This document provides a complete design description of the Space-Time Coordinate (STC) metadata for the Virtual Observatory. It explains the various components, highlights some implementation considerations, presents a complete set of UML diagrams, and discusses the relation between STC and certain other parts of the Data Model. Two implementations are discussed: XML Schema (STC-X) and String (STC-S, also known as LinearSTC).

Status of This Document

This is a Proposed Recommendation. The first release of this document (Version 1.21) was 2005 March 23. The first release of the predecessor Working Draft (Version 1.00) was 2004 July 23.

This is an IVOA Proposed Recommendation made available for public review. It is appropriate to reference this document only as a recommended standard that is under review and which may be changed before it is accepted as a full recommendation.

A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/.

Acknowledgements

I gratefully acknowledge many helpful discussions with Wil O’Mullane, Alex Szalay, Jonathan McDowell, Ray Plante, Ed Shaya, Pat Dowler, David Berry, Anita Richards, Steve Allen, and many others. I am indebted to SAO, ULP, CDS, and the NSF NVO grant for support.

Contents

1 Introduction 3

2 Justification and Scope 4

3 Requirements and Use-Context 6

4 Design 7

4.1 The Metadata Components 7

4.2 Coordinate System (CoordSys) 7

4.2.1 Coordinate Frame (CoordFrame) 7

4.3 Coordinates (Coords) 7

4.3.1 Coordinate 8

4.4 Coordinate Area (CoordArea) 9

4.4.1 ScalarInterval 9

4.5 AstroCoordSys 9

4.5.1 TimeFrame 9

Table 1. Standard Reference Positions 10

Table 2. Timescales 12

4.5.2 SpaceFrame 13

Table 3. Standard Reference Frames 14

4.5.3 SpectralFrame 17

4.5.4 RedshiftFrame 17

4.6 AstroCoords 18

4.6.1 TimeCoord 18

4.6.2 SpatialCoord 19

4.6.3 SpectralCoord 20

4.7 AstroCoordArea 20

4.7.1 TimeInterval 21

4.7.2 SpatialInterval 21

4.7.3 VelocityInterval 21

4.7.4 SpectralInterval 21

4.7.5 RedshiftInterval 22

6 Conclusion and Usage Notes 25

6.1 Methods 25

6.2 Usage 26

6.2.1 Resource Profile 26

6.2.2 Query Constraint 27

6.2.3 Catalog Entry 27

6.2.4 Observational Data 27

7 Implementation 28

7.1 STC-X: XML Schema Implementation 28

7.1.1 Simplifications 28

7.2 STC-S 29

Appendix A: UML Representation 29

Appendix B: Examples 39

B.1 STCResourceProfile 39

B.2 Galaxy Catalog 41

B.3 ROSAT Image 44

B.4 Blue M81 Image from KPNO 47

B.5 Query Specification 50

Appendix C: XInclude Library of Coordinate Systems 52

Appendix D: Changes from Previous Versions 52

References 53

1 Introduction

This document attempts to explain and document the design and implementation of the Space-Time (and related coordinate axes) metadata for the Virtual Observatory. In order to make this specification optimally useful for the VO, we have generalized the Coordinate System concept and we have added a generic Coordinate Frame. Derived from these are the AstroCoordSys and the time, space, redshift, spectral frames that together make up the Space-Time Coordinate (STC) system. Coordinate and Coordinate Area also have generic as well as Astro versions. We shall use “STC” as a proper name in this document which will lead to terms like “STC coordinates”. Although we agree that such usage is linguistically deplorable, we will use it nevertheless because of the clarity it provides.

We present two implementations of STC. The fundamental implementation is provided in the XML schemata and is known as STC-X. A string implementation, based on STC-X, is known as STC-S or LinearSTC.

