Sunday, July 4, 2010

Can you explain the Doppler effect as it applies to satellites? ...

Q Can you explain the Doppler effect as it applies to satellites?

A The Doppler effect is a“relative” thing. The frequencies of satellite signals are affected by relative motion because all satellites are constantly moving relative to each other and to you. As a satellite approaches you its downlink frequency increases. As it passes your position and moves away, the frequency decreases.

Let's say that you are listening for a satellite beacon that has a published frequency of 435.700 MHz. That’s the frequency you would listen to if you and the satellite were moving at the same speed relative to each other. The satellite is moving thousands of miles per hour faster, however, so you must tune your receiver to a higher frequency as the satellite appears above your horizon. You'd have to begin listening about 8 kHz higher, or 435.708 MHz. (The Doppler effect is less pronounced, by the way, at lower frequencies.) You’ll notice right away that the frequency of the beacon signal is sliding steadily downward. When the satellite is at its closest point relative to your station, directly overhead in this example, you will be receiving its beacon on 435.700 MHz, the published frequency. Enjoy it while you can because the frequency will continue shifting downward! By the time the satellite vanishes below your horizon, you’ll be listening at 435.692 MHz.

The Doppler effect is a vivid illustration of the fact that everything is in motion—and we’re not just talking about satellites. The Earth rotates on its axis at approximately 1100 MPH. The Earth orbits the Sun at about 67,000 MPH. The sun circles the Milky Way at a speed of 486,000 MPH. And every object in the universe is moving away from every other object as the Universe expands at a constantly accelerating rate. In absolute terms, you’re never “standing still!”

From QST October 1999