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EPN2024RI
EUROPLANET2024 Research Infrastructure
H2020INFRAIA20191
Grant agreement no: 871149
Document: VESPAWP62053TNv0.1(31)
doi:10.XXXX/abcd1234
This work is licensed under a $name
Documenting coordinate systems in EPNTAP
Date: $action.dateFormatter.formatGivenString("yyyyMMdd",$content.getLastModificationDate())
Start date of project: 01 February 2020
Duration: 48 Months
Responsible WP Leader: Stéphane Erard
Project cofunded by the European Union's Horizon 2024 research and innovation programme  
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CO  Confidential, only for members of the consortium (excluding the Commission Services) 
Project Number  871149 
Project Title  EPN2024  RI 
Project Duration  48 months: 01 February 2020 – 31 January 2024 
Document Number  WP6task2v 
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Title of Document  Documenting coordinate systems in EPNTAP 
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Dissemination level  PU 
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Abstract: 
Document history (to be deleted before submission to Commission)  
Date  Version  Editor  Change  Status 
13 Nov 2023  first draft, adapted from EPN 2020 document VESPAWP112016TNv0.5(47)  DRAFT  
Table of Contents
Reference documents
 [RD1] EPNTAP standard: https://ivoa.net/documents/EPNTAP/
 [RD2] VESPA document in Europlanet 2020 RI: VESPAWP112016TNv0.5(47): Planetary Coordinate Systems
 [RD3] Vocabularies in the IVOA: https://www.ivoa.net/rdf/
 [RD4] Space time and coordinate in IVOA: http://ivoa.net/Documents/latest/STC.html
 [RD5] Reports from the IAU Working Group on Cartographic Coordinates and Rotation Elements of the Planets and Satellites:
https://astrogeology.usgs.gov/Page/groups/name/IAUWGCCRE and references therein  [RD6] Hare and Malapert (2021) 5th PSIDA conference: https://www.hou.usra.edu/meetings/planetdata2021/pdf/7012.pdf
 [RD7] Marmo et al (2018)
 [RD10] Frä̈nz & Harper (2002), section 4.3.1
 [RD11] Thompson (2006)
Other inputs to be included, TBC:
 Chiara's comments, 7/2014
 IMPEx / 3Dview frames doc, 9/2013
 IAU Celes Mech Comm suggestion for historical frames (~ 10/2015)
 Laura, J. R., Beyer, R. A., 2020. Knowledge Inventory of foundational data products in planetary science. Earth and Space Sci., submitted: https://www.essoar.org/doi/pdf/10.1002/essoar.10501479.1
Acronym list
 EPNCore Set of core parameters from EPNDM, mandatory for EPNTAP compatibility
 EPNTAP Specific protocol to access Planetary Science data in EuroplanetVO
 EPNDM Specific Data Model to describe Planetary Science data in EuroplanetVO
 epn_core Name of table / view of a database which contains the EPNTAP parameters. Required for EPNTAP compatibility
 IVOA International Virtual Observatory Alliance
 IPDA International Planetary Data Alliance
 PDAP (Planetary Data Access Protocol) Protocol to access planetary data space archives, developed and maintained by IPDA.
 TAP (Table Access Protocol) One of the protocols developed by IVOA to access astronomical data.
 ObsTAP TAP protocol applied to the Observation Data Model of IVOA
 ObsCore set of core parameters from the Observation Data Model of IVOA
 ADQL (Astronomical Data Query Language)
 RoI Region of Interest
1  Introduction
EPNTAP coordinates
The EPNTAP protocol allows sharing Solar System data in the Virtual Observatory (VO). It is directly derived from IVOA’s TAP [RD3], a simple protocol to access data organized in tables, here adapted for Planetary Science data. EPNTAP and its implicit Data Model EPNcore are described in [RD1].
In EPNTAP, spatial coordinates are provided via parameters named c1, c2, c3. The spatial_frame_type parameter provides the general “flavor” of the coordinate system and defines the nature of the 3 spatial coordinates. It can have the following values: celestial, body, cartesian, cylindrical, spherical, none (this also applies to related optional spatial parameters such as subsolar_longitude, s_region, etc).
When using some values of the spatial_frame_type parameter, the spatial_coordinate_description parameter allows the data provider to specify more precisely the coordinate system in use and the spatial_origin parameter provides additional information. A related parameter is map_projection, which is used to specify the projection adopted by cartographic products. The system in use is known by the data provider, and these parameters are essentially intended to provide this information to the user for search purpose and crossreferences with other data sets.
A first document [RD2] was produced during the Europlanet 2020 RI programme to identify possible values for these parameters from the literature. The present document aims at constraining possible values in terms of IVOA vocabulary, whenever possible.
