A Question about Reliability analizing and Prediction

Dear RiAC

I think RiAC is special to analize and predict electronic system reliability by Mil-hdbk-217F

So I have the question about analizing some system which might be operated in Gm(Ground Mobile) and Gf(Ground fixed)

If the ratio of operating time in Gm environment is 10% and the ratio of operating time in Gf environment is 90%

,eg multi environment, how do i analize reliabilty of that system ?

Is it right equation ? –> failure rate = basic failure rate * (Pi(Gm)*0.1 + Pi(Gf)*0.9)

Plz answer that question ~

Thanks for reading my letter

Best Regard