a question about pull_in function in the open code

hi, there

Many thanks for your sharing the GPS code.

I got a question about the tracking lock loop function in the
CH_PULL_IN procedure.

"  q_sum = c->accum.q_dith + c->accum.q_prompt;
  i_sum = c->accum.i_dith + c->accum.i_prompt;
  if (i_sum != 0 || q_sum != 0)
    theta = fix_atan2 (q_sum, -i_sum); "

Is fix_atan2 (q_sum, -i_sum) the phase detecter?
Why you use Q=q_sum & I=-i_sum instead of rss(c->accum.q_dith ,
c->accum.q_prompt) & rss(c->accum.i_dith ,  c->accum.i_prompt) in the
atan2 part?

Hope I can get answer here.
Thanks again.

//tongtong