Format Position in JSON simulation of SITL

How to convert LLA (lat lon alt) to position (north, east, down) (m) earth frame for JSON

I’d like use LLA as (deg deg meter) in FDM and send to ardupilot.

Or I must to convert first position as home to ECEF and new position to another ECEF and with different of two points to NED?!! or ECI !!