An important point to address in this document is related to a historical ambiguity in the definition of spatial_coordinate_description. This parameter was originally intended to document the spatial frame used in the data products, but this has been constantly challenged to document the coordinate system used in the epn_core table instead (c1/2/3 coordinates, etc), with subsequent reversions. In many cases this is identical, but a complete survey is required to identify possible situations. In short:
 c1/2/3 (which are among the main discovery parameters) need to be provided in a consistent way for each value of spatial_frame_type, to allow for uniform queries
 The coordinate frame used in the data products can also be used as a discovery parameter, but is chiefly intended to plot the data correctly in accurate display tools. It can also be used to check what data products can be overplotted in display tools, and potentially to perform automatic coordinate conversions using specialized tools or libraries.
 The indication of the coordinate frame must therefore be provided in a standard way to be understood by tools and libraries.
IVOA vocabularies
IVOA vocabularies are available here: https://www.ivoa.net/rdf
Those of interest in this context are refframe and refposition, which correspond to spatial_coordinate_description and spatial_origin in EPNTAP.
Vocabularies are limited in content on purpose. When these values are only used to locate data of interest in collections, a certain level of approximation is acceptable.
2  Coordinate systems
2.1 Astronomical / telescopic coordinates
This situation is identified by:
spatial_frame_type = celestial
This provides 2 angles + 1 optional distance counted from an origin (which defaults to the observer location).
Possible values for spatial_coordinate_description in the previous document are included in refframe:
Coord System Description  Fundamental plane  Poles  Coordinates  In IVOA voc refframe? 
(=Alt/Az)  elevation (=altitude) –  AZ_EL, check order  
EQUATORIAL, pre FK4 data only  
ECLIPTIC  
galactic latitude  galactic longitude  GALACTIC 
The STC [RD4] distinguishes 3 types of equatorial systems, also included in the IVOA refframe:
In IVOA voc refframe?  
FK4  Fundamental Katalog, system 4; Besselian  Requires Equinox; default B1950.0 Lefthanded in spherical coordinates  FK4 
FK5  Fundamental Katalog, system 5; Julian  Requires Equinox; default J2000.0 Lefthanded in spherical coordinates  FK5 
ICRS  International Celestial Reference System  (based on radio sources)  Requires Equinox?  ICRS 
refframe also uses UNKNOWN for desperate situations.
Comments
• In EPNCore, spatial_coordinate_description defaults to "Equatorial" when spatial_frame_type = celestial
• For groundbased observations:
 Horizontal coordinates require to specify the observer location, though parameters observer_lon and observer_lat
 All these frames apply to both the data product and spatial parameters (c1/c2)
 The extra specification of FK4/FK5/ICRS provides an unnecessary level of accuracy at the expense of multiple values that need to be tested. For equatorial systems it is proposed to restrain spatial_coordinate_description to the generic value "Equatorial", and provide c1/c2 in the actual data system.
Alternative proposal: use hashlists with Equatorial value present, e.g., "Equatorial#FK5" (not favorite: providing hashlists in this parameter may result in a complete mess… TBC)
• For space borne observations:
 The situation is similar, but the indication of the observer location is required when providing celestial coordinates of close targets. This is introduced by spatial_origin (in the current situation of celestial coordinates), which can be a planet (with coordinates at surface?) or a spacecraft.
 The identification of a spacecraft is provided by instrument_host_name, therefore only a generic mention is required in spatial_origin
 Open question: how do we provide the location of a spacecraft in space in this case? Example: target = Solar system object, c1/2 in RADec, spacecraft located at L1 or in Saturn orbit. We can't use observer_lon and observer_lat which are implicitly tied to the Earth (if not, no way to tell what is the coord system in use), nor subobserver_longitude / subobserver_latitude (which are on the target, not on the sky). Is conveying only the idea through spatial_origin enough?
• For computational data, eg of target coordinates:
 spatial_origin provides the origin of the system, as per refposition. Are other values needed to provide an origin in the Solar System?
The reference position listed in the IVOA vocabulary refposition are:
Reference Position  Description  Comments 
GEOCENTER  Center of the Earth  
BARYCENTER  Center of the solar system barycenter  
HELIOCENTER  Center of the Sun  
TOPOCENTER  The location of the observer  
EMBARYCENTER  EarthMoon barycenter  
UNKNOWN  Unknown reference position  Only to be used as a last resort 
The STC [RD4] (table 1) also includes values of interest for EPNTAP:
Reference Position  Description  Comments 
MOON  Center of the Moon  
MERCURY  Center of Mercury  
VENUS  Center of Venus  
MARS  Center of Mars  
JUPITER  Center of Jupiter  
SATURN  Center of Saturn  
URANUS  Center of Uranus  
NEPTUNE  Center of Neptune  
PLUTO  Center of Pluto  
RELOCATABLE  Relocatable center; for simulations  Could also match spacecraft? 
Open questions:
 Are planets and spacecraft values required to provide celestial coordinates for search purpose? The difference may be significant when observing targets in the Solar System. In addition, only the mention of spacecraft seems required, as it involves an ephemeris of positions, and therefore the identification of the planet, if any (and location on the planet).
