There is a doppler shift when the recipient is moving and this can be a significant issue.

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This is due to a Doppler shift the same phenomenon that causes the pitch of an ambulance's siren to seem to vary as it passes a stationary listener.

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And since a moving object causes a tiny change (known as a Doppler shift) in the frequency of the reflected signal, Silent Sentry can also calculate how fast it is moving.

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The recent conference I attended at the Lowell Observatory in Flagstaff to celebrate the 100th anniversary of his first measured Doppler shift, was a great opportunity to learn more about him.

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Rather than looking for the tiny Doppler shift of a bright star, in other words, Dr Cameron and his team were looking for the much larger Doppler shift of a much fainter planet.

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Again, the Doppler shift is the telltale.

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As is to be expected, ordinary digital transmission is nowhere near robust enough to cope with the problems experienced by a mobile receiver, which can suffer from all sorts of doppler-shift effects and multipath radio interference as it moves around the place.

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