[PROJ] Non-Earth Based Coordinate Transforms

[Martin Desruisseaux]

Hello Daniel

Le 2025-01-29 à 20 h 13, Daniel Garcia-Briseno via PROJ a écrit :

> I'm a software developer on a NASA contract, I'm working on a web 
> based platform that pulls data from various providers in different 
> coordinate systems, but not all my coordinates are earth based. I 
> recently discovered proj4js which led me to you.
> Throughout your documentation there's a lot of "geo", but my data is a 
> mix of "geo" and "helio" where my coordinates are relative to the sun, 
> not the earth. Is there a way I could use proj to transform between 
> earth-relative and sun-relative coordinates? I need to convert between 
> a geocentric system and heliocentric system.

The Open Geospatial Consortium (OGC) has conducted TestBeds in 2022 and 
2023 on exactly those topics. OGC has published the following 
engineering reports:

  * 3D+ Data Space Object Engineering Report (23-011r1) —
    https://docs.ogc.org/per/23-011r1.pdf
  * Reference Frame Transformation Engineering Report (22-038r2) —
    https://docs.ogc.org/per/22-038r2.pdf
  * Non-Terrestrial Geospatial Engineering Report (23-042) —
    https://docs.ogc.org/per/23-042.pdf
  * Extraterrestrial GeoTIFF Engineering Report (23-028) —
    https://docs.ogc.org/per/23-028.pdf

A more complete list of Testbed engineering reports is at 
https://www.ogc.org/completed-cosi-initiatives/

Those engineering reports review some theoretical concepts about 
referencing in space and mention some existing software such as NASA 
SPICE. Then, they review how the OGC Topic 2 / ISO 19111 standard 
(https://docs.ogc.org/as/18-005r8/18-005r8.pdf) can be used for the 
definition of CRS in space. Those reports identify the following classes 
which would need to be added to the existing international standards:

  * InertialCRS (you will need this one for referencing relative to the Sun)
  * MinkowskyCS (for Einstein's relativity)
  * A few others

Above-cited classes are not yet approved standards. However, they are on 
the agenda for discussion in the ISO 19111 working group. It may take a 
few years before they appear in standards, if accepted. A proposal for 
encoding images in space in the GeoTIFF format has also been submitted, 
but I think that the OGC GeoTIFF working group will want to take a 
different approach than the one proposed in the last engineering report.

A prototype exists for the CRS in space and the GeoTIFF extension. As a 
proof of concept, the TestBed implemented a scenario with the NASA DART 
mission, where a satellite intentionally collided with an asteroid for 
measuring the trajectory change. A chain of coordinate operation from 
Earth to the satellite's moon has been implemented using ISO 19111 + 
above-cited extensions. More details in above engineering reports.

Please do not hesitate to contact me if you have any question.

     Regards,

         Martin

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