Headlines: Notable news from the past five years
GPS for the Military and Civilians
The fourth in a series of U.S. Global Positioning System (GPS) replacement satellites, GPS IIR4, was launched aboard a Delta II from Cape Canaveral May 10, 2000. Aerospace reviewed the hardware, software, and procedures, and verified that the vehicle was ready for launch. Aerospace developed the fundamental concept of GPS for the Air Force in 1963. Today, GPS, a constellation of 28 navigational satellites that orbit 11,000 miles above Earth, is used increasingly by civilians.
Civilian owners of GPS receivers found their systems significantly more accurate as of May 2, 2000. That day, President Clinton ordered an end to the intentional degradation of GPS satellite signals by the military. The military will, however, retain its right to selectively deny the GPS signals over any given region. Civilians use GPS for many purposes, including search and rescue operations and airplane and ground-vehicle navigation (GPS sensors placed in cars enable drivers to use the Internet to navigate). Unscrambling the signals should benefit the GPS industry, which is expected to grow to $16 billion in the next three years. (Summer 2000)
Amazing MEMS
Microelectromechanical systems (MEMS), machines so tiny they cannot be seen with the naked eye, are quickly gaining notoriety for their capability and versatility in a variety of areas. MEMS can be used to detect environmental pollutants, monitor the health of a premature newborn, sense an impending car crash and deploy the air bag, and be "woven" into the clothes of soldiers on the battlefield (where the sensors would warn against an attack by chemical or biological weapons). A more aggressive use of MEMS is the potential for manufacturing mass-producible, 1-kilogram-class nanosatellites with microelectronics-processing technology.
More than 30 Aerospace scientists are involved in MEMS research, including Henry Helvajian of the Aerospace Center for Microtechnology and the editor of Microengineering Aerospace Systems. Aerospace researchers sent aloft a MEMS experimental testbed on the space shuttle Columbia last year. Data from 30 of the devices were analyzed to see how the various MEMS performed during launch, orbit, and reentry, compared with their performance in preflight tests. One device, designed and built by Aerospace, contains 15 microthrusters, which act like 15 individual solid rocket motors. The usefulness of MEMS in space has yet to be fully realized, and the analysis by Aerospace was the first systematic testing of MEMS in that capacity. Another experimental MEMS test mission, planned for 2001, will involve the International Space Station. (Summer 2000)
New Tools for Testing Antennas
A new testing facility will enable Aerospace to provide more accurate and secure assessment of large, complex satellite antennas.
The near-field antenna range uses a large planar scanner to characterize the radiation patterns of antennas and scale models. It's also used to investigate new measurement techniques and methodologies. Specialized data-processing routines enable the Aerospace range to capture an antenna's entire radiation pattern—an achievement that would be impossible using conventional planar near-field techniques, said facility director Paul Rousseau.
The previous facility, though much smaller, made key contributions to a National Reconnaissance Office program, determining, for example, that the far sidelobes (unwanted spikes in the radiation pattern) of an antenna system were caused by electromagnetic scattering from struts. Mock-up measurements developed at the range also helped validate a numerical analysis, saving the program an estimated $5 million, Rousseau said.
The near-field range became operational in October 2001, but additional enhancements are planned. New material for absorbing microwaves will be installed, with slightly larger dimensions to allow measurements at lower frequencies. Researchers will also install a larger 12-by-12-foot scanner that will enable them to analyze antennas up to 10 feet in diameter.
Upgrades are also planned for the facility's compact range, which uses a large parabolic reflector to simulate the long-range performance of a satellite antenna. (Winter 2002)
Supporting the Nation's Needs at Ground Zero
When the twin towers of New York's World Trade Center collapsed after the terrorist attacks on September 11, 2001, public officials faced the formidable task of sifting through the wreckage to find victims and collect forensic evidence. The Aerospace Corporation immediately offered its assistance and resources. As a result, a remote-sensing instrument developed by Aerospace helped generate and analyze data to support the investigation.
