The U.S. Navys effort, to adapt its Aegis air defense system for anti-ballistic missile work, is running into some unexpected problems because of the shape of the planet earth and unpredictable gravity. The Aegis radar system was originally designed for spotting hostile aircraft that could be brought down by surface to air missiles fired from ships. The software assumed a perfectly spherical earth with unvarying gravity, as the radar had to take into account the curvature of the earth, and the effects of gravity, when making calculations (based on radar signals bounced back from enemy aircraft) about where the hostile warplane was. But ballistic missiles are much farther away (500 kilometers or more) and higher (over a hundred kilometers). As tests of the anti-ballistic missile version of Aegis (and the longer range Standard 3 missile that is part of it) proceeded, engineers began to notice something that geographers already knew. The earth is not perfectly round, and the gravitational field has slight variations to it. These normally minor and inconsequential variations became meaningful. At the longer ranges, minor irregularities resulted in the missile missing the incoming warhead. The solution is classified, but probably involves databases of these variations and a lot of computing power.