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EPN2024-RI


EUROPLANET2024 Research Infrastructure 

H2020-INFRAIA-2019-1  

Grant agreement no: 871149


Document: VESPA-WP6-2-053-TN-v0.1(31)




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Documenting coordinate systems in EPN-TAP




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Start date of project: 01 February  2020

 Duration: 48 Months

Responsible WP Leader: Stéphane Erard


Project co-funded by the European Union's Horizon 2024 research and innovation programme

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Public

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Restricted to other programme participants (including the Commission Service)

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Restricted to a group specified by the consortium (including the Commission Services)

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Confidential, only for members of the consortium (excluding the Commission Services)

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Project Number

871149

Project Title

EPN2024 - RI

Project Duration

48 months: 01 February 2020 – 31 January 2024

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WP6-task2--v

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Title of Document

Documenting coordinate systems in EPN-TAP

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Abstract:



Document history (to be deleted before submission to Commission)

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Version

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Status

13 Nov 2023

first draft, adapted from EPN 2020 document VESPA-WP11-2-016-TN-v0.5(47)

 DRAFT





Table of Contents

Reference documents

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 EPN-DM, mandatory for EPN-TAP compatibility
  • EPN-TAP Specific protocol to access Planetary Science data in Europlanet-VO
  • EPN-DM Specific Data Model to describe Planetary Science data in Europlanet-VO
  • epn_core Name of table / view of a database which contains the EPN-TAP parameters. Required for EPN-TAP 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

EPN-TAP coordinates

The EPN-TAP 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. EPN-TAP and its implicit Data Model EPNcore are described in [RD1].  

In EPN-TAP, 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 cross-references 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 over-plotted 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 EPN-TAP.

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?

Horizontal

(=Alt/Az)

horizon

zenith/nadir

elevation (=altitude) –

azimuth - meridian

AZ_EL, check order

Equatorial

celestial equator

celestial poles

declination

right ascension or hour angle

EQUATORIAL, pre FK4 data only

Ecliptic

ecliptic

ecliptic poles

ecliptic latitude - ecliptic longitude

ECLIPTIC

Galactic

galactic plane

galactic poles

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 Left-handed in spherical coordinates

FK4

FK5

Fundamental Katalog, system 5; Julian

Requires Equinox; default J2000.0 Left-handed 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 ground-based 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 hash-lists with Equatorial value present, e.g., "Equatorial#FK5" (not favorite: providing hash-lists 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 RA-Dec, 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

Earth-Moon barycenter


UNKNOWN

Unknown reference position

Only to be used as a last resort


The STC [RD4] (table 1) also includes values of interest for EPN-TAP:

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/ogc-to-form-space-standards-domain-working-group-public-comment-sought-on-draft-charter/)


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 non-spherical 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.

IAU-accepted coordinate systems for the planets and satellites listed here: https://planetarynames.wr.usgs.gov/TargetCoordinates

   Coordinate systems may be defined as east-handed or west-handed. In the IAU 2000 standard, planetocentric systems must be east-handed 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/control-networks

***

- 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 E-handed). Variations between successive versions are small enough to be non-relevant 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 E-handed frame in lon/lat

Derived comment: do we need another spatial_frame_type for shape models? This is essentially body-fixed, 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 EPN-TAP doc = Spice body-fixed frames:  IAU_Target  => IAU_Mars. 

2) Early EPN-TAP 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 W-handed).

=> 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 EPN-TAP (then IAU_Mars would do also) - but the data wouldn't match in detail.

3) The current (2023) OGC-compliant 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:5-digit-code

In short, and if we restrain to planetocentric E-handed frames: "IAU:2015:49900" - where 2015 identifies the IAU ellipsoid version, 499 = Mars in NAIF/SPICE, 00 stands for planetocentric 1-axis (02 for 2-axis; 04 for 3-axis). 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 cross-reference WKT2 strings (solution 3) - a resolver is available here: http://voparis-vespa-crs.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 body-fixed - they should appear later in the doc, with spatial_frame_type = "spherical"

AbbrevNameDescription
JUP_IJovian System I

Mean atmospheric equatorial rotation. +Z -axis: pole of rotation. Rotation speed: 877.900 degree/day

JUP_IIJovian System IIMean atmospheric polar rotation. +Z -axis: pole of rotation. Rotation speed: 870.270 degree/day
JUP_IIIJovian System IIIMagnetospheric rotation. +Z -axis: pole of rotation. Rotation speed: 870.53 degree/day
JUP_III_sunJovian System III, fix Sun Line+Z-axis: pole of rotation. +Y-axis: cross-product of +Z-axis and vector (Jupiter–Sun).
JUP_DJovian Magnetic Dipole System 

+Z-axis: 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: cross-product of +Z -axis and vector (Jupiter–Sun).

Magnetic Dipole r-th-ph   System+X-axis: vector (Jupiter-S=C)
+Z-axis: cross product of (dipole axis) and +X-axis.
This system depends on the S=C-position.


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

body-fixed

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- Tangential-Normal coordinates

See F&H (2002)


HPC

Helioprojective Cartesian coordinates

See Thompson (2006), Section 4.1, 2- or 3-dimensional (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); right-handed

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 instrument-frame 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 Solar-terrestrial interactions

Derived from a SPASE document compiling various sources.

