your post is very old, I suppose you found a solution to your problem.
I'm getting started to work with ZTD data (WRFDA v.3.7.1), could you help me to find where is the operator in the 3DVAR code to assimilate it? I've deeply searched in the /var/da directory (da_gpspw).
Do you know where I can find an example of gpsztd dataset to perform a test and to see the format?