Converting cartesian x,y,z positions in an ECEF frame into RA and dec in a topocentric frame?

[MagicConch]

Hi everyone,

I have the position of a satellite in cartesian coordinates measured in the ECEF frame (i.e. the position of the satellite is given in [x,y,z] coordinates with the Earth-center at the origin) and I want to convert this to and RA and Dec position as seen by the observer (topocentric frame).

Can anyone help?

Thanks!

[AstroKeith]

There are python modules that would do this (Pyephem and Skyfield)

If you want to do it the hard way, then first convert to Azimuth & Altitude at your location. Then there are well documented equations to go from AltAz to RA & Dec (requiring your latitude and sidereal time)

[MagicConch]

On 30/01/2023 at 22:45, AstroKeith said:

There are python modules that would do this (Pyephem and Skyfield)

If you want to do it the hard way, then first convert to Azimuth & Altitude at your location. Then there are well documented equations to go from AltAz to RA & Dec (requiring your latitude and sidereal time)

Hi, I managed to do this however I've noticed that the RA and dec have some offsets about 5 degrees and declination by about half a degree of what they should be, any idea what could cause this?

[AstroKeith]

Interesting.

You are so close, that it looks like your basic method is right.

First are you sure your input coordinates, and the final result you are using as a reference are correct?! Are they all at the current epoch (it is now usual to publish some data in the ICRS frame, ie J2000)

A useful check can be to compare your computation of sidereal time, with that from a planetarium App such as SkySafari.

I might guess that the first stage, x,y,z to AltAz is most likely to have an error (AltAz to RADec is more defined).