I knew a guy that built gravity meters for a living, and other geophysical equipment. He told the story of how the missles take into account gravity anomalies over which they will travel into account. Changes in gravity due to mountains, oceans, changes in rock, etc. All of that is programmed into the computer. 9This was before GPS, etc.)
But the missles still were off course a bit. The government had him do some studies on determining if there was such a thing as “anti-gravity”. He did gravity measurements deep in tunnels, and high up on towers at the same location, among other things.
He said he thought it was conceivable that there is “anti-gravity”, but that it was too small for him to measure. And I would think that if it is too small to measure, if it does exist, how you would harness its power.
One of the problems is that the "source" of gravity is still a mystery. Sure, we know its properties, and that one of the properties of mass is gravity, but the source of gravity is one of the things that still puzzles physicists and scientists.
Something interesting is that it was GPS that proved a part of Einstein's theory of special relativity. The problem was that things on the earth kept shifting because their coordinates kept changing. GPS relies on very specific time-keeping. What was discovered was that time "runs" at a different rate in orbit than it does on the surface of the earth, and they realized that regular time updates needed to be made. And that's because gravity effects the rate at which time "runs."
Mark