 Can all value(s) for spacecraft, planetary landers, and orbital telescopes (with moving origin) be accessed through TOPOCENTER, or RELOCATABLE? — PDS3 uses a SPACECRAFT value.
An ephemeris file can be provided to specify the location of moving instruments (done e.g., in the hst_planeto service, through Miriade).
=> Altogether, the values listed in refframe are used and sufficient for spatial_coordinate_description, but there may be a need for extra values in refposition (TBC)
2.2 Solid bodies
This situation is identified by:
spatial_frame_type = body
This provides 2 angles + 1 optional distance (not necessarily from body center, therefore not consistent with the STC).
The constraint to maintain uniformity of the queries is to always provide longitudes ranging from 0° to 360° eastward, and latitudes computed from the body center (= planetocentric system). Therefore, the coordinate system used to provide coordinates c1/2/3 may be different from that of the data product.
In addition, it is important to encode the coordinate systems of the data in a way compliant with OGC standards, so that OGC tools can be used with these data in the future. The Planetary WG at OGC has been working on this since 2022 (https://www.ogc.org/requests/ogctoformspacestandardsdomainworkinggrouppubliccommentsoughtondraftcharter/)
Coordinate systems may differ by the assumed shape of the body, its rotational elements, the location of the prime meridian, the control point network in use, and the definition of latitudes. Coordinate systems may be planetocentric (defined relative to a vector passing by the center of mass) or planetographic (defined relative to the local horizontal plane), resulting in different latitudes if the body is not spherical. The difference in latitude between centric and graphic systems can be significant on nonspherical bodies, so this has to be indicated
The IAU WG reports the consensual values for the ellipsoid, prime meridian and rotational elements for the planets and satellites.
IAUaccepted coordinate systems for the planets and satellites listed here: https://planetarynames.wr.usgs.gov/TargetCoordinates
Coordinate systems may be defined as easthanded or westhanded. In the IAU 2000 standard, planetocentric systems must be easthanded for planets and satellites, while no constraint applies to planetographic ones.
However, IAU does not issue recommendation to use coordinate frames, so that the ellipsoid and prime meridian are independent from such frames and can be used with either convention
The control point network in use can also produce large variations on occasions (seen on the Moon in the early 2000's). Information can be found here: http://astrogeology.usgs.gov/maps/controlnetworks
***
 Open question: how do we provide the location of a spacecraft in space? Example (an actual one): target = Sun, therefore c1/2 in a solar frame; location of spacecraft = location in the solar system. We can't use observer_lon and observer_lat which are implicitly tied to the Earth (if not, no way to tell what is the coord system in use). A solution is to use subobserver_longitude, subobserver_latitude (and possibly radial_distance) to provide the coordinates of the spacecraft projection on the target in the same system as c1/c2 — this may not be always applicable (although this seems to work for landers/rovers on planetary surfaces).
***
Possible nomenclatures:
0) coordinates c1/2 and other spatial parameters need to be provided in planetocentric scale. For regular bodies (planets, large satellites and asteroids), the last IAU planetocentric scale is assumed (ellipsoid, meridian and N pole from last IAU WG report, planetocentric Ehanded). Variations between successive versions are small enough to be nonrelevant for discovery purpose.
00) Irregular bodies are more difficult to handle, especially if the lon/lat system is degenerate (several points having the same coordinates). In this case the only simple way is to use a first order Ehanded frame in lon/lat
Derived comment: do we need another spatial_frame_type for shape models? This is essentially bodyfixed, but coordinates are not longitudes/latitudes (facet indices instead?).
• Spatial_coordinate_description in this case refers to the data, not necessarily to c1/2/3:
1) Recommendation in the EPNTAP doc = Spice bodyfixed frames: IAU_Target => IAU_Mars.
2) Early EPNTAP recommendation (the only one actually used so far): Target_IAUyear of introduction => Mars_IAU2000 (this was derived from SPICE kernel names of early 2000 Mars missions, e.g. MEx, MER, MGS…)
"IAUyear" above refers to the ellipsoid, prime meridian and rotational elements in the IAU WG report of that year, not to a preferred coordinate system (TBC). It therefore does not tell if the system is planetocentric/graphic (can be either depending on body, and may be Whanded).
=> The early recommendation was unambiguous : Mars_IAU2015 means Mars, ellipsoid from 2015 IAU WG report, planetocentric.
It can be considered that variations between successive reports do not affect the search function in EPNTAP (then IAU_Mars would do also)  but the data wouldn't match in detail.
3) The current (2023) OGCcompliant recommendation provides a detailed reference = complete WKT2 strings, which are long and parameterized — we don't want that!