AcronymNameDescriptionReference

CGM

Corrected GeoMagnetic

A coordinate system from a spatial point with GEO radial distance and geomagnetic latitude and longitude, follow the epoch-appropriate 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)
CARCarrington

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)
DMDipole 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)
GEOGeographic - geocentric corotatingA 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)
GSEGeocentric Solar EclipticA 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)
GSEQGeocentric Solar EquatorialA 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)
HAEHeliocentric Aries EclipticA 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)
HCCHeliocentric CartesianA 3-D 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)
HCDHeliocentric of Date
(11)
HCIHeliographic Carrington Inertial
(1,8,11,12)
HCR

Heliocentric Radial

A 3-D 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)
HEEHeliocentric Earth EclipticA 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 Sun-Earth direction.(1,5,8,11)
HG (or HGC)HeliographicA 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)
HGIHeliographic 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)
HPCHelioprojective CartesianA 3-D orthonormal (left-handed) 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)
HPRHelioprojective RadialA 3-D orthonormal (left-handed) 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)
HSHeliocentric SolarX = Intersection between solar equator and solar central meridian as seen from Earth. Z = North Pole of solar rotation axis.(10)
HSEaHeliocentric 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)
HSEbHeliocentric Solar Ecliptic (Earth Oriented)X = Sun-Earth Line. Z = Ecliptic North Pole(10)
J2000J2000An 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)
LGMLocal GeomagneticA 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)
MAGGeomagnetic - geocentricZ 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)
MFAMagnetic Field AlignedA coordinate system spacecraft-centered system with Z in the direction of the ambient magnetic field vector. X is in the plane defined by Z and the spacecraft-Sun line, positive sunward.(1,3)
RTNRadial Tangential NormalTypically 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)
SCSpacecraftA 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)
SESolar 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 Earth-Sun line, positive sunward.(1,3,4,5,9,11)
SRSpin ReferenceA 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 spacecraft-Sun line, positive sunward.(1,3)
SSESpacecraft Solar EclipticA 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_LSelenocentric Solar EclipticThe 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 right-handed set of axes.(1)
SCOPSpacecraft Orbit PlaneA 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 right-handed coordinate system.(1)
VDHVertical Dusk Horizontal system

The V-axis is the outwards local vertical, to the point of observation. The H-axis is parallel to the horizontal local plane, positive to the North. The V-H plane is a geographic meridian plane. The D-axis is azimuthal, eastwards.
As DM system, this system is a local coordinate system, which is dependent of the position of the point of observation from the Earth.

(3)
WGS84World Geodetic System 1984The 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)


[1] http://www.spase-group.org/data/search.jsp?term=Coordinate+System+Name&style=entry&scope=dictionary&version=2.2.0

[2] http://omniweb.gsfc.nasa.gov/vitmo/cgmm_des.html 

[3] https://cdpp-archive.cnes.fr/project/data/documents/PLAS-LO-ROCOTLIB-00428-CET/00428.pdf and software libraries in IDL (tar) or Fortran (tar)

[4] C.T. Russell, (1971) "Geophysical Coordinate Transformations", Cosmic. Electrodyn. 2, 184-196. URL: http://www-ssc.igpp.ucla.edu/personnel/russell/papers/gct1.html

[5] M.A. Hapgood, (1992) "Space Physics Coordinate Transformations: A User Guide", Planet. Space Sci. 40, 711-717.
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://stereo-ssc.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, 217-233 (Feb 2002). URL: http://www.mps.mpg.de/homes/fraenz/systems/

[12] http://helio-vo.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/10.1051/0004-6361:20054262


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

Two coordinate systems are provided:

  • SPRH (Satellite centered planetocentric, right-handed),
  • PhiO (Satellite centered inertial Phi-Omega coordinates),

Distances from the satellites are measured in satellite radii. The radii values used for the various satellites are listed in the following Table:

MoonRadius (km)
Amalthea86.2
Io1818
Europa1560
Ganymede2634
Callisto2409

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, right-handed)

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 Z-axis along the satellite axis of rotation positive in the direction of angular momentum, the X-axis in the equatorial plane in the direction of the prime meridian, with the Y-axis completing the right-handed set. The coordinate system rotates with the satellite (body-fixed).

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:

SatelliteAxis orientation J2000Prime Meridian*
Right AscensionDeclinationConstantRate
Amalthea268.05+64.49231.67+722.6314560
Io268.05+64.50200.39+203.4889538
Europa268.08+64.5135.67+101.3747235
Ganymede268.20+64.5744.04+50.3176081
Callisto268.72+64.83259.73+21.5710715

* PM = const + rate*d where d is days after the J2000 epoch.

This is the general case for body-fixed frames (TBC), so spatial_frame_type = "body"

PhiO (Satellite centered inertial Phi-Omega coordinates)

The basis vectors of the inertial Phi-Omega coordinate systems are defined at an epoch time that is set at the time of spacecraft closest approach to the satellite. The X-direction 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 Z-axis is defined to be orthogonal to the X direction such that the X-Z plane contains the Jupiter spin axis (Omega), positive in the direction of angular momentum. The Y-axis is defined to complete the right-handed 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 body-fixed 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 body-fixed.

  • 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, 10.1051/0004-6361/201834841)