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Ten planetary nebulæ yielded perfectly satisfactory evidence of line-of-sight motion, the swiftest traveller being the well-known greenish globe in Draco, found to be hurrying towards the earth at the rate of forty miles a second.
It was a modification of the devices used to clock artificial satellites in their orbits and measure their distance within inches from hundreds of miles away. The purpose was to make a really accurate map of the park. There were other instruments in other line-of-sight positions, very far away. Lockley's schedule called for them to measure their distances from each other some time this morning.
Over-the-horizon HF radars use the ionosphere as a kind of mirror to "see" around the curvature of the earth. They have a variety of uses, both military and civilian. And they have the advantage over line-of-sight microwave radars of being able to cover enormous areas with much less power and at a fraction of the cost of the latter.
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