There are six companion files to this document:

http://www.ivoa.net/xml/STC/v1.20
http://hea-www.harvard.edu/~arots/nvometa/v1.2/stc-v1.20.xsd
is the top-level schema of the XML implementation STC-X, containing the coordinate system and coordinate area definitions.
http://www.ivoa.net/xml/STC/STCcoords/v1.20
http://hea-www.harvard.edu/~arots/nvometa/v1.2/coords-v1.20.xsd
is the STC-X schema with the coordinate value definitions.
http://www.ivoa.net/xml/STC/STCregion/v1.20
http://hea-www.harvard.edu/~arots/nvometa/v1.2/region-v1.20.xsd
is the STC-X schema for the region specification.
http://www.ivoa.net/Documents/latest/STC-X.html
documents the STC-X (XML) implementation.
http://hea-www.harvard.edu/~arots/nvometa/STCPoster.pdf
is a poster on the STC schema.
http://www.ivoa.net/Documents/latest/STC-S.html
documents the STC-S (string, or LinearSTC) implementation.

The schemas are fully up-to-date.

In the following sections we shall discuss the justification and scope for this metadata specification, the requirements and use context, a detailed design description, and some implementation considerations. Some concepts are only provided in their simplest form and we expect that more sophisticated classes will be derived (and will fit in seamlessly) as work progresses. The emphasis of this document is on the framework of the STC metadata.

2 Justification and Scope

Before we plunge into the requirements and design of these metadata structures, it may be helpful to discuss what is driving this exercise and to provide some context to the design. Our intent in this is to explain why the design needs to be fully general and extensible, leading to a certain level of complexity.

The basic justification for considering the following coordinate axes in a single metadata specification is that they are closely intertwined:

Space (including its time-derivatives, i.e., velocities)
Time
Spectral (frequency, wavelength, energy)
Redshift (Doppler velocity)

It should be emphasized that the STC design distinguishes between the spectral coordinate and the redshift coordinate (one might, after all, have to deal with a dataset containing Doppler velocities measured in different parts of the spectrum) and between redshifts and velocities (the latter are physical quantities – derivatives of spatial positions; the former represent a spectral property that is transformed via one of several formalisms to what is commonly referred to as a Doppler velocity – when we get to relativistic situations the connection with physical velocities becomes tenuous). In the presence of a redshift coordinate, the values of a spectral coordinate, if present, will contain rest frequencies.

We shall refer to these as the STC coordinates. The issue is that time is bound to a position and positions are time-variable. Similarly, spectral and redshift data are tied to reference frames that may or may not be time-variable.

In the past, most data archives have not been extremely concerned with dotting every i and crossing every t, in a meticulous specification of all the details of all coordinate axes. They assumed sets of coordinate system-related defaults. Such context-dependent defaults are quite acceptable: issues are generally well-defined, obvious, and clear for single-observatory observations, even when not all is explicitly specified. However, there are no global defaults in the VO. All implicit assumptions need to be made explicit since they will not be “obvious” anymore. One must be able to transform the coordinates of two observations to a common coordinate system; this means that every little tidbit of information needs to be documented in the metadata.

Here are some simple examples of situations and questions that need an answer, when such context-dependent defaults do not work anymore:

Are Doppler velocities optical or radio?
What is the spatial reference position for the time stamps; if corrected for pathlength, what direction was assumed?
Does “J2000” mean “FK5/J2000” or “ICRS”?
At which spatial position was this observation made?
How can we get information on Galactic background objects from solar system observations, and vice versa?

Reality dictates, though, that we allow for cases where these values are simply unknown. In the XML implementation nillable elements that are nil should be interpreted as unknown values; the burden is then on the client to determine and adopt a sensible default value. This holds true for elements named “UNKNOWN” as well. We sincerely hope that the existence of such nillable and UNKNOWN elements will not be an excuse to provide incomplete metadata in cases where the values are known.

