Hi Sangana,
I'm afraid I don't much (or anything!) about remote sensing but thought I'd offer some general advice instead! When I was trying to work out my PhD topic I found that the most useful thing was to e-mail researchers that specialised in the general area I was interested in, explaining that I would like to do a PhD and asking what they were currently working on and what they would be willing to supervise. Although I'd read around what I thought I wanted to do, I didn't know much about what was likely to get funded or what the current trends in research were, so it helped me develop my ideas with those things in mind and with a bit more focus.
I know this isn't too specific to what you were asking but I hope it helps anyway!