draft 9-21-00 10-13-00 10-20-00

Plenary Talk Oct. 23, 2000 Monday 8 am, San Geronimo Ballroom, Caribe Hilton

IEEE International Ultrasonics Symposium

Rodolfo Almar, General Chair

Peter Wright, Technical Program Chair

INTRO

Slide : Aerial view of the observatory

Introductory remarks. Thank you chairman ALMAR..... Jon Hagen, sr. staff engineer at the observatory and employee of Cornell University, as are all the 140 or so employees at the observatory. I came to Puerto Rico from Cornell in 1971 and have been back and forth between Ithaca and Arecibo for most of the time since then. Happy to have a chance to tell you about the observatory and the science and engineering done there. I see that the title of this plenary talk is "probing the heavens from paradise" - we actually start our probing quite low - in the atmosphere and ionosphere. The ionosphere was the originally intended target and, for the first few years, the observatory was known as the Arecibo Ionospheric Observatory.

QUICK TOUR

This aerial view shows you the bumpy haystack hill topography -limestone karst geography 50 miles west of here. This topography more than any other factor was responsible for Cornell picking Puerto Rico as the site for the observatory - back in 1958.

Here's the reflector - 1000 ft in diameter.

Slide : Aerial closeup view of the observatory

If we zoom in , flying overhead, you'll make out the three towers and the cables that hold

the suspended feed structure at the "focal point" of the reflector. Point out roads, cable car, buildings. Here the feed structure isn't very clear so let's look at it from the ground.

Slide : feed structure viewed from the ground

The telescope is pointed by moving the feed antenna. The main dish remains fixed. (Good thing, as it would be hard to tip and rotate a 1000 ft dish. There are actually two feed antennas on the suspended structure. The original "line feed" and the Gregorian feed inside the dome.

When we say "feed" we are thinking transmitter. If we think receiver, we could say " collector" instead of feed. Reciprocity lets us assume whichever viewpoint is most convenient. A feed antenna at the focal point of a reflector is now a common site - anywhere you go you see home satellite dishes for tv. The general public seems to understand the basic operation. But before they were common, visitors sometimes looked up at the "antenna" and supposed that the dish below was the safety net. A more common notion is that the telescope forms an image whereas it really looks at only one pixel at a time. Images can be built up by raster scanning an area of the sky. I should qualify this: in radar astronomy an image is actually formed. And there are plans to install a feed arrangement with seven horns that will see seven pixels at a time.

Main point here is that the telescope is steerable i.e. the beam is steerable - up to 20 degrees away from the zenith. We can see anywhere within a cone. The earth's rotation moves the cone, so over the course of a day, we can see a belt. While we observe any fixed point in space, we have to counteract the earths rotation by moving the feed both in azimuth and elevation in a way that is equivalent to a polar mount. The antenna is fitted with receivers that cover a frequency range from about 50MHz to about 8GHz. In addition to these radio astronomy receivers, the antenna also has transmitters for 50 MHz, 430 MHz, and 2380 MHz radar work. The 230 MHz transmitter is located here in the suspended structure. It provides one megawatt of cw power. Its prime power comes from an on-site 3 Megawatt gas turbine generator. (200 gal/hour).

The physical structure itself is impressive - The platform is around 500 feet above the center of the bowl. The tallest tower is 365 feet high. The other two are about 260 feet high. There are ladders up the sides but it's a work out to climb up. The suspended structure weighs around 700 tons or 1.4 million pounds which is somewhat heavier that a loaded 747.

Slide: Beneath the bowl, showing reflector above and one of three tie downs

There is a laser radar to complement the atmospheric and ionospheric capabilities of the main telescope, but complement is the word. The 1000 ft telescope is the centerpiece and the science that goes on at Arecibo is all determined by the capabilities of the telescope. Let me quickly show you some examples of results from Arecibo in the three main areas: radio astronomy,

ionospheric studies, and radar astronomy.

FAST LOOK AT THE SCIENCE

The objects most studied at Arecibo are pulsars and interstellar and intergalactic hydrogen clouds. The large collecting area of the dish makes Arecibo the best radio telescope for these studies.

