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eci2aer(utc, x0, y0, z0, lat, lon, alt)
convert ECI to AER (azimuth, elevation, slant range)
parameters: utc: datetime UTC x0, y0, z0: ECI x, y, z lat, lon, alt: latitude, longitude, altiude of observer (degrees, meters)
outputs: az,el,rng: Azimuth (degrees), Elevation (degrees), Slant Range (meters)