awk issue (return an array from user defined function)

I have this function (converting coordinate of an WGS84 geoid to cartesian coordinates...doesn't matter):

  function convert_geo_to_enu(coord_geo) { \
    xi=sqrt(1 - e*e*sin(coord_geo[1])*sin(coord_geo[2])); \
        \
        coord_enu[1]=(a/xi + coord_geo[3])*cos(coord_geo[1])*cos(coord_geo[2]); \
        print coord_enu[1] " hhh " ; \
        coord_enu[2]=(a/xi + coord_geo[3])*cos(coord_geo[1])*sin(coord_geo[2]); \
        coord_enu[3]=(a*(1-e*e)/xi0 + coord_geo[3])*sin(coord_geo[1]); \
    \
    return coord_enu \   # <-- here comes the problem
  } \


problem --> mawk: line 64: illegal reference to array coord_enu

What is the problem to return as an array? Is there any different syntax?

I could use:

  function convert_geo_to_enu(coord_geo, coord_enu) { \
       ...
       coord_enu[1]=...
       ...
  } \

or even:

  function convert_geo_to_enu(coord_geo) { \
       ...
       coord_enu[1]=...
       ...
  } \

and then just use the variable coord_enu as a global?

But it looks better with usage of return statement (esp. for me)