Radiotelescopes and human-generated radio "noise"

[Boyo_Jim]

How do radiotelescopes filter out all (or most of) the signals generated by humans.

Not only do they have local TV/radio stations, cellular cells, shortwave bouncing up and down through the atmosphere, they have dozens, maybe hundreds by now, of earthly satellites buzzing around the planet transmitting on thousands of frequencies 24 hours a day.

Do the have to shut down or reorient when some comsat crosses through their focus. Do they have schedules of all these orbits so they can figure where to point so a GPS satellite DOESN’T run across their field of interest?

[Achernar]

In my experience, this is not as big a problem as you might think. Many radio telescopes are placed in radio-quiet zones, so earthbound signals aren’t much of an issue. I don’t know about satellites, but they certainly wouldn’t have to shut down for one. Even if you do get one, it’s pretty easy to tell the difference between an artificial signal and a natural one, and it won’t be in the focus for more than a few seconds.

[pldennison]

There are also portions of the spectrum that are reserved for radioastronomy use; the bandwidths differ depending on the region of the world.

There was a huge controversy a few years ago when the Iridium satellite network was going up because it was going to be stepping all over some adjacent frequencies, making work difficult:

Even before the first satellite was launched Iridium was the subject of considerable controversy. In order to communicate with small handheld mobiles each spacecraft has to transmit a powerful radio signal and the frequency of that signal is very near to the 1612 MHz resonance of the hydroxyl molecule. In theory this band is reserved for radio astronomers, but since the Iridium wavelength is nearby it has been impossible to prevent interference. Some agreements are now in place restricting Iridium transmissions but the situation is not very satisfactory from a radio astronomy point of view.

[pldennison]

There are also portions of the spectrum that are reserved for radioastronomy use; the bandwidths differ depending on the region of the world.

There was a huge controversy a few years ago when the Iridium satellite network was going up because it was going to be stepping all over some adjacent frequencies, making work difficult:

Even before the first satellite was launched Iridium was the subject of considerable controversy. In order to communicate with small handheld mobiles each spacecraft has to transmit a powerful radio signal and the frequency of that signal is very near to the 1612 MHz resonance of the hydroxyl molecule. In theory this band is reserved for radio astronomers, but since the Iridium wavelength is nearby it has been impossible to prevent interference. Some agreements are now in place restricting Iridium transmissions but the situation is not very satisfactory from a radio astronomy point of view.

[Padeye]

Did you notice those big parabolic reflectors? They do a good job of gathing signals coming from one particular direction and focusing it on the detector. I’m going to throw some arbitrary numbers out there to explain a signal to noise ratio. Say the dish is 100ft across, that’s about 7,854 square feet. Say the detector is 1 foot in diameter, let’s make it 1 square foot to keep it simple. Stray raidio noise coming in from any other direction than what the dish is aiming at will be 1/7,854 as strong as signals coming from where the dish is aimed. Parabolic reflectors for “tele” microphones work the same way. background noise is just as loud but sound coming from the direction the dish is aimed at are so much louder at the detector.

[The_Bad_Astronomer]

Actually, it can be more complicated than that. Antennas have side lobes, which is like signal that leaks around the edges. A car’s spark plug, for example, can cause significant noise in a single dish antenna. At Green Bank, WV, where the National Radio Astronomy Observatory runbs several big dishes, they use old diesel cars without spark plugs. They rpomote bicycles there too. I biked around there once, which was a pain, because it’s subtly downhill all the way to the dishes, making it hard coming back.

If you have more than one dish, like the Very Large Array in NM, then you can filter out noises that occur only in one dish and not in the other ones. It’s called anti-coincidence, and works fairly well.

Still, the best bet is to go to a radio quiet zone. The VLA is way out in the middle of nowhere, and Green Bank ain’t exactly downtown either.

[Achernar]

*Originally posted by The Bad Astronomer *
I biked around there once, which was a pain, because it’s subtly downhill all the way to the dishes, making it hard coming back.

I love that. You can go from the sleeping quarters to the 140-foot without pedaling. If you make sure to miss the gate, that is.

[JRDelirious]

Just as an anecdotal point – shortly before you get to the Arecibo Radio Observatory you used to see a sign on the side of the road requesting you refrain from use of cell phones and other personal communication equipment (haven’t been there in over 4 years, don’t know if that’s still the case). The land immediately around the Observatory is zoned such as to prevent the installation of cells, repeaters or other such comms equipment w/o having NAIC take a look at what you want to install first.