** 4) The expected update to the OGC recommendation (evolution from [RD6] ) is to use a namespace IAU: followed by year:5digitcode
In short, and if we restrain to planetocentric Ehanded frames: "IAU:2015:49900"  where 2015 identifies the IAU ellipsoid version, 499 = Mars in NAIF/SPICE, 00 stands for planetocentric 1axis (02 for 2axis; 04 for 3axis). This seems manageable as a vocabulary, although slightly more complicated than solutions 1 and 2, and will also fulfill the requirement of compliance with OGC standards (when validated). Beware that the namespace IAU: may be replaced by PDSSP: in the final OGC standard.
Such IDs can be used to crossreference WKT2 strings (solution 3)  a resolver is available here: http://voparisvespacrs.obspm.fr:8080/ (implemented for the 2015 IAU report only, so far)
References for small bodies
According to IAU standards, small bodies are all considered prograde and their north pole is defined accordingly.
Coordinate system name should state shape model/prime meridian in use?
The STC document distinguishes 2 options related to the meaning of the third coordinate:
To be introduced through the spatial_origin parameter??
GEO_C  Geographic (geocentric) coordinates: longitude, latitude, geocentric distance 
GEO_D  Geodetic coordinates: longitude, latitude, elevation 
In many cases the surface is used as origin, e.g., in atmospheric databases – this corresponds to the geodetic option above.
+ Need different origin for planetographic coordinates?
** Comment: this seems to be covered by the general body case above.
2.3 Giant planets
This situation is identified by:
spatial_frame_type = body
Provides 2 angles + 1 optional distance (from body center, for consistency with STC?? TBC)
STC distinguishes between planetographic and planetocentric:
 Planetocentric  Planetographic 
Jupiter_C_III  Jupiter_G_III  
Saturn_C_III  Saturn_G_III  
Uranus_C_III  Uranus_G_III  
Neptune_C_III  Neptune_G_III 
In the PDS3 standard reference there is only one value for each planet in the coordinate_system_ID keyword:
"JUPSYS3", "SATSYS3", "URNSYS3" + VSO
• Open questions:
Can we assume that system III is used to provide the coordinates c1/2 in all case? That would be the planetocentric one: *_C_III
Spatial_coordinate_description would provide the actual system in the data product.
Jovian System frames are available in from Fraenz and Harper (2002): the last two are not bodyfixed  they should appear later in the doc, with spatial_frame_type = "spherical"
Abbrev  Name  Description 

JUP_I  Jovian System I  Mean atmospheric equatorial rotation. +Z axis: pole of rotation. Rotation speed: 877.900 degree/day 
JUP_II  Jovian System II  Mean atmospheric polar rotation. +Z axis: pole of rotation. Rotation speed: 870.270 degree/day 
JUP_III  Jovian System III  Magnetospheric rotation. +Z axis: pole of rotation. Rotation speed: 870.53 degree/day 
JUP_III_sun  Jovian System III, fix Sun Line  +Zaxis: pole of rotation. +Yaxis: crossproduct of +Zaxis and vector (Jupiter–Sun). 
JUP_D  Jovian Magnetic Dipole System  +Zaxis: dipole axis defined by its System III latitude and longitude: lat_D = (90° − 9.8°); lambda_D = 200° +X axis: intersection of System III prime meridian and magnetic equator. 
Jovian Centrifugal System  +Z axis: centrifugal axis defined by its System III latitude and longitude: lat_C = (90° − 7.0°); lambda_C = 200°. +X axis: intersection of System III prime meridian and centrifugal equator.  
Magnetic Dipole System x Sun line  +Z axis: dipole axis. +Y axis: crossproduct of +Z axis and vector (Jupiter–Sun).  
Magnetic Dipole rthph System  +Xaxis: vector (JupiterS=C) +Zaxis: cross product of (dipole axis) and +Xaxis. This system depends on the S=Cposition. 
• Proposition:
Spatial_frame_type = "spherical"
Spatial_coordinate_description would provide the actual system in the data product, and this system is used to define c1/c2
2.4 Plasma / dynamic coordinates
This situation may correspond to
spatial_frame_type = cartesian or spherical — may be celestial in some cases, see notes in table below (some unclear…)
From STC document:
name  note  type  

MAG  Geomagnetic coordinates  See F&H (2002)  
GSE  Geocentric Solar Ecliptic coordinates  See F&H (2002)  
GSM  Geocentric Solar Magnetic coordinates  See F&H (2002)  
SM  Solar Magnetic coordinates  See F&H (2002)  
HGC  Heliographic coordinates (Carrington)  See Explanatory Supplement, Section 7.2 Thompson (2006), Section 2.2  bodyfixed 
HGS  Heliographic coordinates (Stonyhurst)  See Explanatory Supplement, Section 7.2 Thompson (2006), Section 2.2  Origin on subterrestrial meridian Spherical? 
HEEQ  Heliographic Earth Equatorial coordinates  See F&H (2002); related to Heliographic (Stonyhurst), see Thompson (2006), Section 2.1  Origin on subterrestrial meridian cartesian? 