Slide : Radio Astronomy result (pulsar) - mention Nobel Prize

Slide: shift of periastron - slowdown of pulsar due to emission of gravity waves.

Mention pulsar planets, 1991

Slide : Ionosphere result (O+, He+ H+ profiles)

This is an example of incoherent scatter data. The observatory was originally built as an ionsophereic probe to use a new technique - incoherent scatter. Needed lots of antenna area and tx power. It wasn't long before radio astronomy became more important.The high sensitivity allows us to see more reflective targets at very long ranges. Radar has provided information on planets, planetary moons, asteroids, comets, and even the rings of Saturn. The resolution can be extremely good:

Slide : Radar image of an asteroid 1999 jm8 60 sec light time (25 times moon distance when observed) size 5 km (large for a near earth asteroid. All are optically discovered.

Slide : Radar Astronomy image of the moon

ADMINISTRATION, HISTORY

NAIC Viewgraph Facilities, Research Areas, Operation

National center - open to visiting scientists. Other national centers include NRAO, optical observatories, various high energy particle accelerators, magnet lab, etc. Cornell now runs NAIC for the NSF under a cooperative agreement (somehow different from a contract) but the obs was originally conceived of and built by Cornell between 1958 and 1963. The proposal was immediately funded by the government - probably because the soviets had just launched sputnik and here was a chance for the US to probe outer space using high-power ground-based radar.

Three areas of science: 75% Radio Astronomy (passive reception of pulsars, hydrogen line, etc.)

5% Radar Astronomy (planets, asteroids, comets)

15% Ionospheric studies (the original mission).

Viewgraph of the Visitor Center - Donated by the Angel Ramos Foundation of Puerto Rico,

gets more than 120,000 visitors each year.

OPTICS

2. Why is the main reflector spherical?

...... so that pointing can be done without moving the reflector

How does that work?

Viewgraph of vertically incident rays on sphere.

Here we have parallel rays (from a distant source) incident vertically on a spherical

reflector. These rays come to an approximate focus at a distance halfway up to the center of

curvature. For us, this is 435 ft above the center of the bowl. (The radius of curvature of the bowl is 870 ft.) Let's look at a close up of the focal region. (point it out first)

Viewgraph of the caustic

Here we get a good look at the spherical aberration up close. While the rays don't come to a point focus, as they would if the reflector were parabolic, there is an area through which they all pass. This region is called the caustic. In fact, all the rays pass through a line at the center of the caustic. This line is 40 ft long if we consider the rays from a 700 ft diameter circle on the reflector. We can correct the spherical aberration if we can build an extended collector that spans the caustic region to collect all the rays and phase them all up properly.

Viewgraph of the line feed

Here is a collector that does the job. It's known as a line feed (again thinking from the transmitting standpoint). Feed, pickup, transducer, collector. It is a tapered waveguide with radiating slots cut strategically along its length. This one is 96 ft long and corrects all the rays over the complete 1000 ft aperture.

So why go to all this trouble?

Viewgraph of tilted caustic.

Here's what happens when the source is not overhead. The rays now come in at an angle.

But, because of the spherical symmetry of the reflector, they again form the same caustic. Except, of course, that the caustic is rotated and translated. But, if we move our line feed into the new position, we can receive these rays as well as we received vertically incident rays. (Not quite - notice the vignetting. What motion is required of the line feed? As if it were gimbelled from the center of curvature. Let's see how bad an upward pointing parabolic antenna would have been:

Viewgraph of focal point of parabola for vertically incident and 3degree off vertical rays. Here's the wonderful point focus for vertical rays, but over here is a very blurry region where 3 deg rays are concentrated. We could build a special collector for this particular concentration , but it would

be good only for sources at 3 degrees off vertical!

Viewgraph of antenna geometry - drawing

So here is our geometry.

The feed is mounted on a carriage that moves along a curved beam a radius of 435 ft. That beam rotates about a vertical axis. These two motions allow the feed to move as though it were gimbelled from the center of curvature.