3 Requirements and Use-Context

The top-level requirement for the design of the Space-Time Coordinate metadata can be formulated in a very simple way. It is that they:

provide sufficient and necessary information
are self-consistent

That is really all. However, to fit it into the larger VO Data Model context, we should leverage two more requirements:

the system should be extensible to include other coordinates and accommodate coordinate metadata descriptions that include axes other than the STC ones
it must be possible to define relocatable frames to accommodate, for instance, simulation data

From this one can derive secondary requirements: the metadata must include complete descriptions of the following three concepts:

the coordinate frames that anchor the STC coordinate axes, collectively referred to as the Coordinate System
a specific position in these frames, i.e., a coordinate object, that includes not only coordinate values, but also essential attributes such as errors and resolutions
the volume in coordinate space that is taken up by the data object that these metadata are attached to

In all, there are four different contexts for the use of the STC metadata:

STC Resource profile – to specify the spatial, temporal, redshift, spectral coverage of a data collection which is ultimately used by clients to determine whether or not the resource is of interest for their purposes
Query – to specify the STC constraints of a particular request
Catalog information – clearly, the response to a catalog query may contain coordinate information (e.g., positions), but it should also contain coverage information, specifically, the intersection of the catalog’s coverage and the query’s STC constraints
Observational data – here we need the volume in coordinate space that is occupied by the observational data, as well as the coordinates of the observatory; note that these may not necessarily be given in the same Coordinate System; a Pixel Coordinate System is needed for pixilated data

We will return to the use of STC in these contexts in Section 6. As commented before, the emphasis of this document is on the structure of the STC metadata. Hence, though the classes and attributes are laid out in detail, little is said about methods.

4 Design

It may be helpful to refer to the UML diagrams in Appendix A: UML Representation, while reading this section.

4.1 The Metadata Components

There are three metadata components in the design, as already noted in the derived requirements:

Coordinate system
This object contains the coordinate frames that together define a complete coordinate system. There may be one or more per document.
Coordinate values
This is an object that specifies a specific position in the space defined by the coordinate system it refers to. The object contains values, units, errors, resolution, size, and pixel size, though not all of these need be present.
Coordinate areas or ranges
This is an object that defines a volume in the coordinate space specified by the coordinate system it refers to. Regions are a special case, specifically designed for 2-D (and 3-D unit-sphere) spatial coordinates.

4.2 Coordinate System (CoordSys)

The CoordSys class is nothing more than a collection of one or more Coordinate Frames, where each frame describes a set of one or more coordinate axes that belong together.

4.2.1 Coordinate Frame (CoordFrame)

A CoordFrame object provides the metadata about one or more coordinate axes. Usually, we will be dealing with one axis. The exception is the spatial frame which may control up to three position coordinate axes, as well as their time-derivatives (velocity). CoordFrame is really the base class for coordinate frames. To ensure its full generality, this base class should only contain a name. Real-life coordinate frames should be derived from this class.

4.3 Coordinates (Coords)

The Coords object specifies a particular position in coordinate space. It requires a reference (IDREF in the XML implementation) to a CoordSys object and it is made up of one or more Coordinate objects, corresponding to the Coordinate Frames defined in the Coordinate System. All objects are optional. However, in order to constitute a meaningful Coords object, one would expect at least one of the Coordinate objects to be present.

4.3.1 Coordinate

A Coordinate is an aggregate of up to 6 objects: CoordName, CoordValue, CoordError, CoordResolution, CoordSize, and CoordPixSize. Of these, only CoordName is required to be present. The others, which in their simplest forms consist of a scalar numerical Value, are all optional, though one may doubt whether a Coordinate object that contains none of these has any meaning. Although at first sight it would seem that at least CoordValue is indispensable, we shall show that there are legitimate cases where the value may be absent. Coordinate has a single Unit attribute that applies to all components. Classes that are derived from Coordinate may differ in the data types of the objects, their structure and meaning, as well as restrict allowable values for Unit. There are currently two derived classes that have only a Name and a Value: StringCoordinate and PixelCoordinate.

The numeric elements, other than the Coordinate Value, may appear singly or in pairs. In the former case it will be obvious that a single specific value is indicated. In the latter case the pair indicates a range of values.

4.3.1.1 CoordName

This required member of Coordinate is a simple string that acts as a label, presumably taken from a limited enumerated list

4.3.1.2 CoordValue

The value of the Coordinate consists of a scalar numerical or string value and a Unit string.

