ECI2ECEF_NAIVE rotate ECI coordinates to ECEF

because this doesn't account for nutation, etc. error is generally significant

x_eci, y_eci, z_eci: eci position vectors utc: Matlab datetime UTC

x,y,z: ECEF position (meters)

function [x,y,z] = eci2ecef_naive(utc, x_eci, y_eci, z_eci)

% Greenwich hour angles (radians)
gst = matmap3d.greenwichsrt(juliandate(utc));

x = nan(like=x_eci);
y = nan(like=y_eci);
z = nan(like=z_eci);

for j = 1:length(x)
  ecef = matmap3d.R3(gst(j)) * [x_eci(j), y_eci(j), z_eci(j)].';
  x(j) = ecef(1);
  y(j) = ecef(2);
  z(j) = ecef(3);
end
end