 The left image shows a nighttime false-color infrared picture constructed from data gathered in 15 passes over lower Manhattan in a Twin Otter aircraft on October 24, 2001. In the right image, the infrared data have been processed to show the debris-fallout pattern, which appears not to cross the East River into Brooklyn. |
The instrument, known as the Spectrally Enhanced Broadband Array Spectrograph System (SEBASS), is a midwave and long-wave hyperspectral imager optimized for airborne sensing. It was flown over the site in October to characterize the distribution of gas and materials.
The information was used to confirm or refute the presence of asbestos in and around the wreckage area as well as the landfill where the debris was deposited. The concentration of asbestos was not high enough for the sensor to detect, but the distribution of fiberglass (which would be expected to have a distribution pattern similar to that of asbestos) was mapped out. SEBASS also detected and mapped the spread of freon and ammonia gas. (Winter 2002)
Researchers Track Mir's Reentry
After 15 years in orbit, the Mir space laboratory took a fiery plunge toward Earth, and Aerospace was on hand to examine the event. The Center for Orbital and Reentry Debris Studies (CORDS), under the direction of William Ailor, has been gathering data to characterize how Mir broke up and what parts survived reentry.
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Mir disappeared into the South Pacific, so the prospects for examining debris are naturally limited. But Ailor's team has gathered useful details from a wide variety of sources—including tracking data, video clips, eyewitness reports, and radar measurements. Using this information, the researchers have been reconstructing Mir's final descent. NASA has contracted Aerospace to perform this analysis.
The information should hold immediate benefits for designers and operators of space systems, who need to understand what will happen to a satellite or space vehicle when it reaches the end of its mission life. Knowledge of how complex structures disintegrate during reentry can also help designers optimize satellite configuration. "If you have critical components, and you really want to make sure they get cooked on the way down," said Ailor, "you can put them in places where you know they'll disintegrate. Similarly, if there are things you want protected, you can keep them on the interior, keep heating to them low."
Ailor further noted that satellite operators are increasingly being asked—though not yet required—to examine reentry risk. In fact, Aerospace has been working with NASA to develop thresholds and guidelines for satellite deorbiting. "Basically, if your footprint on the ground is bigger than eight square meters, you'll need to exercise a controlled deorbit," Ailor said.
CORDS also helped NASA in reconstructing the demise of the Compton Gamma Ray Observatory, which was successfully brought down in June 2000 (see Crosslink, Winter 2000/2001). Of course, that observatory weighed only 17 tons, whereas Mir—the "granddaddy of them all," as Ailor puts it—weighed about 140 tons. Moreover, the observatory left orbit in a very precise manner, coming down "right on the money," said Ailor. Mir, in contrast, fell slightly ahead of its mark, primarily because Russian controllers were very conservative in their calculations.
Aerospace provided technical support for the reentry. Wayne Hallman is leading the Mir trajectory reconstruction effort. (Summer 2001)
Beaming Power to Satellites
Batteries add mass, cost, and uncertainty to satellite missions—but such concerns may soon be a thing of the past. A recent study by Aerospace indicates that lasers can be used to beam power from an orbiting space structure to a constellation of satellites. What's more, preliminary results show that with the expected advancements in optical technologies, total system cost can be lower than that of traditional power systems.
The concept builds upon the Air Force Research Laboratory's PowerSail, a large free-flying thin-film solar array. By adding solid-state lasers, thermal controllers, and optical telescopes to this platform, Aerospace researchers were able to create a satellite—the PowerSat—with optical power-beaming capability. Several architectures are being investigated to determine the cost and performance advantages of each.
One model found that just two PowerSats could provide a full-time energy supply to a constellation of 12 low-Earth-orbit satellites. In this configuration, each mission satellite keeps its existing solar array, but loses its battery and most of its power-management system. This arrangement substantially reduces the mass, volume, and cost of each satellite. Moreover, the power available to the mission satellites can be varied in real time, enabling better power optimization based on the requirements of the constellation payloads. (Summer 2001)
Galileo Goes Forward
The European Union (EU) has decided to press forward with plans to develop Galileo, a European version of the Global Positioning System (GPS). The European Commission approved funding for the project despite resistance from the United States, which sees "no compelling need" for it, according to a U.S. State Department announcement.