4.3.1.3 CoordError

Again, this consists of a scalar Value, representing an rms[1] error. However, we expect a variety of derived classes to appear with more sophisticated error descriptions. And clearly, the multi-dimensional coordinates require more information.

4.3.1.4 CoordResolution

The resolution along the Coordinate is expressed as a FWHM [2] Value. In cases where this is a poor description, one should derive a more sophisticated class.

4.3.1.5 CoordSize

The size along the Coordinate is also expressed as a FWHM2 Value, though one might expect derived classes with more precise definitions (e.g., Holmberg[3] diameter). This object is used in three contexts: in a resource profile it indicates the typical size (e.g., FOV[4]) of datasets in the resource; in queries it serves the same purpose; and in catalog datasets it allows the server to provide Coordinate positions of objects as well as their sizes.

4.3.1.6 CoordPixSize

The pixel size (scalar Value) is often a useful figure to characterize resources, queries, and datasets, even if it only provides a typical value.

4.4 Coordinate Area (CoordArea)

The Coordinate Area class defines the volume in coordinate space that is occupied by the object it is attached to. It is an aggregate of one or more ranges in individual coordinates and is required to have a reference (IDREF in the XML implementation) to a CoordSys. Multiple ranges along each coordinate axis are OR-ed together; the ranges of different axes are logically AND-ed together.

4.4.1 ScalarInterval

A ScalarInterval consists of a LoLimit and/or a HiLimit, both of which contain a double value and a unit. Note that they do not both have to be present, but at least one of them needs to be. If only one is present, an upper or lower limit is indicated. ScalarInterval has Boolean attributes LoLimInclude and HiLimInclude and a FillFactor attribute with a value between 0 and 1 that indicates what fraction of the interval is actually covered by the data.

4.5 AstroCoordSys

The AstroCoordSys is derived from CoordSys. It still is a collection of coordinate frames, but it is required to contain specific frame classes derived from the CoordFrame base class, in addition to zero or more additional (generic) frames. The required frames are TimeFrame, SpaceFrame, and SpectralFrame; RedshiftFrame is optional.

4.5.1 TimeFrame

The Time Frame contains two or three objects: a reference position, a time scale, and, optionally, a time reference direction.

4.5.1.1 ReferencePosition

The reference position, or spatial coordinate origin, may be chosen from a standard list of such origins (StdRefPosition) or specified as a particular coordinate position (CoordOrigin). In the latter case, one may nest the origins, but eventually they should be referenced to a known standard origin. The list of standard origins includes (see Table 1): GEOCENTER, BARYCENTER, HELIOCENTER, TOPOCENTER, LSR, GALACTIC_CENTER, LOCAL_GROUP_CENTER, EMBARYCENTER, MOON, MERCURY, VENUS, MARS, JUPITER, SATURN, URANUS, NEPTUNE, PLUTO, RELOCATABLE; additional values (e.g., planetary satellites, non-planet solar system objects) are allowed, provided they are identified in a referenced ephemeris or other authoritative source (see: Section 4.5.1.4). Ultimately, one must be able to tie the coordinate origin down to the geocenter, be it through a geographic position, an IAU resolution, an orbit ephemeris, or a solar system ephemeris – unless one deals with simulation data that have a RELOCATABLE origin. Planetary Ephemeris is required for any position related to a solar system entity other than the geocenter. ReferencePosition is a standard class that is being used for all four STC frames, though there are different restrictions for the four usages. For the Time Frame LSRK, LSRD, GALACTIC_CENTER, LOCAL_GROUP_CENTER, and RELOCATABLE are not allowed values; for simulations one may want to use a custom position at the origin of the relocatable spatial frame.

