Millimeter-Wave Radiometry in the 1990s
The trend to large single element antennas and arrays for radio astronomy continued to accelerate during the 1990s. The goal was to improve spatial resolution. Minimizing the (wavelength / aperture diameter) ratio became the driver. Large antennas with a millimeter wave capability became the vogue. As the observing frequency moved up into the millimeter wave region, antenna installations headed for the hills to escape the atmospheric attenuation of the lower troposphere.
As radio astronomy was moving up in frequency and antenna size, the Air Force was approaching the need for improved spatial resolution in a manner unique to their needs. The allowed antenna aperture size was specified, along with the footprint size needed to identify man made targets of interest, when viewed from the flight altitude of Unmanned Aerial Vehicles (UAVís). The goal was to improve the detection and localization of "non GPS targets" (mobile targets), under adverse weather conditions that limit the effectiveness of optical and IR sensors. The answer was millimeter wavelengths. The "stand-off" mode of manned aircraft was being replaced by a "stand-in" philosophy for the UAV. Not all UAVís qualified for the stand-in posture, simply because of cost. Though man was no longer aboard, some very expensive guidance equipments were introduced in some of the early and larger UAVís like the Global Hawk. At the turn of the Century, UAVís with a much broader spectrum of capabilities were in the fleet or on the drawing board.
On May 5, 1994, President Clinton approved the tri-agency National Polar-Orbiting Operational Satellite Systems, NPOESS. That action converged the meteorological satellite activities of the DoD Defense Meteorological Satellite Program (DMSP), the National Oceanic and Atmospheric Administration (NOAA) Polar-Orbiting Operational Environmental Satellite (POES), and the NASA Earth Observing System (EOS). The Department of Commerce, through NOAA, became the lead agency responsible for operation of the NPOESS constellation of satellites. Under NPOESS, DoD assumed responsibility for major system acquisitions and NASA was assigned as the lead agency responsible for the development and insertion of new technologies to meet operational requirements. This timely convergence of major satellite systems looking at the same atmosphere at the same time from essentially the same polar orbit should save hundreds of millions of dollars and provide a coordinated national approach to satellite meteorology. The first launch of the NPOESS satellite constellation in 2008 is scheduled to coincide with the expected lifetimes of the DMSP and POES constellations.
Satellite meteorology has become an important partner in the move to millimeter waves. The radio meteorologistís need for robust millimeter wave spectrometers identified satellite meteorology as a driver in the advanced development of millimeter wave receivers. It is not surprising that talented engineers like Sandy Weinreb were attracted to NASA/JPL. By the mid-1990s a 1.5 THz satellite receiver was on the drawing board at JPL and joined the challenge to move beyond the upper SWAS frequency at 560 GHz.
Satellite meteorologists became the new partner of millimeter wave radio astronomy as well as the all weather smart munitions of DoD. The enhancement in receiver sensitivity during the 1990s was awesome. Receiver noise temperatures plummeted to less than 1000K at all frequencies below 120 GHz while MMIC technology reduced receivers to postage stamp size.
Radio astronomers could now turn their attention to increasing the size of their antennas. The radiometric receiver was no longer a challenge, thanks to the common denominator needs of satellite meteorologists. Antennas, when used in an array configuration, require communication links and novel correlators to tie the network together. An international program launched in the 1990s and known as the Square Kilometer Array (SKA) became a leader in solving many of the large array problems.
The Square Kilometer Array (SKA)
During the 1940s and 1950s radio astronomers world wide were viewed by many in the scientific community as a weird breed of ham radio operators and WWII spare part operators. Shunned by optical astronomy organizations, radio astronomers turned to the International Union of Radio Science (URSI) for recognition and a forum for the interchange of technical information. By the mid-1950s all of that changed and the welcome mat was out, as the American Astronomical Society led the way to recognize radio astronomy as one of its own. The fearless and tenacious leadership of astronomers like Bart Bok broke down the wall and opened the door.
In September of 1993 URSI formed the "Large Telescope Working Group" to define and implement the "next generation radio observatory", as a worldwide effort. In 1997, research institutions from six countries, including the United States, signed a Memorandum of Agreement to cooperate in a study leading to a future very large radio telescope. On August 10, 2000 the International Astronomical Union (IAU) issued a Memorandum of Understanding signed by eleven countries, including the United States, to establish the International Square Kilometer Array Steering Committee (ISSC). The ISSC is the recognized coordinating body to establish agreed goals and schedules for the SKA project. Selection of the design concept will be made in 2008, followed by initiation of full construction in 2012 and full operation in 2020. The cost estimate is one billion dollars. The SKA will provide a view of the microwave spectrum of sources of space radiation with a sensitivity 100 times greater than now possible. The SKA will measure the hydrogen gas content and magnetic fields of the same galaxies that will be observed in their dust and molecular structure by the Atacama Large Millimeter Array (ALMA). SKA will fill the need for improved spatial resolution of microwave data to complement other planned instruments in the optical, infrared and millimeter wavebands. The SKA will have a significant impact on the worldwide direction of radio astronomy for at least the next half century.