The development phase of Galileo is expected to run from 2002 to 2005, allowing researchers to test the technology on orbit before implementing the complete 30-satellite constellation. A deployment phase will follow, leading to a full operational capability in 2008.
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The Aerospace Corporation has been helping define U.S. position with respect to Galileo. For example, Aerospace analyzed potential interference to GPS from Galileo's proposed navigation signal structure and assessed options for making the time and space reference frames interoperable. These reference frames define time and position calculations for system users. The navigation signals provide ranging signals, tied to the time and space reference frames, that allow a receiver to determine its position and time. The Aerospace work had two goals: to prevent GPS and Galileo from adopting signal designs that interfere with each other, and to identify opportunities for making the signals and reference frames interoperable. By making them interoperable, the United States and EU would enable manufacturers to build inexpensive receivers that can simultaneously use signals from both systems.
After identifying a range of approaches and assessing their technical and practical impact, Aerospace recommended that each system develop and maintain its own reference frames but provide users with the data needed to remove intersystem errors. Greater levels of coordination were viewed as technically desirable but would have required revisions of U.S. and EU policy. Aerospace also assessed several alternative Galileo signal designs in light of technical and national policy goals. The assessment identified candidate signals that would be compatible with existing GPS civil signals and that provide the opportunity for establishing a new common standard structure for future civil satellite navigation signals. These recommendations were provided to the GPS program office for eventual use by the Defense and State Departments.
The EU has pledged that Galileo will be a civil program under civil control, independent of, but interoperable with, the civil components of GPS. Although the initial funding approval freed up 4.5 million euros, the total system cost is estimated at 3.4 billion euros. (Summer 2002)
Improving GPS Theater Support
In preparation for Operation Iraqi Freedom, the 14th Air Force tasked the 50th Space Wing to develop and deploy an extended type of GPS support to sustain an intensive precision munitions push. Aerospace supported the 2nd Space Operations Squadron (2SOPS) by developing an innovative tactic to enhance theater accuracy and integrity.
As explained by P. J. Mendicki of the Navigation Division, the new technique is a variation of the GPS enhanced theater support (GETS), which was implemented just a few years ago. Using traditional GETS, field personnel would contact 2SOPS with a generalized target location and a strike time window. The 2SOPS office would predict which satellites would be overhead, monitor their performance, and update their broadcast navigation message. The system worked well, but the improvements were short-lived, lasting only about an hour, and planning required adequate advanced notice. "Traditional GETS," said Mendicki, "is very limiting—we can't do it 24/7. Just a few years ago, round-the-clock enhancement wasn't a major concern, because GPS-guided weapons weren't as prolific as they are today."
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Aerospace proposed a new approach. "We know when satellites will be visible to the theater, and we control our contact schedule, so why not proactively schedule uploads to maximize theater performance?" Mendicki asked. Thus, those satellites approaching the area would be uploaded with a new navigation message shortly before entering the theater of operations. "Rather than do it ad hoc, or on the fly, we made it a routine scheduled activity, which helped smooth out operations." As an added bonus, he said, "the new approach allows war planners to attack targets of opportunity," such as those that began the air campaign; the old GETS approach could not.
Aerospace went to 2SOPS with the proposal, and within four days the 2SOPS team tested this new tactic with the operational GPS constellation. The results were so promising that the technique was implemented 48 hours later in support of the opening salvos of the air campaign. Throughout Operation Iraqi Freedom, in which thousands of GPS-guided munitions were employed, the GPS in-theater accuracy was improved by more than 20 percent. "It worked out very well," Mendicki said.
Mendicki has since been researching whether the technique would yield similar results in other theaters, and how it might be applied during two simultaneous conflicts. "Geography may limit our support to other theaters," he said, "but overall, it looks good." (Summer 2003)
Aerospace Aids Shuttle Investigation
Scientists from The Aerospace Corporation provided technical support and analyses to NASA earlier this year in its investigation of the space shuttle Columbia accident. William Ailor, director of Aerospace's Center for Orbital and Reentry Debris Studies (CORDS), testified before the Columbia Accident Investigation Board in a closed session March 13 on the history of space hardware reentry and breakup and what can be learned about the breakup from debris recovered on the ground. Ken Holden, general manager of the Aerospace Launch Verification Division, briefed board members May 21 on the corporation's basic launch verification process.
