Microwave and Millimeter Wave Applications in the 1970s and 1980s
During the late 1960s and 1970s the trend to millimeter wavelengths was well underway. Satellite cross link communication at 60 GHz was the driver. The concept depended on a strong atmospheric line. Oxygen was a natural, with a cluster of about 40 lines near 60 GHz that provided 300 dB of attenuation from orbit to earth. The investment in 60 GHz hardware technology was significant, not only to the cross-link but to all applications with frequencies extending up to 60 GHz. It was no surprise that the up-down link to the cross-link would be in the 20 to 50 GHz range to capitalize on the common denominator technology developments at 60 GHz and the lack of atmospheric line structure in that region, except for the weak water line near 22 GHz.
As we entered the 1970s, space-based radio meteorology had become a burgeoning business of both NASA and DoD, supported by MIT at Lincoln Lab and Cal Tech at JPL. The atmospheric gas constituents, water, oxygen and ozone, were the molecular targets, and the ocean surface became the principal terrain target for the measurement of wind speed and direction at the surface. Molecular atmospheric oxygen had become the thermometer that measures the vertical temperature profile around the globe. It is not surprising that space radio meteorologists began to use a sensor instrument called a radio telescope.
During the 1970s the DoD invested in millimeter wave sensors to support the smart munition concept. The limited size of the antenna aperture that would fit in a small munition, in combination with the need for spatial resolution, identified millimeter waves as the potential solution. This led to an intensification of hardware development efforts at higher frequencies. By the mid-1980s, submillimeter wave capability was no longer a lab oddity. There was considerable confidence by the late 1980s that a 600 GHz sensor could be space-qualified. At about the same time MMIC technology became a reality and the postage stamp size millimeter radiometer at frequencies up to 100 GHz moved out of the lab into the field. I recall flying a Sandy Weinreb
MMIC radiometer over the Pioneer Valley in the mid-1990s.
To the visionaries in the field of radio astronomy, this explosion in receiver technology meant a rethinking of missions and observing platforms. Radio telescopes in space had become commonplace by the 1980s. Radio meteorologists were careful not to refer to their instruments as radio telescopes, to minimize any confusion with radio astronomy and budget line items. Clearly the focus of radio astronomy was in the direction of millimeter waves and space platforms, to look out into space as well as back at earth, to explore and learn more about our part of the solar system. For NRAO this was an opportune time to extend technical leadership to space-based radio telescopes in a way that did not reduce or in any way limit the responsible effort now devoted to ground based telescope activities.
In the companion slide presentation, I have selected three sensor developments during the 1970s and late 1980s that are characteristic of activities underway at that time. The first two programs were important technical precursors needed to support the successful development of the Submillimeter Wave Astronomy Satellite (SWAS). SWAS is a self contained submillimeter wave radio observatory. It has been operating flawlessly in earth orbit for 6 years (Dec 2004), providing a fresh new look at deep space. The story of SWAS is presented as the third sensor development in the accompanying slides.
Buzzards Bay Bridge, Cape Cod Canal (BC)3
The arrival of the Space Age with Sputnik in the late 1950s led to the moon race in the 1960s driven by an "I can do better than you can" philosophy. Predictably, the successful achievement of the man on the moon goal and the return of moon rocks to earth challenged the significance of a venture that was clearly not part of a grand plan to explore the final frontier. It did, however, generate the need to develop a plan. After the moon, the focus shifted from the achievement of a single narrowly defined goal to a number of diverse initiatives. Space science became the beneficiary of earth-orbiting platforms, from the Small Explorer to giant observatories like Hubble. Unmanned exploration of the solar system with probes like Mariner, Cassini and Galileo set new standards as stepping stones in a grand plan. The goals of the Shuttle and Space Station initiatives were less easily defined, other than as logical steps in the plan to support man-in-space. Meteorological and communication satellites became line items in the budget. The time was ripe for satellites designed to view the ocean surface.
An early goal was the derivation of wind speed at the ocean surface inferred from the measured surface roughness. Analytical models suggested a close relationship between wind speed and surface roughness. A demonstration of that relationship was the next logical step. A unique observing platform was made available: the railroad lift bridge that spans the Cape Cod Canal in Bourne, Massachusetts. The bridge has a 540 foot span. When raised, it is 135 feet above the mean sea level, and is 10 feet above when lowered. The canal is 35 feet deep. The maximum current is 4 kt and reverses direction with the tide. This "Cape Cod Satellite" set new records for low earth orbit simulation.
Installing and operating sensitive electronic equipment on a railroad lift bridge shared with rail traffic was a significant technical challenge. The administrative management of the effort was equally challenging. It was 1970 and the operator of the lift bridge, Penn Central Railroad, had just gone into bankruptcy. Penn Central operated the bridge under subcontract from the Army Corp of Engineers, the builders of the bridge. Surveillance of the waterway was under the direction of the US Coast Guard, which had recently been relocated within the Department of Transportation. The program was sponsored by NASA with technical overview provided by the Naval Research Lab. The Electronics Research Center (ERC) of NASA in Cambridge was the lead activity in charge of the contract effort. Just hours prior to the initiation of the contract effort, President Nixon ordered Jim Webb, the NASA Administrator, to close ERC. Coincidentally, ERC was located in Tip OíNeillís home district. Tip, at that time, was the Democratic Speaker of the House. As a backup for ERC, which became the Transportation Systems Center of Department of Transportation during the course of the bridge program, the Langley Research Center of NASA was assigned a joint supervisory capacity. Cal Swift was a key member of the Langley Team. Cal is now a colleague at University of Massachusetts. Calís contributions to the bridge effort are well documented in technical journals, and he shares some additional recollections in the section below on A Personal Perspective.
