Chasing RFI Waves – Part Two

Here is part two of “Chasing RFI Waves”, my unfinished non-fiction work about the National Radio Astronomy Observatory. You can also read parts one, three, four, five, six and seven.

So, how in the world can they get these weak signals? How can they see them over the man-made noise? There are certain requirements whose details are given below.

A quiet area

That’s why NRAO is in a high mountain valley, where it has fairly good terrain shielding. Radio signals don’t like to go through dirt, forests and mountains.

A rural area in which the population growth will be limited into the future

How do you get that? Through a national forest. You cannot build a permanent dwelling in a national forest. You have difficulty even running power lines through a national forest. If you look at a map of the Green Bank area, you will see that they are surrounded by a lot of state and national forests. The population of Pocahontas County, which is one of the larger counties in West Virginia, is around 9,000. There are three stoplights in the entire county. NRAO was placed in this rural area – that will remain rural into the future – for a good reason.

A way to control transmitters around you

Hence the National Radio Quiet Zone. That’s what Mr. Wesley Sizemore has cared for over the past two decades. The NRQZ was enacted by the two frequency-regulating bodies in the United States: one of them is the FCC (Federal Communications Commission); the other is the NTIA (National Telecommunications and Information Administration), which regulates government users such as the military, Justice department, ATF, FAA, etc.

Those two bodies formed the NRQZ in 1959, which is an area of 13,000 square miles in WV and VA, in which all permanent, licensed radio transmitters must meet NRAO criteria. That means that NRAO enjoys the privilege of reviewing the specifications for proposed transmitters in the NRQZ and commenting on them to the FCC and NTIA. The enforcement authority stays with the FCC/NTIA. They make the final decision, but the comments/recommendations provided by NRAO play a large role in that decision.

There are instances where those limits are exceeded. Radio astronomy recognized that the protection of life and property is more important than radio astronomy. The best example are the local 911 services in Pocahontas County. They have a standing waiver from NRAO for emergency communications only. The dispatch is monitored by the Sheriff’s Department, and they use it only for emergency communications. There’s no chit-chat over the air, such as “John, your wife called, bring home a loaf of bread,” and other such nonsense. Any such waivers are granted on a case by case basis. For example, if two towers need to be built in order to get proper coverage for an approved service, that might be okay. But if a thousand towers need to be built, that won’t happen. NRAO entertains waivers for emergency communications only. If folks are out to make a profit from using the radio waves in the NRQZ, their application will not get approved.

Helpful local legislation

A WV House Bill, a Zoning Act, also gives NRAO a 10-mile radius of protection around any radio telescope in that state. The NRQZ only regulates permanent, licensed fixed transmitters. It doesn’t regulate things like a digital camera, or a family’s own radio service (think cheap walkie-talkies you can get at department stores), or radios, or emissions from motors on ski lifts at Snowshoe, or leakage from the cable TV system, or leakage from power lines, etc. The House Bill gives NRAO legal standing on everything else in the community that isn’t regulated by the NRQZ legislation.

This all sets the groundwork for the reception of weak signals in terms of outside factors. But what about internal ones?

Cryogenically-cooled, vacuum-enclosed receivers

The next thing needed is a way to receive those weak signals without adding any more noise to the reception. The Robert C. Byrd GBT (Green Bank Telescope, not “Great Big Telescope”) is a 100-meter dish. NRAO would like to have a 1,000-meter or a 10,000-meter dish, to be able to collect as much of the weak signals as possible, but that’s not structurally or economically feasible. The GBT collects the very weak signals and runs them through a cryogenically cooled amplifier.

Why cryogenically cooled? Because of static, or internal noise from the components. Have you ever tuned your radio inbetween stations? What did you hear? That’s the noise made by the internal components – it’s little molecules, bumping into another and making noise. If you take that radio and put it into a deep-freeze, the molecules will slow down and the static will be greatly reduced, if not disappear altogether. You will have less thermal noise generated by the internal electronics.

So with the GBT, the first amplifier that the signal hits is cryogenically cooled. It’s cooled down to almost absolute zero (0K or -273.15°C). They use liquid helium to get it around 7K or -266.15°C. When they do that, the very weak signals can be amplified without adding any more noise to them. The amplifier is also in a vacuum chamber, because it acts as a very good thermal insulator. Once that’s done, they can shift it to an ambient-temperature amplifier, filter it, massage it, do what they need to do with it. It’s now big enough to work with.

Band-optimized receivers

NRAO receivers are also optimized for a particular bandwidth. For example, one of the NRAO engineers can take any FM receiver and tweak the electronics in it so much that it will bring in your favorite station perfectly, but you won’t be able to get any other station. FM receivers on the market are tuned for compromise. They are tuned in such a way that a range of stations can be received, but the quality of the reception for each of those stations is reduced.

Every one of the components in an NRAO receiver is optimized for a certain band. That means there are different receivers for different bands. It’s like using different radios for different stations. Receivers are then mounted on turrets that can bring each of them into the focal point of the telescope depending on what band they need to observe. Right now, NRAO can observe anything from 100 MHz to 50 GHz.

Paraboloid dish surface

NRAO works with two types of dishes: a “perfect paraboloid” and a “partial paraboloid” (also called an “offset feed” or “clamshell design”). Their biggest dish, the GBT (Green Bank Telescope), uses a “partial paraboloid” design; its advantages will become clearer in Part Four of this series.