Viewgraph of antenna geometry -photo

Here is the arm. It rides on a circular track that hangs from the support triangle. The tallest tower is 365 ft high. The other two are 265 feet high. There are ladders going to the top - you wear a belt that clips to a vertical bar, allowing you to lean back and relax. By the way, the 1000 ft

dish covers about 20 acres. Its aluminum surface has a radius of curvature with an rms error

of about 2 millimeters.

Now just when you think the optics is over - there's more.

Viewgraph of Gregorian rays in 3-d

The line feed arrangement is an electromagnetic solution to the aberration of the sphere - it actually works so well that the gain of the antenna is somewhat greater than typical parabolic antennas - an aperture efficiency of 70% vs. 50%. But it works only over a limited frequency range - about 20 MHz - no matter what the center frequency is. Different line feeds had to be made for different frequencies. In a recent upgrade program that took several years, a new feed system was installed that corrects the spherical aberration with geometric optics. This is a frequency-independent solution. High freq limitation is the primary surface and the

accuracy of the feed positioning. The rms accuracies should be about 1/20 lambda, so for 3 cm (10GHz), we need accuracies of about 1.5mm rms - over distances of hundreds of meters.

Viewgraph of Gregorian ray tracing with outline of dome and feed tower

Two additional reflectors succeed in bringing all the rays to a point focus where simple horn feeds can be used - the same horns that are used with parabolic reflectors. (It could be done

with one additional reflector but two allows you to distribute the rays evenly on the main dish).

In a Gregorian system, the secondary reflector is beyond the prime focus and is concave.	In a Cassegrain system, the secondary reflector is inside the prime focus and is convex.

Viewgraph - photo of Gregorian aperture

Viewgraph - photo of receivers and underside of rotating floor.

Viewgraph - photo of Varian S-band klystron

METHODS ON MORE DETAIL

INCOHERENT SCATTER

VIEWGRAPH - INCOHERENT SCATTER RADAR DIAGRAM

Now I'd like to show you in more detail how the instrument accomplishes its various measurements. The original mission was to probe the ionosphere - the ionized layer of the atmosphere in the region of 200 km above the surface of the earth. In that region the atmosphere is partially ionized by solar ultraviolet radiation. At lower heights the atmosphere is denser and recombination occurs quickly. Finally the UV is used up and at lower heights the atmosphere is completely neutral. Historically, the ionosphere may have been the first target ever detected and studied with pulsed radar. It was an idea target: as reflective as a metal plate and far enough away for primitive pulse circuitry. Reflection of radio waves from the ionosphere is what lets you listen to distant short-wave stations during the day when the ion densities are high. But this kind of reflection takes place at relatively low frequencies. Above about 50 MHz, the ionosphere becomes transparent to radio waves. (Conduction current becomes less than displacement current and the index of refraction becomes real). At Arecibo the ionosphere is probed with a frequency more than 8 times higher - 430 MHz. And it is true that the probing beam passes almost completely through the ionosphere. But a very small fraction is scattered back, due to the statistically random positions of the electrons in the ionosphere. One part in ten to tenth. And evev that tiny amount woiud vanish if the ionosphere were not random. If these electrons could be located precisely in three dimensional grid - a lattice - there would be no net scattering. The radiation from successive planes in the lattice would cancel that of nearer planes and the net result would be zero.

The same principle works in the atmosphere. If the positions of the molecules in the atmosphere were not random, the sky would be black. Why the sky is blue, rather than white, is a second- order question. Thus "incoherent scatter".

Since the electrons are random in space and time, the echo strength is a random variable; the scattered signal random - noise-like signal.

To get an idea of how weak the returned radar echo is, consider the cross section of a 1 km cube of ionosphere with an ionization density of 10^6 electrons per cc. This cube can represent the kind of target we observe. The cube contains 10^21 electrons. But the scattering cross section per electron is only 10-25 cm2 , so the entire cube is a radar target equivalent to a half-inch diameter ball bearing. Well, we can easily detect such a ball bearing at a range of several hundred kilometers, thanks to 2.5 MW of power and 20 acres of antenna.

