Add Earth Center Earth Fixed (ECEF) reference frames

It would be useful to be able to define an orbit using a state vector in Earth Center Earth Fixed (ECEF) reference frames, such as ITRF2014.

🐞 This is a new feature to facilitate the usage of the package.

🎯 Precise GPS orbits are normally given in ITRF reference frame. Therefore it would be of great help to be able to use ECEF state vectors to define an orbit, to be able to perform mission analysis for GPS-based positioning.

💡 This is just sample code using astropy to do the reference frame transformation

     # These are state vectors in Earth Center Earth Fixed (ECEF reference frame)
    pos = [SV.X[0], SV.Y[0], SV.Z[0]]
    vel = [SV.VX[0], SV.VY[0], SV.VZ[0]]

    from astropy import coordinates as coord
    cartdiff = coord.CartesianDifferential(*vel, unit='m/s')
    cartrep  = coord.CartesianRepresentation(*pos, unit = u.m, differentials=cartdiff)

    # Transform state vector from ECEF to J2000
    itrs = coord.ITRS(cartrep, obstime = epoch)
    gcrs = itrs.transform_to(coord.GCRS(obstime=epoch))

    #This should be coordinates in J2000
    pos = gcrs.cartesian.xyz
    vel = [gcrs.velocity.d_x.to_value(u.km/u.s), gcrs.velocity.d_y.to_value(u.km/u.s), gcrs.velocity.d_z.to_value(u.km/u.s)]

    # Generate orbit using J2000 state vector
    orbit = Orbit.from_vectors(Earth, pos, vel * u.km / u.s )

📋
I am not familiar with the package yet to be able to propose a solution.

[astrojuanlu]

Hi @jtegedor! With the change we just landed in master, thanks to @hrishikeshgoyal in #554, now you can do this (copying and pasting from your original example):

     # These are state vectors in Earth Center Earth Fixed (ECEF reference frame)
    pos = [SV.X[0], SV.Y[0], SV.Z[0]]
    vel = [SV.VX[0], SV.VY[0], SV.VZ[0]]

    from astropy import coordinates as coord
    cartdiff = coord.CartesianDifferential(*vel, unit='m/s')
    cartrep  = coord.CartesianRepresentation(*pos, unit = u.m, differentials=cartdiff)

    # Transform state vector from ECEF to J2000
    itrs = coord.ITRS(cartrep, obstime = epoch)

    # Generate orbit using J2000 state vector
    orbit = Orbit.from_coords(Earth, itrs)

Hopefully, a bit easier. For your information, the orbit will be internally converted to ECI (GCRS). We will release poliastro 0.12 with this functionality in two weeks, but you can test it using pip install git+https://github.com/poliastro/poliastro.git if you want to give us early feedback. With your permission, I am closing the issue, but feel free to comment. Thanks again!

[astrojuanlu]

That’s because ICRS has a fixed definition that does not depend on the time of observation 😃 Further reading in https://github.com/poliastro/poliastro/wiki/Reference-frames#links