More than 80,000 pieces of debris—roughly 40 percent of the shuttle's mass—were recovered from the Columbia. Here, they are placed within an outline of the shuttle, indicating where they were on the vehicle before breakup. |
The disintegration of the Columbia occurred February 1 during the reentry phase of the Space Transportation System (STS)-107 mission. The resulting debris field has characteristics similar to those seen for other reentry breakups, Ailor said. Aerospace has been involved in analyses of reentry breakups for many years and established CORDS in 1997 to lead this work.
Ailor's testimony covered the kinds of evidence of the cause of the accident that might have survived the extreme reentry environment and included recommendations for how individual pieces of debris and the distribution of debris within the debris field might help reconstruct events leading to the accident. The investigation board invited Ailor to provide a similar briefing to a public session, broadcast live on CSPAN March 17. Aerospace scientists, including Ailor, Douglas Moody, Gary Steckel, and Michael Weaver, later visited the hangar where the recovered debris was cataloged to evaluate the debris and provided recommendations for analysis.
During his briefing to the board, Holden described the elements of the launch verification process, which Aerospace uses to provide unbiased independent technical assessments to support all Air Force space launches. "Unparalleled Aerospace scientific and technical capabilities for analyses and modeling and simulation provide the Air Force with a second opinion on virtually every technical issue," he told the board. "The effectiveness of Aerospace's role is significantly enhanced by contractors willing to listen to a second opinion and an Air Force customer that puts mission success above any other objective." (Summer 2003)
Satellite Sentries
Spurred by a need for greater "situational awareness" in space, the Air Force is moving ahead with development of the Space-Based Space Surveillance (SBSS) system. The Initial Operating Capability version of this system has been used to detect, track, identify, catalog, and observe man-made objects in space, day or night, in all weather conditions. The complete system will enable key warfighter decisions based on collection of data regarding military and commercial satellites in deep space and near-Earth orbits without the inherent limitations (e.g., weather, time of day, location) that affect ground systems.
"The SBSS system will provide the ability to find smaller objects, precisely fix and track their location, and characterize many objects in a very timely manner," said Dave Albert, Principal Director, Space Superiority Systems, and Jack Yeatts, Future System Director. During the creation of the program, Aerospace performed key mission-assurance risk assessments for the Air Force Space and Missile Systems Center (SMC). During the technical requirements development and source selection, "Aerospace's technical evaluations led to convincing risk-mitigation actions on the launch vehicle and the focal planes," said Arthur Chin, SBSS Program Lead.
A near-term operational pathfinder, which will operate in low Earth orbit, has completed source selection and is scheduled for launch in June 2007 to significantly improve the current on-orbit capability. It will be launched by a Peacekeeper space-launch vehicle that is under SMC/Aerospace mission-assurance and launch-readiness review. The follow-on constellation will begin acquisition in 2005, with initial operational capability slated for 2012. (Summer 2004)
A Ringside Seat
After years of traveling through the lonely depths of space, the Cassini spacecraft finally reached its destination this summer, surviving a critical insertion into near-perfect orbit around Saturn on July 1. Since then, Cassini has been transmitting remarkable images of the planet's rings and principal moon, Titan. The success of this mission, managed for NASA by Caltech's Jet Propulsion Laboratory (JPL), has given scientists around the world a cause for celebration—including some at Aerospace, who provided technical support during various phases of the program.
For example, from approximately 1995 through launch in 1997, Aerospace and Lincoln Laboratory jointly conducted an external independent readiness review of the satellite for NASA. James Gilchrist, Aerospace cochair of the review, said it encompassed the spacecraft design, most of the instruments built by U.S. manufacturers, and the Huygens probe (sponsored by the European Space Agency). Aerospace also conducted the independent review of the Cassini ground operations.
The review lasted more than two years and began with an early independent assessment of the trajectory design, which included an Earth flyby. This trajectory held potential risk because the spacecraft carried about 33 kilograms of radioactive plutonium dioxide to power its thermal generators.