Table 1. Standard Reference Positions

Reference Position	Description	Comments
GEOCENTER	Center of the earth
BARYCENTER	Center of the solar system barycenter
HELIOCENTER	Center of the sun
TOPOCENTER	“Local”; in most cases this will mean: the location of the telescope
LSR or LSRK	Kinematic Local Standard of Rest: 20 km s^-1 in the direction of GALACTIC_II(56,+23)	Only to be used for redshifts and Doppler velocities, and spectral coordinate
LSRD	Dynamic Local Standard of Rest: 16.6 km s^-1 in the direction of GALACTIC_II(53,+25)	Only to be used for redshifts and Doppler velocities, and spectral coordinate
GALACTIC_CENTER	Center of the Galaxy: 220 km s^-1 in the direction of GALACTIC_II(90,0) wrt LSRD
LOCAL_GROUP_CENTER	Center of the Local Group: 300 km s^-1 in the direction of GALACTIC_II(90,0) wrt BARYCENTER	Only to be used for redshifts and Doppler velocities, and spectral coordinate
EMBARYCENTER	Earth-moon barycenter
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	Only to be used for spatial coordinates
UNKNOWN	Unknown reference position	Only to be used as a last resort The client is responsible for assigning a suitable default

4.5.1.2 TimeScale

The time scale must be chosen from among the list recognized by the IAU (see Table 2): TT, TDT, ET, TAI, IAT, UTC, TDB, TEB, TCG, TCB, LST. TT is the default; TDT and ET are obsolete synonyms for TT; IAT is an unofficial synonym for TAI; and use of LST is to be discouraged. At some point other time scales may need to be added as they become recognized, such as planet- or moon-based time scales. In most cases where TDB is specified, the likely intent was TEB, so TDB may be considered a synonym of TEB; its use requires the presence of a Planetary Ephemeris. For further details, see Seidelmann & Fukushima (1992)[i] and Standish (1998)[ii].

We will allow the value LOCAL, only to be used with RELOCATABLE space frames (see: Section 4.5.1.1).

For a handy explanation of time scales, see: http://tycho.usno.navy.mil/systime.html

For a glossary of fundamental astronomy terms, see: http://syrte.obspm.fr/iauWGnfa/NFA5_B3.html

In the XML implementations the Timescale element is nillable.

Table 2. Timescales

Timescale	Description	Comments
TT	Terrestrial Time
TDT	Terrestrial Dynamic Time; synonym for TT
ET	Ephemeris Time: predecessor of, and continuous with, TT
TAI	International Atomic Time; 32.184 s behind TT
IAT	Synonym for TAI
UTC	Coordinated Universal Time; 32 s behind TAI in 2000-2005	Includes leap seconds Pre-1972 times will be assumed to be UT/GMT
TDB	Barycentric Dynamical Time; synchronous with TT, except for variations in earth’s orbital motion	In most cases where TDB is specified, TEB is really the one used
TEB	Barycentric Ephemeris Time; independent variable in solar system ephemeris, linear function of TT	Requires specification of the solar system and planetary ephemeris used
TCG	Geocentric Coordinate Time; properly relativistic time, running a factor 7∙10^-10 faster than TT
TCB	Barycentric Coordinate Time; properly relativistic time, running a factor 1.5∙10^-8 faster than TDB
LST	Local Siderial Time	Ground-based observations only
LOCAL	“Local” time	Only to be used for simulations, in conjunction with RELOCATABLE spatial coordinates

4.5.1.3 TimeRefDirection

If the Time Frame’s Reference Position is not TOPOCENTER, times have clearly been transformed from observed time at the location where the observation was made to some other spatial location. In order to effect such a transformation, a direction of origin must have been assumed for the observed phenomenon; that direction is given in TimeRefDirection.

4.5.1.4 Planetary ephemeris (if needed)

The planetary ephemeris object PlanetaryEphem indicates which solar system ephemeris was used in the AstroCoordSys. In general, this will be JPL’s DE200 or DE405 (default). It should only be present if a solar system ephemeris was used, for instance because of a planet-based ReferencePosition or the application of barycenter corrections. In addition, the information in Sections 7.2-7.5 of the Explanatory Supplement to the Astronomical Almanacⁱⁱⁱ may be considered a known authoritative source for the purpose of spatial reference frames. If the SpaceRefFrame (see: Section 4.5.2.3) is defined on a body, ephemeris information on its coordinate system (pole, primary meridian) must be included in the PlanetaryEphem.