HRTN  Heliocentric Radial TangentialNormal coordinates  See F&H (2002)  
HPC  Helioprojective Cartesian coordinates  See Thompson (2006), Section 4.1, 2 or 3dimensional (angular coordinates); left handed  celestial 
HPR  Helioprojective Polar coordinates  See Thompson (2006), Section 4.1, 2 dimensional (angular coordinates); left handed  
HCC  Heliocentric Cartesian coordinates  See Thompson (2006), Section 3.1 (linear coordinates); righthanded  cartesian? 
HGI  Heliographic Inertial coordinates  See F&H (2002) 
• Proposition:
Spatial_coordinate_description provides the actual system in the data product, and this system is used to define c1/c2
2.5 Magnetospheric coordinates
This situation may correspond to
spatial_frame_type = cartesian or spherical
Three types of reference frames are used:
 planetocentric solar magnetic (X towards Sun; Z towards planetary magnetic North pole). Only exist when the body has an intrinsic magnetic field.
 planetocentric solar equatorial (X towards Sun; Z perpendicular to the equator of the planet, towards the planetary North pole)
 planetocentric solar ecliptic (X towards the Sun; Z perpendicular to ecliptic plane, in the northern celestial hemisphere)
Each of them is centered on the planet barycenter. Y axis is completing the orthogonal direct reference frames.
Planet  Name  Acronym 

Mercury  Hermian Solar Ecliptic Hermian Solar Magnetic Hermian Solar Equatorial  HSE HSM HSQ  
Venus  Venus Solar Ecliptic Venus Solar Equatorial  VSE VSQ  
Earth  Geocentric Solar Ecliptic Geocentric Solar Equatorial Geocentric Solar Magnetic  GSE GSQ GSM  
Mars  Martian Solar Ecliptic Martian Solar Equatorial  MSE MSQ  
Jupiter  Jovian Solar Ecliptic Jovian Solar Equatorial Jovian Solar Magnetic  JSE JSQ JSM  [see F&H 2002, section 4.3.1] 
Saturn  Kronian Solar Ecliptic Kronian Solar Equatorial Kronian Solar Magnetic  KSE KSQ KSM  
Uranus  Uranian Solar Ecliptic Uranian Solar Equatorial Uranian Solar Magnetic  USE USQ USM  
Neptune  Neptunian Solar Ecliptic Neptunian Solar Equatorial Neptunian Solar Magnetic  NSE NSQ NSM 
• Proposition:
Spatial_coordinate_description would provide the actual system in the data product, and this system is used to define c1/c2
2.6 Native coordinates
Data are projected in a frame related to the instrument or acquisition process.
Typical examples include: X/Y coordinates for a camera, X/time coordinates for an imaging spectrometer.
Possible situation:
spatial_frame_type = cartesian
spatial_coordinate_description = "native". Are Spice instrumentframe names acceptable?
Practically, it means that data products are provided in a native instrument frame, and therefore are not spatially registered – they are not searchable on a coordinate basis in general.
In practice, there is no point in providing c1/2 coordinates.
2.7 Landers/rovers coordinates
This specific situation is expected to be correctly described in the PDS3/4 reference, TBC.
2.8 Solarterrestrial interactions
Derived from a SPASE document compiling various sources.
Acronym  Name  Description  Reference 

CGM  Corrected GeoMagnetic  A coordinate system from a spatial point with GEO radial distance and geomagnetic latitude and longitude, follow the epochappropriate IGRF/DGRF model field vector through to the point where the field line crosses the geomagnetic dipole equatorial plane. Then trace the dipole magnetic field vector Earthward from that point on the equatorial plane, in the same hemisphere as the original point, until the initial radial distance is reached. Designate the dipole latitude and longitude at that point as the CGM latitude and longitude of the original point.  (1,2,11) 
CAR  Carrington  A coordinate system which is centered at the Sun and is "fixed" with respect to the synodic rotation rate; the mean synodic value is about 27.2753 days. The Astronomical Almanac gives a value for Carrington longitude of 349.03 degrees at 0000 UT on 1 January 1995.  (1,11) 
DM  Dipole Meridian  A coordinate system centered at the observation point. Z axis is parallel to the Earth's dipole axis, positive northward. X is in the plane defined by Z and the line linking the observation point with the Earth's center. Y is positive eastward.  (1,3,11) 
GEI (or GCI)  Geocentric Equatorial Inertial  A coordinate system where the Z axis is along Earth's spin vector, positive northward. X axis points towards the first point of Aries (from the Earth towards the Sun at the vernal equinox).  (1,3,4,5,8,9,10,11) 
GEO  Geographic  geocentric corotating  A coordinate system where the Z axis is along Earth's spin vector, positive northward. X axis lies in Greenwich meridian, positive towards Greenwich.  (1,3,4,5,8,9,11) 