As Principal Investigator, I found it necessary on occasion to locate more than one red phone on my desk. I quickly learned where the buck stops. One evening, shortly after midnight, I was called by the US Coast Guard to remove some debris from the canal. A sensor boom mounted on the rip-rap by the Woods Hole Oceanographic Institute had broken away from its support and fallen into the canal. The Coast Guard did not want the telephone number for Woods Hole. They made it clear that Woods Hole was working for me, and it was my problem.
We had put several tons of electronic and other support equipment on the bridge when Penn Central informed us that we were endangering the balance of the bridge and, if one end were forced downward more than the other, the span would lock up. Releasing a span-jam is a major event that affects the rail traffic schedule, among other things. We immediately requisitioned large amounts of lead and concrete block to support our balancing act. As we were completing the installation and about to initiate the data accumulation phase, Penn Central informed us that it would be necessary to assign a Signalman outside of the normal 8-5 working hours if we had people on the bridge outside those hours. The control system was quickly redesigned to accommodate remote unattended operation.
The (BC)3 slides provide highlights of the installation on the bridge. The final configuration included an equipment trailer on the ground at either end of the bridge, and an equipment room on the bridge, opposite the antenna complex that supported the four observing frequencies at 750, 1400, 4000 and 7500 MHz and automatically scanned back and forth from nadir to +72o above the horizon.
A PERSONAL PERSPECTIVE: A personal perspective on (BC)3 by Cal Swift.
The Eight Channel (30-110GHz) Co-Boresighted Receiver System (ECCB)
The (BC)3 program described above was just one of many research efforts in the 1970s and 1980s that turned to radio telescope technology to develop the instrumentation for non-astronomy applications. These efforts frequently produced enhancements in instrument capabilities that were a benefit to radio astronomy. The ECCB receiver system is another example of technology enhancements that aided the development of the Submillimeter Wave Astronomy Satellite (SWAS) receiver.
The ECCB was an eight channel receiving system that simultaneously covered the frequency bands from 30 to 110 GHz. The eight receivers used the same cassegrain antenna which viewed signals in the direction of the horizon while rotating about orthogonal elevation and azimuth axes. The received signal is split into the eight frequency bands and fed to the eight receiver channels by means of a Gaussian Optical Multiplex (GOMUX) unit, specifically designed and developed for this application. In addition, the front end of each receiver was cooled by a self contained single stage closed cycle helium system to obtain an improvement in sensitivity. Noise temperatures at all frequencies were below 1000 K.
The ECCB was a success. A concomitant achievement was the development of a millimeter wave Team of Expertise, which included notables such as G. Richard Huguenin, Paul Goldsmith, Ellen Moore, Alan Parrish, Karl Stephan, Sandy Weinreb and Neal Erickson. Goldsmith and Erickson became members of the Science Team for SWAS. The ECCB program is typical of activities that provide a unique bonding of technical expertise that continues to work together and participate as team members on future system development programs. The Team that developed the SWAS included members of the ECCB Team. I was fortunate to serve on both teams and experience the close knit respect and mutual admiration that makes technical problems evaporate. It was one of those times when you knew that anything could be done.
The ECCB slides provide an introduction to the quasioptical GOMUX concept and assembly details.
Submillimeter Wave Astronomy Satellite (SWAS)
The SWAS Program was funded and supported by the NASA Goddard Space Flight Center under the Small Explorer (SMEX) Program. The principal investigator was Gary Melnick of the Smithsonian Astrophysical Observatory. Co-investigators included representatives from NASA/Ames Research Center, Ball Aerospace Systems Group, Millitech Corporation, University of Cologne, Johns Hopkins University, University of Massachusetts at Amherst, the National Air and Space Museum, and Smithsonian Astrophysical Observatory.
The SWAS instrument was designed to perform a submillimeter wavelength survey of molecular clouds in interstellar space as observed from the vantage point of low earth orbit. SWAS observes and analyzes submillimeter spectral line emission associated with oxygen molecules (O2), neutral carbon atoms (CI), carbon monoxide molecules (13CO), and water molecules (H2O). The design goal was two years on orbit. In December 2004, SWAS completed 6 years of flawless operation on orbit.
The SWAS program began in the late 1980s as a Phase I Small Business Innovative Research (SBIR) Program. Under that initial program several critical capabilities were developed and demonstrated, such as a tripler from 100 GHz to 300 GHz, a subharmonic mixer in the 450 to 600 GHz band, and a low loss polarizing grid for operation in the 450 to 600 GHz frequency band. These were essential to the satellite receiver development, which was approved in 1990. Four years and 9 million dollars later, Millitech delivered the 35 lb SWAS Satellite Receiver to Ball Aerospace for introduction into the satellite and integration with the launch vehicle.
The SWAS was scheduled for a 1994 launch on a Pegasus XL vehicle released from the underside of a Lockheed-1011 at an altitude of about 40,000 feet. The launch was delayed four years due to problems with the Pegasus. During that time the satellite was moth-balled at Goddard. In December 1998 SWAS was launched into a 450-mile earth orbit from a Lockheed-1011 based at the Vandenberg Western Test Range.
View Doc Ewen's SWAS slides. There are additional slides in the Personal Perspective by Paul Goldsmith.
A PERSONAL PERSPECTIVE: A personal perspective on SWAS by Paul Goldsmith.