SoulFlight, there's a dedicated Civa INS thread in this forum. Find it here, just one page away. http://www.captainsim.org/yabb2/YaBB.pl?num=1285100008  8-)
The 707 has a INS/Doppler switch and is driven by the Civa in AP "Nav" mode only, when that switch is set to INS.
You will find that being explained in the mentioned thread, so apologies for my short answer with the big, big recommendation to dive into that thread and find out about e. g. DME updates and procedures.  

Feel free to post any new Civa related question there, but please take the time to search for already given answers.  :)