GSE  Geocentric Solar Ecliptic  A coordinate system where the X axis is from Earth to Sun. Z axis is normal to the ecliptic, positive northward.  (1,3,4,5,8,9,10,11) 
GSEQ  Geocentric Solar Equatorial  A coordinate system where the X axis is from Earth to Sun. Y axis is parallel to solar equatorial plane. Z axis is positive northward.  (1,3,9) 
GSM  Geocentric Solar Magnetospheric  A coordinate system where the X axis is from Earth to Sun, Z axis is northward in a plane containing the X axis and the geomagnetic dipole axis.  (1,3,4,5,9,10,11) 
HAE  Heliocentric Aries Ecliptic  A coordinate system where the Z axis is normal to the ecliptic plane, positive northward. X axis is positive towards the first point of Aries (from Earth to Sun at vernal equinox). Same as SE below.  (1,5,8,11,12) 
HCC  Heliocentric Cartesian  A 3D orthonormal coordinate system that is primarily intended to specify with two dimensions a point on the solar disk. The Z axis points toward the observer. The Y axis lies in the plane defined by the solar spin vector and the Z axis, positive northward. The X axis is perpendicular to the Y and Z axes, positive toward solar west. Standard representation for this system is via the point's x and y values, expressed either as physical distances or as fractions of the solar disk radius.  (1) 
HCD  Heliocentric of Date  (11)  
HCI  Heliographic Carrington Inertial  (1,8,11,12)  
HCR  Heliocentric Radial  A 3D orthonormal coordinate system that is primarily intended to specify with two dimensions a point on the solar disk. The Z axis points toward the observer. The Y axis lies in the plane defined by the solar spin vector and the Z axis, positive northward. The X axis is perpendicular to the Y and Z axes, positive toward solar west. Standard representation for this system is via the point's distance rho from the Z axis [Rho = SQRT(x**2 + y**2)] and its phase angle psi measured counterclockwise from the +Y axis [psi = arctan (y/x)]  (1) 
HEE  Heliocentric Earth Ecliptic  A coordinate system where the Z axis is normal to the ecliptic plane, positive northward. X axis points from Sun to Earth.  (1,5,8,11) 
HEEQ  Heliocentric Earth Equatorial  A coordinate system where the Z axis is normal to the solar equatorial plane, positive northward. X axis is generally Earthward in the plane defined by the Z axis and the SunEarth direction.  (1,5,8,11) 
HG (or HGC)  Heliographic  A heliocentric rotating coordinate system where the Z axis is normal to the solar equatorial plane, positive northward. X, Y axes rotate with a 25.38 day period. The zero longitude (X axis) is defined as the longitude that passed through the ascending node of the solar equator on the ecliptic plane on 1 January, 1854 at 12 UT.  (1,6,11,12) 
HGI  Heliographic Inertial  A heliocentric coordinate system where the Z axis is normal to the solar equatorial plane, positive northward. X axis is along the intersection line between solar equatorial and ecliptic planes. The X axis was positive at SE longitude of 74.367 deg on Jan 1, 1900. (See SE below.)  (1,6) 
HPC  Helioprojective Cartesian  A 3D orthonormal (lefthanded) coordinate system that is primarily intended to specify with two dimensions a point on the solar disk. The Z axis points from the observer to the center of the solar disk. The Y axis lies in the plane defined by the solar spin vector and the Z axis, positive northward. The X axis is perpendicular to the Y and Z axes, positive toward solar west. Given as the distance between the observer and the center of the solar disk, the standard representation of an (x,y) point on the solar disk is via the point's longitude angle [arctan (x/d)] and latitude angle [arctan y/d].  (1) 
HPR  Helioprojective Radial  A 3D orthonormal (lefthanded) coordinate system that is primarily intended to specify with two dimensions a point on the solar disk. The Z axis points from the observer to the center of the solar disk. The Y axis lies in the plane defined by the solar spin vector and the Z axis, positive northward. The X axis is perpendicular to the Y and Z axes, positive toward solar west. Given as the distance between the observer and the center of the solar disk, the standard representation for this system of an (x,y) point on the solar disk is via the point's latitude angle theta {= arctan [SQRT(x**2 + y**2)]/d]} or equivalent declination parameter delta (= theta  90 deg), and its phase angle psi as measured counter clockwise from the +Y axis [psi = arctan (y/x)].  (1) 
HS  Heliocentric Solar  X = Intersection between solar equator and solar central meridian as seen from Earth. Z = North Pole of solar rotation axis.  (10) 