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Formal risk assessment was required because of the presence of this nuclear power source onboard the spacecraft, said Sergio Guarro, director of Aerospace's Risk Planning and Assessment office. Guarro developed the risk assessment methodology to support the environmental assessment and launch approval process for the mission. Aerospace assisted with the risk assessment from early phases of the mission planning and development until launch approval. The importance of this work was recognized by NASA with a project award signed by the former administrator, Daniel Goldin.
William Ailor, director of the Aerospace Center for Orbital and Reentry Debris Studies, was chair of the Interagency Nuclear Safety Review Panel's Reentry Subpanel for the Cassini mission. Ailor's group focused on how well the material protecting the radioisotope would perform under reentry velocities approaching 20 kilometers per second—far beyond the reentry velocities from standard Earth orbits, which range closer to 7.5 kilometers per second.
Aerospace participated in launch readiness tests and the Titan IVB launch-vehicle processing and was instrumental in developing procedures to support the design, installation, and test of a modified Solid Rocket Motor Upgrade actuator. Aerospace supported integration of the payload, including special acoustic tests, thermal analysis, electromagnetic compatibility analysis, loads analysis, targeting, and software testing for the first Centaur launched on a Titan IVB.
In 1998 and 1999, at the request of JPL, Aerospace implemented a number of software enhancements to its Satellite Orbit Analysis Program (SOAP) to model the Cassini mission, said David Stodden, senior project engineer in the Software Assurance and Applications Department. Aerospace developed Cassini solid models and trajectories in 2002 and rendered them to help visualize maneuvers and scientific observation opportunities. JPL used SOAP for visualization and analysis of the June 11 Phoebe flyby, and Cassini is using it to visualize pointing and camera fields of view.
Aerospace also supported in October 2003 a review of the Saturn orbit insertion, the climax of Cassini's long journey and the crux of mission success. "These maneuvers were performed very efficiently, so it appears that the spacecraft may have sufficient propellant to conduct an extended mission beyond the planned four years," said David Bearden, Aerospace Systems Director, Jet Propulsion Laboratory Program Office. "Aerospace congratulates JPL on Cassini's successful seven-year journey to Saturn and insertion into orbit, and looks forward to the tremendous scientific return during the coming years," he said. (Summer 2004)
Popular Science Picks "Picosats"
The smallest operational satellites ever flown-built by Aerospace with Defense Advanced Research Projects Agency (DARPA) funding-were selected by Popular Science as one of the top 100 technologies for the year 2000. About the size of cellphones, these picosatellites, or "picosats," were featured in the magazine's December 2000 issue in the "Best of What's New" section. Project director Ernest Robinson of the Aerospace Center for Microtechnology accepted an award for Aerospace at a Popular Science exhibition in New York in November 2000.
A pair of these picosats flew a groundbreaking mission in February 2000 with the primary goal of demonstrating the use of miniature satellites in testing DARPA microelectromechanical systems (MEMS). Two more picosats, launched in July 2000, are scheduled for orbital release during the summer of 2001. (Winter 2000/2001)
Precision Window for the Space Station
When astronauts view Earth from the International Space Station, they will look through a glass porthole developed by Karen Scott of Aerospace.
 Karen Scott of the Aerospace Houston office, flanked by Astronaut Mario Runco and Dean Eppler of Science Applications International Corporation (SAIC), looks through the space station's optical research window. (Photo courtesy of NASA) |
This 20-inch-diameter window provides a view of more than three-quarters of Earth's surface and is the highest quality window ever installed in a crewed spacecraft. Astronauts will be performing long-term global monitoring with remote-sensing instruments and Earth science photographic observations.
As primary optical scientist for developing the window, Scott tested the viewing glass originally planned for the window and found that it would not support high-resolution telescopes or precision remote-sensing experiments. Her recommended upgrade was approved for the four-piece window, now consisting of a thin exterior "debris" pane, primary and secondary pressure panes, and an interior "scratch" pane.