Note that the fraction of power scattered out of the beam by this cube is only

5e-4 cm2 /(4pi) x ( 8/3) / (103 x 102 )2 = 1.1 x 10-14. In the total column of ionosphere, the total scattering would be less than 1000 times this or 1 part in 10 to the 11th!

(note that in the above calculation, 1/(4pi) gets us back to re 2 and 8/3 then gives us the total

cross section.

The radar echo from the ionosphere is broadened - the incident wave is a sharp spectral line at 430 MHz, but the return signal is a few kHz wide, due to the electron velocities. It is important to note that the echo is really only from the electrons. The electron mass is so much lower than the ion masses, so the electrons undergo much more acceleration and therefore radiate much more. Nevertheless, it turns out that in this plasma, there is a coupling between electrons and ions. A result of this coupling is that the echo spectrum is narrowed, as if the ions, not the electrons, had done the scattering. Analyzing the exact shape the spectrum, one can determine the ion mass, the ion temperature, the electron temperature (higher than the ion temperature during the day). Using a pulsed radar, we can measure all these parameters as a function of height and produce profiles of ion densities, and ion and electron temperatures.

PLANETARY IMAGING BY DELAY- DOPPLER RADAR

Viewgraph of radar image of venus.

This image appears to be a photograph but it's radar image of the north pole of the moon made with the 70 cm radar (430 MHz). The size of the radar beam is roughly the size of Venus. So obviously this image was not made by doing a point by point scan. The technique used is called Delay-Doppler mapping.

View graph of delay doppler method

This slide shows the idea. The radar can operate with very short pulses to give the desired resolution , at least in range. If we send out a short pulse, the return echo, at a given moment is the sum of the echos from a ring centered on the sub-radar point. Cross range resolution comes from Doppler shift. If we were to use a cw radar (continuous, rather than pulsed), the return signal's frequency spectrum would correspond to lines of constant doppler shift. As the figure shows, these are actually rings, since the target is a sphere. Combining the two measurements using a pulse-Doppler radar lets us get both range resolution and cross-range resolution. The method is really different from side-looking radar (or side-looking sonar) which are known as "synthetic apertura" methods. This delay doppler technique is sometimes called inverse synthetic aperture radar - I guess because it is the target and not the radar that is doing most of the moving. A broader name for this is interferometry. In as much as radar astronomy amounts to probing the target with a man-made coherent radio waves, a Radar Astronomer effectively performs a lab experiment on the target. That is a luxury not available in the rest of astronomy.

photo-like images even though the main beam is "large"

INSTRUMENTATION

Receivers, Transmitters, Signal processing

Receivers

Low-noise amplifiers probably came out of radio astronomy. In radio astronomy, the noise output of the transducer (the antenna) approaches the sky noise, which can be as low as 3deg. Kelvin, the cosmic black body background radiation. This means that the receiver looks into a source impedance (typically 50 ohms) which has an effective physical temperature within a few degrees of absolute zero. The noise of the source is kT watts/Hz (-134 dBm /MHz for 3 Kelvins i.e. 10-13 milliwatts.

This sounds like a very small power, yet it is common to observe millidegree sources - pulsars, or whatever, that only increase the antenna temperature by, say, a millidegree. Signal averaging allows us to pull weak sources out of the background noise, but obviously we would like to keep the receiver noise as low as possible since it adds to the background noise. Our present receivers still have noise temperatures on the order of 5 to 10 degrees Kelvin so they contribute more noise than the background noise from the antenna. When radio astronomy got started, receiver temperatures were thousands of degrees. Diode mixers, then vacuum tube and transistor amplifiers, parametric amplifiers, masers, and now, cooled MESFET transistor amplifiers.

Viewgraph of receiver & compressor

A typical receiver at Arecibo uses a helium refrigerator (Gifford McMahon cycle) to cool a pair of amplifiers in a dewar. These refrigerators typically use a 5 hp helium compressor and produce about 1 watt of cooling at around 10 deg. K. The need for cryogenic temperatures makes them difficult to maintain (and to design - since the transistors must be characterized at the low temperatures). So today's amplifiers are still noisy in radio astronomy. Maybe in ultrasonics, your background noise is high enough that amplifier noise has become negligible. I hope so.