The Army and Air Force investigated the use of millimeter wave sensors during the early 1970s, driven by the need for image data during adverse weather conditions that precluded the use of optical and IR sensors. Both the Army and the Air Force shared a common need for higher spatial resolution using apertures of fixed diameter. Both services came to the table with aperture diameters that were fixed in size by their observing platforms. A 30 cm antenna became a common denominator. The results of the studies in the 1970s were an abysmal failure. The low point occurred when a "smart munition" dropped from a helicopter found a puddle of water next to a tank to be more attractive than the tank. For this and similar reasons, DoD did not initiate a sustained and serious study of passive millimeter wave sensors until the STAG program was launched in the early 1990s by Bryce Sundstrom at Eglin AFB. Bryce named the program Tactical Autonomous Guidance. When he realized the implications of the TAG acronym, he quickly added the word "smart" and the program name became Smart Tactical Autonomous Guidance (STAG).
During the mid-1970s, I became interested in the passive radiometerís fascination with water. As a senior scientist on the staff of Cincinnati Electonics (CE), we were looking for a new market for CE as a follow-on to the very successful RSTN program. RSTN included five stations each operating passively at seven discrete frequencies while looking at the sun. RSTN was part of a global solar network designed to warn astronauts of solar flare activity that might effect the scheduling of EVA activities.
A "smart weapon" equipped with a passive millimeter wave imager was included among the next (CE) projects. Developing a practical analytical model of the passive millimeter wave scenario, as viewed from the vantage point of a high speed missile became a high priority challenge. The target was a tank located in a large puddle of water. Development of the analytical model occupied much of my "spare time" during the next five years.
In the early 1980s, the analytical model was completed and ready for testing. I assembled the test set-up at my home in Weston, MA. It consisted of a homemade T-62 Russian tank model supported on wooden tracks crossing a 30 ft x 60 ft bin of water approximately 4 inches deep. The test set-up was located on my driveway in Weston. The sensor was rope supported from the branch of a birch extending over the water bin. The control station was located in the garage. The 30 ft x 60 ft water bin was made from a 3 mil vinyl sheet supported on a rectangular 2 inch x 6 inch wooden frame. The entire set-up was automated with the aid of a simple PC program. The tank moved back and forth under the sensor and over the water. Sensing of the turret center was routinely accomplished with the aid of the analytical model. In 1983 I presented the results at a tri-service meeting held at Eglin. The fascination of a passive millimeter wave sensor for water had been defeated. The new buzz word was jamming. It was thought that a radiometer would be too sensitive for reliable operation since it would be an easy target for jamming. That myth took another 15 years. It died with the development of the PADLOC system which operates in the Radarometer mode that allows simultaneous operation of a radar and radiometer in the same frequency band while sharing a common antenna. Operation in both modes is truly simultaneous, there is no synchronization or time sharing. Passive, Active, Detection and Localization (PADLOC) was demonstrated under the STAG program in 2000.
Photographs of the Weston measurement equipment and test set-up are linked from the thumbnails.
My participation in the STAG program began in 1990 when I received a call from Bryce Sundstrom suggesting that I respond to a Small Business Innovative Research Program (SBIR) topic currently in the solicitation phase. I did and as a result I enjoyed one of the best decades of research in my career. Working with Eglinís team of specialists, under the enthusiastic support and direction of Byron Belcher, was a real pleasure. John Walker on the Eglin Team and Killer Kocinski on the Millitech Special Projects Team were Supertechs that made things happen, along with John Kapitzkyís magical way of developing needed software changes over night. STAG helped take the edge off the minute by minute emergencies of the big program SWAS, which was moving on a parallel track in time. The day that I arrived in the Pioneer Valley of western Massachusetts in 1988 to enjoy my retirement days at UMass, Richard Huguenin, an old friend from Harvard, called and invited me to lunch. He explained that he had given up his tenure position at UMass to form Millitech, a company devoted to the development of millimeter wave systems. He asked that I provide temporary part time assistance while they searched for a VP of Engineering. From the time I accepted the position of "Acting VP" my retirement ended and I returned to the 60 hour week. Three months later, I was appointed Executive Vice President and moved to the familiar 80 hour week. In 1992 John Youngblood was elected President of Millitech and I became the VP of Special Projects.
||Weston test set-up overview.
||Weston test set-up operations.
||Weston measurement equipment.
