in one of the fourth out rings. by doing this, they were able to establish this idea that the plane moved through the rings in this fashion. why do they think it didn't go to the north? well, there's a lot of radar up there in different cities and towns and they probably would have seen it up there but the shape of the earth is not exactly round. you get a slightly different mathematical equation if you go that way as compared to the south. but just to make sure, they checked this equation they came up with against existing planes, planes that they knew were out there and they specifically knew the location of. it's sort of like if you did a test problem in arithmetic and you had the answers for the examples. then you get to the actual problem where row don't have the answer. the one where they didn't have the answer to is where was the malaysian air flight, then they ran this equation and they said it's right there, somewhere in that region. that's what's allowed them to cut a gigantic search area down into something much more manageable and much more