Transmitters

Viewgraph of 430 transmitter

Arecibo has powerful transmitters for radar operation at 430 MHz and 2380 MHz. The 430 transmitter has been in place from the start of operations. It has a peak power of 2.5 MW and an average power of 150 kW. It uses two 10ft high klystrons - the same type as used for several decades in the BMEWs defense radar sites in Alaska, Greenland and England. We now run this transmitter on surplus tubes - available since the BMEWs sites were converted to phased array radars.

Viewgraph of 2380 transmitter

The 2390 transmitter has an output of 1 MW CW. The transmitter is phase-code modulated so its output is a constant megawatt. The efficiency is somewhat less than 50% so more than 2MW of input power are required. We have a 3 MW gas turbine generator to supply power for this transmitter, so the overall system has a kerosene input and a microwave output.

Viewgraph of turbine generator

Signal Processing

View graph of correlator chip and processor rack

Digital signal processing used almost exclusively. Exceptions have included a SAW transversal filter - 13-bit Barker Decoder, one or two processors using charge-coupled devices, and one or two accost-optic spectrometers. The most demanding processing operations are spectral analysis and decoding the phase-encoded radar signal. We use special digital processors for these jobs. In both cases the processors are correlators - used as auto correlators for spectrometry (where the spectrum is then the Fourier transform of the averaged ACF) and as cross correlators for radar pulse compression and for polarization measurements in radio astronomy. The present generation of processors uses a custom CMOS chip that has 1024 correlator channels and a clock rate of 100 MHz. Input signals are coarsely cantiest to only 3 levels (1.6 bits). We blocked out the logic design of the chip. The actual chip design (transistor circuitry) was done by a NASA sponsored lab at the U. of New Mexico, and the chips were fabricated by HP.

Viewgraph of correlator chip

In one second this chip does 1024 x 108 = 1011 multiplications and replace adds.

Several other observatories adopted the chip and a total of around 1000 have been made. The next generation correlator chip is being designed by NRAO and will have 4000 (maybe 8000) channels and use 4-level quantization.

Sensitivity of backscatter radar: compare 1km cube of ionosphere with a ball bearing

jbh 12 Oct. 2000

Suppose the electron density in the cube of ionosphere is N=106/cc. The total radar cross section is given by

radar = ½ N V 4 re2

where re is the classical electron radius: re = e2 /(4 0 mc2 ) =2.82 x 10-15 m. The factor of ½ comes from collective effects (i.s. theory for short debye length case).

Evaluating the total radar cross section we have

radar = ½ N re2 = 0.5(10^6)(10^3 x 100)^3 x 4 (2.82 x 10^-13)^2 = 5 x 10-4 cm2

For a metal sphere whose radius a is much smaller than , the radar cross section is

radar =4 9/4 a2 (ka)4 = 9 a6 k4 = 144 5 a6 / 4 .

For =70 cm, this radar will equal 5 x 10-4 cm if a = 0.805 cm, so the diameter of the sphere is 1.6 cm = 0.6 inch.

IS Spectrum:

Electrostatic forces from ions suppress fast fluctuations in electron density. They cause slow fluctuation in electron density that neutralize the slow fluctuation of ion density.

Slow fluctuations in electron density produce a narrow spectrum. The question becomes:

If the electron gas still has a maxwellian velocity distribution, how is it that the large doppler shifts don't contribute to the scattering? How do we reconcile large thermal velocities and, at the same time, slow density fluctuations? Since N(k, ) for large values of seems to decrease when the electrons and ions cooperate, N for smaller values of must be increasing. The is the same as saying that, for the given k value, the fluctuation spectrum slows way down.

__________________________

0. Map - Arecibo is a city of about 100,000, 50 miles west of San Juan, on the North coast.

The observatory is about 8 miles inland, in the foothills at an elevation of about 800 feet above sea level.