They are currently attempting to receive frequencies up to 100 GHz, but that depends on how accurate they can keep the surface of the telescope’s dish. Any imperfections in the surface of the dish make it harder to focus the waves on the receiver. Surface accuracy becomes crucial for higher frequencies such as 50-100 GHz.

That’s because the higher the frequency, the shorter the wavelength. Think about this in very simple terms. If you were to stick a plank in a wading pool, and you caused a ripple, then a splash, and counted the waves (big and small) hitting the plank, in which case did more waves hit the plank? Was it the ripple, or the splash? It’s the same with radio waves. Their size needs to decrease if they are to have higher frequencies. And if they’re smaller, any irregularity in the telescope surface will deflect them at a different angle than the expected one, thus causing them to veer off the receiver’s field of view, smearing the focal point.

Ability to track the signal

This is where movable telescopes earn their keep! Some of the telescopes only move North and South, and depend on the earth’s rotation to bring objects into focus, and yet others, like the GBT, are fully steerable. The source can be picked up as soon as it comes across the horizon, and it can be tracked all day long. The added advantage to that is the ability to do a long-term integration, where numerous scans from days of tracking are combined in order to isolate and eliminate any random noise. As study over study is pasted on top of each other, the noise floor can be pushed down, and the true signal can be brought above it. However, there’s a caveat to integrations. If there is a burst of interference during those times, it drives the noise floor up unexpectedly, obscuring the astronomical signal. Fortunately there are techniques to work around that, but it is something that researchers watch out for, such as taking short looks with the telescope and adding things together later.

Given the tiny strength of the signals received, and the problems inherent in capturing them, it makes sense to really worry about RFI, or Radio Frequency Interference. It doesn’t take much to swamp the astronomical signals. One analogy Mr. Sizemore uses is that radio astronomy is like trying to see a flashlight in front of a spotlight. That is why the NRQZ is maintained. Interference is interference is interference. It doesn’t matter where it comes from. NRAO and Mr. Sizemore work hard to maintain an interference-free environment on-site, in the local community and the larger area of the NRQZ. There is no one else in the world looking for signals as weak as the ones chased by NRAO.

They are now even concerned about digital cameras. Any electronics can emit signals. While they comply with FCC Part 15 rules about unlicensed devices, and won’t interfere with any normal devices in the world, they will most definitely interfere with NRAO’s telescopes. Things that don’t interfere with anybody else in the world interfere with NRAO!

The higher the frequency of a wave, the harder it is to shield against it. While it’s relatively easy to shield against AM waves by simply building windows with frames 1-2 meters apart, and to shield against cellphone signals by planting pine trees whose needles are about the same length as a phone’s antenna, it’s much harder to shield against smaller waves.

Most electronics nowadays are in the lower frequency range, which is below a few GHz, approximately 1.5 GHz. But as computers become faster – think about your 3.0 GHz Pentium processor, signals get higher and higher in frequency and they become more difficult to shield. But the good thing is that higher-frequency signals don’t like to go through dirt and trees, and they become a little more attenuated.

NRAO has no legal standing when it comes to mobile transmitters and satellites, unless they’re in an allocated radio astronomy band or happen to be encroaching into an allocated band. Even though the NRQZ exists, it’s not totally RFI-free, but compared to the rest of the world, NRAO has a unique location. They have better access to the spectrum than anywhere else. There are quieter areas in the world, such as the Amazon, the Arctic and Antarctic, but it’s not economically feasible to operate a radio telescope in those environments.

Water vapor can absorb high frequency waves. On foggy or cloudy days, high frequency work can’t be done. That’s why NRAO is building a telescope array in Chile, where the site is above the clouds. They’ll get much better reception of high-frequency waves there.

Any electrical arc emits interference. Whether it’s lightnining, at the lower frequencies, or spark plugs on a vehicle, or an arcing thermostat, RFI is emitted.

The NRAO is funded through the NSF (National Science Foundation). One area where they always have to watch for interference is their pocketbook. There are always people who demand “bang for their buck”, or rather the taxpayers’ bucks, since the NRAO is ultimately funded by them. Since NRAO is engaged in basic research on a daily basis, and this kind of research doesn’t pay off immediately, they are constantly in need of justifying their existence. If you are a scientist, you know that knowledge for knowledge’s sake is worth the pursuit, but for some people, like the politicians or the irate taxpayer, that isn’t necessarily on their radar.

Here are just a couple of arguments in their defense.

In any area of basic research, there are always predictable and un-predictable spin-offs. The predictable spin-off in radio astronomy is radio receivers. The amplifiers that NRAO uses are the best in the world, period. Therefore, they are the proving ground for receiver technology. What they have in their receivers today will be in your home entertainment system, your cellphone, in 5-10 years.

The unpredictable spin-off could be the next Teflon, the material that was originally used in the Apollo project and is now used by all of us. The shining star in the area of radio astronomy, as far as unpredictable spin-offs are concerned, are the original MRI (Magnetic Resonance Imaging) algorithms. When molecules in our bodies are placed in a big magnetic field that is turned on and off, they will emit radio signals. To a receiver, it makes no difference whether the signals are coming from space or our bodies. That receiver will record the signals and transmit them to a computer for processing. The algorithms and computer programs that astronomers were using to make sense of their data were directly translated to medical imaging technology.

While current-day MRI technology has advanced far beyond this, numbers from an MRI scan can still be plugged into NRAO programs today, and they can generate a false-color image of that particular radio signal. They can color it, set levels, so you can obtain contoured images. So the original MRI technology was a direct spinoff that couldn’t be predicted yet turned out to be very useful.

That was part two. You can also read parts one, three, four, five, six and seven.