An Earth Occultation calculation for Satellites

I am trying to figure out if a satellite’s view of a particular RA/DEC target is occulted by the Earth over a large range of times. I have some code which I think should do this, but I am not sure the results make perfect sense. I am also uncertain as to whether or not the coordinates I am choosing for each part of the calculation make sense. Here is the code snippet that calculates the minimum distance between the Earth center and the line of sight between

        target_pos = skyfield.positionlib.position_from_radec(target_ra, target_dec)
        target_ecliptic = target_pos.ecliptic_position().au
        earth_ecliptic = Earth.at(ut).ecliptic_position().au
        sat_ecliptic = satellite.at(ut).ecliptic_position().au

        
        limit_dist = (EARTH_RADIUS + limiting_altitude) / AU_TO_KM
        #print(limit_dist)
        dist_list = []
        n_times = numpy.shape(ut)[0]
        for j in range(n_times):
            this_sat = sat_ecliptic[:,j]
            this_earth = earth_ecliptic[:,j]
            # this_sat is satellite position relative to Earth, whereas this_earth
            # and target_ecliptic are positions in ecliptic coordinates.
            # This is why we add this_sat and this_earth below.

            diff_sat_target = this_sat + this_earth  - target_ecliptic
            diff_target_earth = target_ecliptic - this_earth

            cross = numpy.cross(diff_sat_target,diff_target_earth)

            lcross = numpy.sqrt(numpy.dot(cross,cross))

            dist_list.append(lcross / numpy.sqrt(numpy.dot(diff_sat_target,diff_sat_target)))
        dist_arr = numpy.array(dist_list)

with the elements of dist_arr being tested against the limit_dist assumed above. The behavior I am finding, however, does not necessarily jive with my expectations given the satellite’s nearly equatorial orbit. So three questions:

1. Are all of these positions in a consistent coordinate system?
2. If not, what changes need to be made to put them in one?
3. Would this be something you would be interested in integrating into skyfield moving forward?

[steveehlert]

I looked over the example and it looks great! I think we have all of the math figured out so I suppose it is just a matter of putting the proper functionalities in at this point. Let me know if you need any help with that!

[steveehlert]

If you are interested in identifying what point on the Earth gives you the first “impact” then yes you will want to keep min instead of max. I am only interested in knowing whether either intersection point was positive for this particular project, so I kept max.

Also, I have now done the time bin by time bin comparison, which provides perfect agreement on 10,000 time bins.