By John E. Blyler
The huge RF radio observatory at Arecibo, Puerto Rico has all of the key ingredients for a high-tech adventure movie. First, its location is remote, as it’s buried deep within the rainforest of a Caribbean island. Second, the sheer size of the radio telescope renders it sublime. It measures 305 m (1001 ft.) in diameter and more than 500 m from the jungle floor to the top of the moveable radio feed platform (see Figure 1). Unlike other astronomic R&D facilities in the United States, the observatory at Arecibo also is more than just a radio telescope. It also is a complete R&D facility. Its mission – in part – is to search for the stuff of science fiction stories ranging from extraterrestrials and gravity waves to asteroids that could devastate the Earth.
We will return to the cool sci-fi aspects of Arecibo later. For now, let’s explore the technology that makes all of this possible—starting with an overview of the RF telescope and the critical electronics. Radio astronomy studies celestial objects using radio transmissions. Often traveling great distances, these radio waves are reflected from the objects of study. The returning signal is analyzed and developed into amazing images. Although this may seem like a straightforward task, the returning signal is typically so weak as to be almost indiscernible from the cosmic noise.
Thus, the successful detection of the returning signal requires the very best that modern electronics has to offer. Indeed, the noise generated by even the most modern low-noise amplifier (LNA) and other sources are orders of magnitude greater than the signals being examined. Dana Whitlow, research technician at Arecibo, estimates that the return signals may be over 40 dB below the overall system noise level—a factor of 10,000 lower!
Critical Sensitivity To Noise
Simply put, everything that can be done is done to maximize the sensitivity of the receivers. The front-end electronics are cytogenetically cooled in 99.99% pure Helium to between 10 and 15 Kelvin. These temperatures can only be achieved in a vacuum. As a result, all of the specially designed electronic systems must be evacuated before the cooling can begin.
The front-end electronic systems consist of amplifiers, filters, and mixers. The amplifiers are specifically designed to minimize noise. Toward that end, Ganesan Rajagopalan, a senior receiver engineer and head of the Electronics Deptartment at the observatory, has been improving the sensitivity of the receivers by slowly replacing the existing gallium-arsenide (GaAs) monolithic microwave integrated circuits (MMICs) with indium-phosphide (InP). MMICs are devices that operate at microwave frequencies between 300 MHz and 300 GHz.
InP-based amplifiers have lower noise and higher gain than their GaAs counterparts. Yet these circuits also must be customized for the lowest noise possible. The Cornell University-based team at Arecibo collaborated with the experts at CalTech’s JPL team to make these customized application-specific integrated circuits (ASICs) tailored to a cryogenic environment. The CalTech design also has been implemented at the Allen Telescope Array (ATA) in California. ATA is a “large number of small dishes” (LNSD) array that’s designed to be highly effective for simultaneous surveys of conventional radio-astronomy projects and Search for Extraterrestrial Intelligence (SETI) observations at centimeter wavelengths.
With such innovative LNA devices, it’s no wonder that the Arecibo Observatory is considered state of the art in receiver technology. In terms of the available bandwidth per receiver, however, the facility is playing catch-up. The receivers used at Arecibo are 2 GHz wide, ranging from 2 to 4 GHz and another from 4 to 8 GHz. The goal is to widen the current 2-GHz signals, which are being received using Ultra Wideband (UWB) technology. Here too, the R&D team is working with other scientists and engineers around the globe to develop a UWB feed that will operate from 1 to 10 GHz. Such a feed would reduce the number of existing receivers from 8 down to 1, which would further reduce the collective number of noise generators in the system.
A Noisy Planet
Reducing the noise sensitivity of the receiving electronics is critical to analyzing the radio signals returning from deep space. But another challenge exists closer to home— namely, the effective “noise” created by wireless devices ranging from cell phones to data devices. The RF telescope operates to 10 GHz and includes receivers in the S-, C-, and X-bands. Wi-Fi technology occupies a relatively small bandwidth centered around 2.4 GHz—right in the middle of the lower S-band space. Another source of radio interference comes from a much more powerful source—namely, the various airports on the island. These sources are mission critical and cannot be turned off at select times during the day.
To help reduce the opportunities for radio noise interference, the Arecibo team actively works with the Puerto Rico Spectrum users’ group. In cases involving mission-critical systems like airport radar, the team has coordinated the on-off time of the radar. The airport radar goes blank for a short period of time when it points in the direction of the Arecibo observatory. Unfortunately, this well-intentioned gesture has proven to be of limited value. The radar signal has more power located in the back lobes of the radar signature than in the front lobes.