In 1995, Huguenin formed Millivision Corp and moved his team to Northampton to build millimeter wave imaging cameras. I was in the midst of SWAS, STAG, SPINNER, etc and stayed at Millitech. In 1997, Youngblood decided to focus all of his attention on millimeter wave wireless communication. He sold everything except the Engineering Dept, and the Special Projects Office (my operation) to Bill Hanley. Bill moved south to Northampton and reformed Millitech. In the meantime Youngblood renamed the wireless communications operation in South Deerfield, Telaxis and took the operation public on NASDAQ.
In 2001, Youngblood decided to terminate all government work and focus the total effort of Telaxis on wireless communications. Special Projects was in the midst of the PADLOC development. Two Program Managers with a contracting officer from Eglin, along with three government representatives from DCAS Hartford, Ct. descended on Telaxis in June 2000 to negotiate a two year extension of the PADLOC program. Telaxis refused and asked that the current program be completed in September 2000, three months ahead of schedule, and thatís what happened. During all of these changes in names and management, my work on STAG and SWAS continued with a tone of business as usual. Names changed, but the game remained the same.
In 2002, YDI bought Telaxis and moved into Building I with what had been the Engineering Dept. of Millitech. For the better part of a decade John Youngblood had been commuting on weekends from his home in San Antonio. He was happy to return, and begin a well deserved retirement. John was, and will always be, a great personal friend. Together, we have been through the fire and back, more than once. As John left, another old friend returned. In early 2003 Richard Huguenin moved the Millivision Corp back into Building II. By that time I had retreated to the RAVIN trailer. In fact, that is where PADLOC was born. In 2004, Air Force asked that I organize a study of the application of passive millimeter wave sensors for the guidance of hypersonic air vehicles. Kent Whitney then president of Millitech thought the effort a bit too high tech and suggested I do it with the UMass Team. UMass was unable to handle it because of potential security overtones. So, I took the problem to the new owners of Millivision and they agreed to form an LLC with a pass-through, no charge contract from their parent company. They asked that I name the LLC. The name I selected was Special Projects LLC.
My ten year experience with the STAG Program sponsored by Eglin AFB, was focused on five major tasks: Analytical Model, MAPS, RAVIN, ROSCAM, and PADLOC. All of the work was performed in either Building 1 or II of the original Millitech Corporation located in the Research Park in South Deerfield, MA
Four slide pages on topics from the STAG Program include:
For STAG Program Publication References, see the Bibliography.
- MAPS slides: MAPS is a 50 foot trailer facility. It was designed to investigate through measurement, the predictions of the Analytical Model of the Passive Millimeter Wave Scenario. Roger Smith was the Eglin Program Manager appointed by Bryce Sundstrom.
- RAVIN slides: RAVIN is a field measurement facility used to support the PADLOC development, and later reconfigured as a mobile 5 foot steerable radio telescope. During the MAPS measurements several sites equipped with unique targets were visited in Florida and Mississippi. While at Stennis AFB, in Mi, MAPS measured the radio brightness temperature of rocket plumes generated on the test stand. Confirmation of the >1000K results observed simultaneously at 35, 60 and 95 GHz, suggested the need for measurements during the boost phase of a launch. RAVIN was reconfigured for these measurements by the addition of a five foot WWII Search Light with a ROSCAM radiometer located at the prime focus.
- ROSCAM slides: ROSCAM is an airborne 95 GHz imager, based on a unique antenna designed by Millitech for an automobile collision avoidance radar. The system was named ROSCAM by Eglin, based on their goal to generate a radiometric image in less than five seconds. The Radiometric One Second Camera (ROSCAM) exceeded that requirement by a factor of five. When we asked about the significance of the five second image time, We were told that it was the retention time of a congressman. Roger Smith was the very capable Program Manager of the ROSCAM. He was also appointed the first Chairman of the SPIE session on Millimeter Wave Imaging. We lost a great team member when Roger decided to pursue Fuse Technology.
- PADLOC slides: PADLOC (Passive Active Detection and LOCalization) was a major milestone of the STAG program. The opportunity to selectively or simultaneously image a target actively or passively in the same frequency band using the same antenna brought new meaning to "sensor data fusion". Extension of that capability to the use of selective spatial communication links, while passively imaging the terrain, was viewed as a block buster advance. Darryl Huddleston replaced Roger Smith as Eglin Program Manager of the 95 GHz PADLOC. Darryl was a vigorous supporter of the program. He was the right man for the job.