1. If you were to fly over the site, you would spot the 1000 foot diameter reflector antenna, nestled in a natural bowl, flat on its back, pointing vertically upward. With a 1000 ft diameter, the reflector occupies 20 acres and is easy to spot. If you flew over every day, you might notice that the dish is always flat on its back and conclude , wrongly, that the telescope is not steerable. These hills and depressions are in a limestone geology known as Karst. Underground water erods the limestone and the collapsed areas form depressions like the one holding the Arecibo dish. Not much digging was needed. And what was needed was easy because the limestone can be cut with the blade of a bulldozer. Cornell concluded that building the telescope in one of these puertorican natural bowls would save $200,000 over building it at a prospective site in Texas and it seems that this was the single most important reason PR was selected.

Another reason was that PR is at 18 deg latitude so that a telescope able to steer only 20 degrees away from vertical would be able to point at the ecliptic plane - the planets. I suspect a third reason was that PR seemed an interesting and exotic place to engineers in the winters of upstate New York. (Hawaii was considered).

2. Coming in closer we have a close up aerial view of the site.

The reflector is made up of 30,000 shaped aluminum panels.

There is a 50 foot fence surrounding the edge of the dish - a ground screen to keep thermal radiation from the ground from reaching the primary feed antennas on the

feed platform. The feed platform is hard to see from this view, but you can see its shadow.

This platform holds the payload - feed antennas at the focal point, 435 feet above the bottom of the dish. machinery to move the feed antennas (which is how the telescope is pointed) transmitters, and receivers. The platform weighs about 1.5 million pounds (750 tons), about the same as two loaded 747s.

It is suspended by a total of 18 wire ropes (3 1/2 inch diameter bridge strands) which pass over the tops of three concrete towers.

On any given working day, there might be a dozen or more workers on the feed platform. They get there either on a cable car (which is a metal basket that can hold four people) or on a catwalk - a suspended bridge.

The only way up the towers is hand-over-hand up steel ladders. That's a good work out - this tower is 365 feet high and the other two are about 260 feet high.

3. From the edge of the dish, the feed platform looks like this. Here's the shadow again. Here's the catwalk. You can't see the track rope for the cable car, but it's here. Below the support structure are two feeds. The original line feed is still used for 430 MHz transmitting and receiving. The new Gregorian feed system consists of two more reflectors, hidden inside an aluminum dome whose function is to lower wind resistance. This mirror system is wide band - covering from 300 MHz to 10 GHz. The feed in use is moved to the focal point while the unused feed serves as a counterweight. I'll get back to the feeds and the optics later and discuss them in some detail.

You'll see how the pointing is done - it may be obvious already. The feeds can move along the bottom of this curved beam - it's an upside down cog railroad. In addition, this feed arm rotates about a vertical axis. The two motions are needed to point to a source and to keep the beam on the source while the earth rotates.

A pair of vertical tiedown cables runs from a boom at each of the three corners of the triangle.

These cables are anchored to the ground, so the platform is both pulled up and down. It is very stable - except under hurricane force winds. You feel as if you are on the ground (Unless you

are bothered by heights - then you wish you were on the ground).

The final view is a shot taken below the reflector. The 30,000 panels are mounted on a cable suspension system. The headroom varies from zero, at the edges of the reflector, to as much as 20 or 30 feet near the bottom. The aluminum panels are perforated and allow enough light to get through to grow plants and avoid eroision.

The big structure in the foreground is the anchorage for one of the three tiedowns. These are motorized so that the height of the platform can be kept constant as the temperature changes.

Without this active tiedown system, there would be about a 1 foot change in height from day to night.

Nearly anybody feels uneasy about pulling downward on a suspended structure of 1.5 million pounds. The pull on each cable pair is limited to 120,000 lbs by tieing the cables to concrete blocks. If the pull reaches 120,000 lbs, the block simply lifts off the ground.

___________________

That's the general layout. Let me give you some idea of how the telescope is used. There are three main research areas - radio astronomy 75%, ionospheric research 15%, and planetary studies 5%. Somebody will want to ask about SETI - there is some of that which I'll describe later.

For this telescope, the most important radio astronomy is the observation of pulsars and hydrogen clouds.