or output to a file. Format can be specified using either Fortran or C format codes. For instance, “F10.3” indicates that the data should be displayed across 10 characters where 3 of those characters are to the right of the decimal. For a description of FORTRAN formatting codes see the docs here: https://docs.oracle.com/cd/E19957-01/805-4939/z40007437a2e/index.html	False	True		data support_data metadata
FORM_PTR	FORMAT	False	The value of this field is a variable which stores the character string that represents the desired output format for the associated data.	False	False
LABLAXIS	LABL_PTR_1	True	Used to label a plot axis or to provide a heading for a data listing. This field is generally 6-10 characters. Only one of LABLAXIS or LABL_PTR_i should be present.	False	True		data support_data
LABL_PTR_i	LABLAXIS	False	Used to label a dimensional variable when one value of LABLAXIS is not sufficient to describe the variable or to label all the axes. LABL_PTR_i is used instead of LABLAXIS, where i can take on any value from 1 to n where n is the total number of dimensions of the original variable. The value of LABL_PTR_1 is a variable which will contain the short character strings which describe the first dimension of the original variable. The value of the attribute must be a variable in the same CDF data set and is generally 6-10 characters. Only one of LABLAXIS or LABL_PTR_i should be present.	False	False
SI_CONVERSION		True	The conversion factor to SI units. This is the factor that the variable must be multiplied by in order to convert it to generic SI units. This parameter contains two text fields separated by the “>” delimiter. The first component is the conversion factor and the second is the standard SI unit. Units are defined according to their standard SI symbols (ie. Tesla = T, Newtons = N, Meters = m, etc.) For data variables that are inherently unitless, and thus lack a conversion factor, this data attribute will be “ > “ where ‘ ‘ is a blank space and the quotation marks are not included. Units which are not conveniently transformed into SI should follow the blank syntax “ > “ described above.	False	False		data support_data
UNITS	UNIT_PTR	True	A 6-20 character string that identifies the units of the variable (e.g. nT for magnetic field). Use a blank character, rather than “None” or “unitless”, for variables that have no units (e.g., a ratio or a direction cosine).	False	True		data support_data
UNIT_PTR	UNITS	False	The value of this field is a variable which stores short character strings which identify the units of the variable. Use a blank character, rather than “None” or “unitless”, for variables that have no units (e.g., a ratio or a direction cosine). The value of this attribute must be a variable in the same CDF data set.	False	False
VALIDMIN		True	The minimum value for a particular variable that is expected over the lifetime of the mission. Used by application software to filter out values that are out of range. The value must match the data type of the variable.	False	True		data support_data
VALIDMAX		True	The maximum value for a particular variable that is expected over the lifetime of the mission. Used by application software to filter out values that are out of range. The value must match the data type of the variable.	False	True		data support_data
VAR_TYPE		True	Used in CDAWeb to indicate if the data should be used directly by users.	False	True	data support_data metadata ignore_data	data support_data metadata
WCSAXES		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. The value field shall contain a non-negative integer no greater than 999, representing the number of axes in the associated data array.	False	False
MJDREF		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. The value shall contain a floating point number representing the reference time position of the time stamps along the 0’th axis of the measurement.	False	False
TIMEUNIT		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. The value shall contain a character string giving the units of the time stamps along the 0’th axis of the measurement. The TIMEUNIT should match the CUNITi along the time axis of the measurement	False	False
TIMEDEL		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. The value shall contain a floating point number representing the resolution of the time stamps along the 0’th axis of the measurement. The TIMEDEL should match the CRDELi along the time axis of the measurement.	False	False
CNAMEi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value shall contain a charachter string represnting the name of the i’th axis. The name is used for comment/documentation purposes only and is not used as a part of the i’th axis coordinate transformations.	False	False
CTYPEi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value field shall contain a character string, giving the name of the coordinate represented by axis i.	False	False
CUNITi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value shall be the units along axis i, compatible with CTYPEi to be used for scaling and coordinate transformations along the i’th axis.	False	False
CRPIXi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value field shall contain a floating point number, identifying the location of a reference point along axis i, in units of the axis index. This value is based upon a counter that runs from 1 to NAXISn with an increment of 1 per pixel. The reference point value need not be that for the center of a pixel nor lie within the actual data array. Use comments to indicate the location of the index point relative to the pixel.	False	False
CRVALi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value field shall contain a floating point number, giving the value of the coordinate specified by the CTYPEn keyword at the reference point CRPIXi.	False	False
CDELTi		True	This is a FITS WCS Keyword being repurposed for handling WCS transformations with high-dimensional or spectral CDF data variables. This metadata attribte should be used for the i’th dimension (1-based) and reapeated for all WCSAXES dimensions. The value field shall contain a floating point number giving the partial derivative of the coordinate specified by the CTYPEi keywords with respect to the pixel index, evaluated at the reference point CRPIXi, in units of the coordinate specified by the CTYPEi keyword.	False	False