Sci-Fi Becomes Reality
As fascinating as the engineering work at Arecibo is, does it really have any practical value? Can it turn science fiction into science fact? Some would suggest that the jungle-hidden facility will play an important role in saving humanity from near-earth objects (NEOs) like asteroids, which may be on a collision course with earth. The RF Observatory has the capability to pinpoint the orbit of NEOs as far away as Jupiter or Saturn and then calculate whether that object poses a threat to humanity. Such knowledge could be used to evacuate populations and move important property to a safe location. This is just one reason why the U.S. Congress is interested in keeping the Arecibo radar telescope working.
“We are also doing a lot of work on pulsars,” explains Rajagopalan. “Pulsar timing is very important in the detection of gravitational wave radiation.” Described as a fluctuation in the curvature of spacetime, which propagates as a wave, gravitational waves were predicted by Albert Einstein’s theory of general relativity. Sources of gravitational waves include binary star systems (e.g., white dwarfs, neutron stars, or black holes).
Pulsar astronomers believe that they can detect gravitational waves. Telescopes at Arecibo, PR and the mainland US, Europe, and Australia are all part of an array that’s being used to carefully time pulsars. All of these facilities make very long, simultaneous observations of the same deep-space source using long baselined interferometry (LBI). Precise synchronization timing among the global facilities is achieved using a hydrogen maser atomic clock. Thus, the research being done here is not just astronomy. It’s planetary radar science and ionospheric as well.
What happens to the signal returning from the reflection off of nearby planets or from signals originating from a deep-space pulsar? The signal comes into the feed in a concentrated form after reflection from the big reflector (see Figure 3). An ortho-mode transducer (OMT) —some more than 3-ft. long—splits the signal into two separate channels. Noise-injection couplers are connected to one channel. These couplers inject a weak but carefully calibrated noise source into the main signal.
The injected noise signal is switched on and off at a rapid rate that’s called a “winking” rate calibration, says Dana Whitlow, a senior receiver engineer. “By a measurement of the levels later in the system with the cal on and the cal off, we can determine the system noise temperature. Also, this calibration allows us to track unique time-dependent changes and gain of the amplifiers.”
The signal then travels through isolators, which flatten out the frequency response. Effectively, they remove reflections from the amplifiers back into the earlier part of the signal path. Finally, the signal is amplified in the LNAs mentioned earlier.
All of these electronics are contained with a dewar, which is used to cool the amplifiers down to 15 Kelvin. Cables connect the dewar to the next signal-conditioning module, which contains a pulse amplifier module to provide additional amplification. Computer-selectable filters are used to exclude unwanted frequency bands, limiting the bandwidth from radio-interference sources like Wi-Fi and airport radar.
What happens if the ionospheric, planetary, or deep-space phenomena that a researcher is trying to study occur at the same frequency as the radio-interference sources—perhaps centered at 2.4 GHz (same as Wi-Fi)? To study these signals, researchers would have to go to one of the other RF telescope facilities on the mainland United States. For example, the Robert C. Byrd Green Bank Telescope in West Virginia operates in a radio quiet zone.
Aside from rejecting unwanted interference signals, filters also help to prevent the interference from compressing the gain of the subsequent signal chain. If it’s strong enough, an interfering signal could drive an amplifier into saturation. This forces the gain to go down, says Whitlow. “If there’s anything that radio astronomers hate, it’s unexpected gain changes in their signal path. It’s difficult, if not impossible, to deal with from a perspective of obtaining calibrated data of their signal or source they are looking at.” After more filtering and amplification, just to increase the signal strength, the signal is then downconverted to a lower, intermediate frequency.
One might wonder if all of these filters don’t attenuate the signal even further—especially because they are passive filters, which contain no power source to help boost the signal strength. While it’s true that passive filters attenuate the signal slightly, these attenuations can be corrected by the numerous amplifiers. Active filters would have their own problems, such as the introduction of extra noise and distortion.
Finally, the conditioned signal is sent down from the receiver platform to the control-room area some 500 m below using analog optical fiber cable. Fiber-optic cable is used because it has a much broader frequency response. Plus, it doesn’t pick up electrical noise due to the imperfect shielding of coaxial cable. Fiber cables are typically much less lossy than coaxial—especially at the higher frequency ends.
Perhaps the most compelling reason for fiber over coaxial cable is that the former doesn’t conduct lightning down to the control room, explains Whitlow. “I haven’t been down here to see this firsthand, but I’ve been told by many people that in the early days of the observatory, when lightning struck the platform, there would be sparks jumping around things inside the control room.”
Coming in Part II: We’ll delve into the technology used in the control room and laboratory, where the data is digitized and analysis is performed. Of particular interest to chip and embedded designers will be the evolution taking place from ASIC- to FPGA-based systems.