I have experience in working with Spirent GNSS Simulators. I have also myself designed and developed the IRNSS Simulator for our Indian Regional Satellite Service (IRNSS) project.  I am not sure about the exact nature of the problem you want to simulate.  However, one needs to understand that there are a certain difference with the way simulators generate the signals and as it happens in reality. Majority of the artificial synthesis of the impairments in a simulator use sophisticated models which are close approximation of the reality. Hence you should  always prepared for some amount of difference with the simulator generated signals and what actual happens in the real scenario.

All the best of your Research.

Great to know that you already have a vast experience with simulators and thank you so much for your reply.

The problem I face with the simulator has to do with RTK fix from a modem. I am using a modem that provides me with RTK fixes from popular sources like Leica and when I simulate a position on the simulator I do get the RTK fixes for my receiver under test. However, these fixes are not constant and so the solution I get keeps switching between both RTK float and RTK fix. Also when I use the same modem and a same receiver (without any simualtor) I get the fixed solution constatntly. 

I need to know or somehow be able to simulate RTK corrections constantly for a position I want to.

I hope you get to know what I am trying to do. 

thanks,

appreciate your time,

best Regards!