In the XML implementations this element is nillable.

4.5.2 SpaceFrame

The Space Frame contains a reference position, a reference frame, and a coordinate flavor object.

4.5.2.1 ReferencePosition

This object is of the same class as described in Section 4.5.1.1, with the following restrictions. The value RELOCATABLE is allowed, but LSRD, LSRK, GALACTIC_CENTER, and LOCAL_GROUP_CENTER are not. Obviously, if an explicit coordinate origin is provided, those coordinates should be referenced to a different CoordSys; one may still nest them, but eventually they should be referenced to a known standard origin.

4.5.2.2 CoordFlavor

This class is peculiar to spatial coordinates. It indicates the intrinsic dimensionality of the coordinate system (1, 2, or 3), and the type of coordinates: Cartesian, spherical, unitsphere, or POLAR. For 3-dimensional SPHERICAL (because of geographical applications) we allow the special Units value “deg deg m”. The UnitSphere has default units “”. This list will likely be expanded with, for instance, cylindrical coordinates.

4.5.2.3 SpaceRefFrame

While the Reference Position pins down the origin of the spatial coordinate system, the Space Reference Frame specifies its orientation. In most cases this will be a StdFrame, consisting of a RefSystem (FK4, FK5, ECLIPTIC, ICRS, GALACTIC_I, GALACTIC_II, SUPER_GALACTIC, AZ_EL, BODY; the first three require a CoordEquinox, which is nillable in the XML implementation). The Standard Frame may be BODY if the Reference Position is specified as a solar system object. In addition, we allow solar, lunar, planetary, and planetary satellite coordinate frames (planetocentric and planetographic) as defined by Sections 7.2-7.5 of the Explanatory Supplement to the Astronomical Almanac (Seidelmann, 1992)[iii]; see also Fränz & Harper (2002)[iv]. For the most current update of cartographic coordinates and rotation rates, see Seidelmann et al. (2002)[v].

A full list of Standard Reference Frames is provided in Table 3.

Alternatively, a CustomFrame may be specified by way of the pole (Z) axis and the longitude zero-point (X-axis) directions.

We want to provide some caveats on subtle differences between various coordinate systems that may look similar.

First, one should note that the 2-dimensional spherical versions of all celestial coordinate systems are left-handed, but that their 3-dimensional Cartesian versions are right-handed.

Second, because of the definition of longitude (IAU 1970), following astronomical tradition, longitude increases westward in the planetographic coordinate frames for Mercury, Mars, Jupiter, Saturn, and Neptune (MERCURY_G, MARS_G, JUPITER_G, SATURN_G, and NEPTUNE_G – ignoring planetary satellites), since their rotation is direct (prograde); hence, these coordinate systems are also left-handed. Note that earth, sun, and moon are exempted from this rule because of even older conventions. In addition, many coordinate systems that are based on video devices are left-handed. All other coordinate systems (including all planetocentric systems) are right-handed.

Finally, beware that planetographic and planetocentric coordinates may differ in three respects:

For some, the longitudinal directions are opposite
Planetocentric latitude is based in the vector from the planet’s center, while planetographic latitude is measured with respect to the local vertical (and hence a reference spheroid needs to be adopted)
Planetocentric coordinates are longitude, latitude, and radius; planetographic coordinates are longitude, latitude, and elevation (with respect to the reference spheroid)
The origin of planetographic longitude is tied to a particular feature on the surface; planetocentric longitude’s origin is defined purely in terms of the rotational ephemeris of that feature, which may result in slight differences.

For further details, see Seidelmann (1992, Sections 4.2 and 7)ⁱⁱⁱ, Seidelmann et al. (2002)^v, and Duxbury et al. (2002)[vi].