HSEa  Heliocentric Solar Ecliptic (Inertial)  X = First point of Aries (Vernal Equinox, i.e. to the Sun from Earth in the first day of Spring). Z = Ecliptic North Pole  (10) 
HSEb  Heliocentric Solar Ecliptic (Earth Oriented)  X = SunEarth Line. Z = Ecliptic North Pole  (10) 
J2000  J2000  An astronomical coordinate system which uses the mean equator and equinox of Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT. (aka J2000) to define a celestial reference frame.  (1) 
LGM  Local Geomagnetic  A coordinate system used mainly for Earth surface or near Earth surface magnetic field data. X axis northward from observation point in a geographic meridian. Z axis downward towards Earth's center. In this system, H (total horizontal component) = SQRT (Bx^2 + By^2) and D (declination angle) = arctan (By/Bx)  (1) 
MAG  Geomagnetic  geocentric  Z axis is parallel to the geomagnetic dipole axis, positive north. X is in the plane defined by the Z axis and the Earth's rotation axis. If N is a unit vector from the Earth's center to the north geographic pole, the signs of the X and Y axes are given by Y = N x Z, X = Y x Z.  (1,3,4,5,11) 
MFA  Magnetic Field Aligned  A coordinate system spacecraftcentered system with Z in the direction of the ambient magnetic field vector. X is in the plane defined by Z and the spacecraftSun line, positive sunward.  (1,3) 
RTN  Radial Tangential Normal  Typically centered at a spacecraft. Used for IMF and plasma V vectors. R (radial) axis is radially away from the Sun, T (tangential) axis is normal to the plane formed by R and the Sun's spin vector, positive in the direction of planetary motion. N (normal) is R x T.  (1,10,11,12) 
SC  Spacecraft  A coordinate system defined by the spacecraft geometry and/or spin. Often has Z axis parallel to spacecraft spin vector. X and Y axes may or may not corotate with the spacecraft. See SR and SR2 below.  (1) 
SE  Solar Ecliptic  A heliocentric coordinate system where the Z axis is normal to the ecliptic plane, positive northward. X axis is positive towards the first point of Aries (from Earth to Sun at vernal equinox). Same as HAE above.  (1,6) 
SM  Solar Magnetic  A geocentric coordinate system where the Z axis is northward along Earth's dipole axis, X axis is in plane of z axis and EarthSun line, positive sunward.  (1,3,4,5,9,11) 
SR  Spin Reference  A special case of a Spacecraft (SC) coordinate system for a spinning spacecraft. Z is parallel to the spacecraft spin vector. X and Y rotate with the spacecraft.  (1,3) 
SR2  Spin Reference 2  A special case of a Spacecraft (SC) coordinate system for a spinning spacecraft. Z is parallel to the spacecraft spin vector. X is in the plane defined by Z and the spacecraftSun line, positive sunward.  (1,3) 
SSE  Spacecraft Solar Ecliptic  A coordinate system used for deep space spacecraft, for example Helios.  X axis from spacecraft to Sun. Z axis normal to ecliptic plane, positive northward. Note: Angle between normals to ecliptic and to Helios orbit plane ~ 0.25 deg.  (1,11) 
SSE_L  Selenocentric Solar Ecliptic  The X axis points from the center of the Earth's moon to the sun, the Z axis is normal to the ecliptic plane, positive northward. And the Y axis completes the righthanded set of axes.  (1) 
SCOP  Spacecraft Orbit Plane  A coordinate system where X lies in the plane normal to and in the direction of motion of the spacecraft, Z is normal to this plane and Y completes the triad in a righthanded coordinate system.  (1) 
VDH  Vertical Dusk Horizontal system  The Vaxis is the outwards local vertical, to the point of observation. The Haxis is parallel to the horizontal local plane, positive to the North. The VH plane is a geographic meridian plane. The Daxis is azimuthal, eastwards.  (3) 
WGS84  World Geodetic System 1984  The World Geodetic System (WGS) defines a reference frame for the earth, for use in geodesy and navigation. The WGS84 uses the zero meridian as defined by the Bureau International de l'Heure.  (1,9) 
[2] http://omniweb.gsfc.nasa.gov/vitmo/cgmm_des.html
[3] https://cdpparchive.cnes.fr/project/data/documents/PLASLOROCOTLIB00428CET/00428.pdf and software libraries in IDL (tar) or Fortran (tar)
[4] C.T. Russell, (1971) "Geophysical Coordinate Transformations", Cosmic. Electrodyn. 2, 184196. URL: http://wwwssc.igpp.ucla.edu/personnel/russell/papers/gct1.html
[5] M.A. Hapgood, (1992) "Space Physics Coordinate Transformations: A User Guide", Planet. Space Sci. 40, 711717.
M.A. Hapgood, (1997) "Corrigendum to Space Physics Coordinate Transformations: A User Guide", Planet. Space Sci. 45, 1047.