Table 3. Standard Reference Frames

Reference Frame	Description	Comments
FK4	Fundamental Katalog, system 4; Besselian	Requires Equinox; default B1950.0 Left-handed in spherical coordinates
FK5	Fundamental Katalog, system 5; Julian	Requires Equinox; default J2000.0 Left-handed in spherical coordinates
ECLIPTIC	Ecliptic coordinates	Left-handed in spherical coordinates
ICRS	International Celestial Reference System	Left-handed in spherical coordinates
GALACTIC_I	Old Galactic coordinates	Left-handed in spherical coordinates
GALACTIC[_II]	“New” Galactic coordinates	Left-handed in spherical coordinates
SUPER_GALACTIC	Super-galactic coordinates: pole at GALACTIC_II (47.37,+6.32) origin at GALACTIC_II (137.37,0)	Left-handed in spherical coordinates
AZ_EL	Local azimuth and elevation	Ground-based observatories Azimuth: from north through east
BODY	Generic “BODY” coordinates
GEO_C	Geographic (geocentric) coordinates: longitude, latitude, geocentric distance	3-D spherical or 3-D Cartesian
GEO_D	Geodetic coordinates: longitude, latitude, elevation	Semi-major axis and inverse flattening of the reference spheroid may need to be provided; default is IAU 1976 (6378140 m, 298.2577)
MAG	Geomagnetic coordinates
GSE	Geocentric Solar Ecliptic coordinates
GSM	Geocentric Solar Magnetic coordinates
SM	Solar Magnetic coordinates
HGC	Heliographic coordinates	See Explanatory Supplement, Section 7.2
HEE	Heliocentric Earth Ecliptic coordinates
HEEQ	Heliocentric Earth Equatorial coordinates
HCI	Heliocentric Inertial coordinates
HCD	Heliocentric coordinates of Date
MERCURY_C	Planetocentric coordinates on Mercury	See Explanatory Supplement, Section 7.4
VENUS_C	Planetocentric coordinates on Venus	See Explanatory Supplement, Section 7.4
LUNA_C	Selenocentric coordinates	See Explanatory Supplement, Section 7.3
MARS_C	Planetocentric coordinates on Mars	See Explanatory Supplement, Section 7.4
JUPITER_C _III	Planetocentric coordinates on Jupiter, system III	See Explanatory Supplement, Section 7.4
SATURN_C _III	Planetocentric coordinates on Saturn, system III	See Explanatory Supplement, Section 7.4
URANUS_C _III	Planetocentric coordinates on Uranus, system III	See Explanatory Supplement, Section 7.4
NEPTUNE_C _III	Planetocentric coordinates on Neptune, system III	See Explanatory Supplement, Section 7.4
PLUTO_C	Planetocentric coordinates on Pluto	See Explanatory Supplement, Section 7.4
MERCURY_G	Planetographic coordinates on Mercury	See Explanatory Supplement, Section 7.4 Left-handed
VENUS_G	Planetographic coordinates on Venus	See Explanatory Supplement, Section 7.4
LUNA_G	Selenographic coordinates	See Explanatory Supplement, Section 7.3
MARS_G	Planetographic coordinates on Mars	See Explanatory Supplement, Section 7.4 Left-handed
JUPITER_G _III	Planetographic coordinates on Jupiter, system III	See Explanatory Supplement, Section 7.4 Left-handed
SATURN_G _III	Planetographic coordinates on Saturn, system III	See Explanatory Supplement, Section 7.4 Left-handed
URANUS_G _III	Planetographic coordinates on Uranus, system III	See Explanatory Supplement, Section 7.4
NEPTUNE_G _III	Planetographic coordinates on Neptune, system III	See Explanatory Supplement, Section 7.4 Left-handed
PLUTO_G	Planetographic coordinates on Pluto	See Explanatory Supplement, Section 7.4
UNKNOWN	Unknown reference frame	Only to be used as a last resort The client is responsible for assigning a suitable default

4.5.2.4 OffsetCenter

In order to accommodate positions that are given as offsets in Right Ascension and Declination from some specific position, the optional OffsetCenter object is provided. N