[6] http://cohoweb.gsfc.nasa.gov/helios/coor_des.html
[7] http://sspg1.bnsc.rl.ac.uk/SEG/Coordinates/refs.htm
[8] http://stereossc.nascom.nasa.gov/coordinates_explanation.shtml
[9] http://www.spenvis.oma.be/help/background/coortran/coortran.html
[10] http://www.srl.caltech.edu/ACE/ASC/coordinate_systems.html
[11] M.Fraenz and D.Harper, Heliospheric Coordinate Systems, Planet. Space Sci., 50, 217233 (Feb 2002). URL: http://www.mps.mpg.de/homes/fraenz/systems/
[12] http://heliovo.eu/documents/public/HELIO_Coordinates_100322.pdf
[13] Thompson, W. T. Coordinate systems for solar image data. A&A 449, 791–803 (2006). http://www.aanda.org/10.1051/00046361:20054262
• Proposition:
Spatial_coordinate_description would provide the actual system in the data product, and this system is used to define c1/c2
2.9 Galilean Moon Frames
 Source = PDS Data_set_id = GOJPOS6SCTRAJMOONCOORDSV1.0
 Current URL: https://pds.nasa.gov/dsview/pds/viewDataset.jsp?dsid=GOJPOS6SCTRAJMOONCOORDSV1.0
Two coordinate systems are provided:
 SPRH (Satellite centered planetocentric, righthanded),
 PhiO (Satellite centered inertial PhiOmega coordinates),
Distances from the satellites are measured in satellite radii. The radii values used for the various satellites are listed in the following Table:
Moon  Radius (km) 

Amalthea  86.2 
Io  1818 
Europa  1560 
Ganymede  2634 
Callisto  2409 
Satellite centered coordinate systems names are preceded by the first letter in the name of the satellite, in order to indicate which satellite is used as the center. In order words, PhiO coordinates are called EPhiO at Europa and CPhiO at Callisto
SPRH (Satellite centered planetocentric, righthanded)
This coordinate system is the basic J2000 definition of planetocentric coordinates, as applied to each of the satellites. In it's Cartesian form the coordinate system has its Zaxis along the satellite axis of rotation positive in the direction of angular momentum, the Xaxis in the equatorial plane in the direction of the prime meridian, with the Yaxis completing the righthanded set. The coordinate system rotates with the satellite (bodyfixed).
All of the SPRH data in this data set are based on the International Astronomical Union (IAU) definitions of the satellite axes orientations and rotation rates from the Report of the IAU/IAG/COSPAR Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 1994 [IAU1994]. The specific orbit elements for each satellite are summarized in the following Table:
Satellite  Axis orientation J2000  Prime Meridian*  

Right Ascension  Declination  Constant  Rate  
Amalthea  268.05  +64.49  231.67  +722.6314560 
Io  268.05  +64.50  200.39  +203.4889538 
Europa  268.08  +64.51  35.67  +101.3747235 
Ganymede  268.20  +64.57  44.04  +50.3176081 
Callisto  268.72  +64.83  259.73  +21.5710715 
* PM = const + rate*d where d is days after the J2000 epoch.
This is the general case for bodyfixed frames (TBC), so spatial_frame_type = "body"
PhiO (Satellite centered inertial PhiOmega coordinates)
The basis vectors of the inertial PhiOmega coordinate systems are defined at an epoch time that is set at the time of spacecraft closest approach to the satellite. The Xdirection is defined to be in the direction of corotation, at the center of the satellite at the epoch time (System III Phi direction [DESSLER1983]). The Zaxis is defined to be orthogonal to the X direction such that the XZ plane contains the Jupiter spin axis (Omega), positive in the direction of angular momentum. The Yaxis is defined to complete the righthanded set. Since the jovian satellites all lie very close to the jovian equatorial plane, it is often convenient to visualize this coordinate system as follows: X lies in the direction of plasma flow, Y points towards Jupiter, and Z points 'up'.
Here, spatial_frame_type is not "body" but = "spherical" (TBC)
3  Map projection
The indication of the projection of a data product may be used by display tools, and must be provided to avoid superpositions of data in different frames. This situation is mostly (only?) relevant for bodyfixed coordinates, although not necessarily relative to surface features (ex: maps of clouds, winds, etc). If may apply to any 2D spatial product, such as maps, images, spectral cubes. See if maps of magnetospheric features are relevant, in coordinate systems not bodyfixed.
 A list of possible projection types is included in the fits standard, see e.g. Marmo et al 2018 table 1. This could make a vocabulary (limited to 13 values), although some of these require parameters.
 In practice however, some providers insist to include a proj4 string here, as this seems an unambiguous way to describe the projection in use.
An example from HRSC3nd (including the ellipsoid definition) is "+proj=eqc +lat_ts=0 +lat_0=0 +lon_0=0 +x_0=0 +y_0=0 +a=3396000 +b=3396000 +units=m +no_defs"
This defines the ellipsoid (a sphere in this case), but apparently the prime meridian definition is not referenced here  it could be provided through spatial_coordinate_description though.
 The complete codes proposed in [RD6] include a projection type (last 2 digits) followed by parameters, e.g. "IAU:2018:49960,100,45,1.0"
This seems to encompass the information provided in the proj4 strings and more (reference to the ellipsoid and prime meridian). Even without parameters however, the values hardly fit a vocabulary. So this must be considered an informational parameter — for use by specialized tools.
 Other, exotic projections may be used for irregular bodies, e.g., quincuncial or QuACK projections (see Grieger 2019, 10.1051/00046361/201834841)