Scott led a 30-member team from Johnson and Kennedy Space Centers, Marshall Space Flight Center, and the University of Arizona Remote Sensing Group that conducted calibration tests on the upgraded window before it was installed in the Destiny module scheduled for launch in January 2001. The team determined that the window could support a wide variety of research, including the monitoring of coral reefs and Earth's upper atmosphere. Scott's efforts in completing the tests on a tight schedule brought her a Johnson Space Center group achievement award. (Winter 2000/2001)
Successful Launch of NRO Satellite
The National Reconnaissance Office (NRO) successfully launched its GeoLITE satellite from a Delta II rocket on May 18, 2001, from Cape Canaveral. This was the first time an NRO satellite was launched on a Delta II. Aerospace provided critical support throughout the launch campaign, from source selection to early-orbit operations.
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The GeoLITE program broke from traditional NRO practices, said Tom Darone of the Advanced Technology Group, moving instead toward a more streamlined acquisition of the spacecraft and a more commercial procurement of the launch vehicle. The satellite was designed and built in less than four years at a cost of $130 million.
The new approach initially limited Aerospace's involvement to the satellite itself, but late in the program, Aerospace was asked to certify the Delta II launch vehicle as well. In doing so, engineers grew concerned about the stability of the rocket's third-stage engine because it was unclear how GeoLITE's 2000 pounds of propellant would affect the spinning stack. Aerospace performed independent analyses of the third-stage control and the fluid dynamics of the propellant tank; these analyses were key to the Air Force's decision to fly.
The actual launch was delayed one day to replace four of eight flex hoses in the first-stage rocket engine. These hoses became suspect when routine factory testing yielded leaks in two similar hoses. Aerospace worked to understand the factory failure, verify that the replacement hoses were flightworthy, and exonerate the hoses that weren't replaced.
GeoLITE will perform a dual mission: For the first 15 months of its nine-year mission life, it will test an advanced laser communications system. Afterward, it will provide operational support data to the military using a conventional ultrahigh-frequency transmitter. (Summer 2001)
Aerospace Takes Part in NASA CRYSTAL-FACE Mission
The Aerospace Corporation played a key role in the recently completed airborne cloud sampling campaign of the Cirrus Regional Study of Tropical Anvils and Cirrus Layers—Florida Area Cirrus Experiment (CRYSTAL-FACE).
The campaign was part of a continuing interagency effort to better understand the ways in which aerospace propulsion-system combustion emissions affect atmospheric chemistry and radiation. Sponsored by NASA, the CRYSTAL-FACE effort extends previous joint Air Force and NASA work under the Rocket Impacts on Stratospheric Ozone (RISO) program.
"Preliminary analysis of the data has provided new insights into the size, shape, and chemical composition of cirrus and contrail ice crystals and how these clouds could affect global warming," said Martin Ross, Aerospace RISO program manager.
A south Florida thunderstorm takes on the classic "anvil" form with clouds composed of ice crystals forming a high-altitude "shield" over the entire area. A key objective of CRYSTAL-FACE was to investigate the microphysical and radiative properties of the ice particles that make up the shield. The WB-57F extensively sampled the shield cloud of this system on July 27, 2002. (NASA) |
Ross served as co-flight scientist (with Randall Friedl of NASA's Jet Propulsion Laboratory) for high-altitude aircraft WB-57F payload integration and operations during CRYSTAL-FACE deployment to Key West Naval Air Station, Florida, in the summer of 2002. He directed a team of more than 100 scientists and engineers operating 27 instruments carried by the WB-57F to study how high-altitude cirrus clouds are formed, dissipate, and affect the heat balance of the lower atmosphere.
William Engblom of Aerospace applied state-of-the-art computer models to simulate the flow of air around the WB-57F during flight in an effort to understand how aircraft-induced changes in air pressure and temperature could influence the response of instruments carried by the aircraft. Highlights of the month-long CRYSTAL-FACE mission included sampling a variety of thunderstorm-related cirrus clouds, close high-altitude formation flying with the NASA ER-2, and sampling of the WB-57F's own contrail.
More detailed results from CRYSTAL-FACE will be presented at the spring meeting of the American Geophysical Union in Nice, France. The continuing collaboration between the Air Force, NASA, and other agencies under the RISO program provides the Air Force with important credibility, as well as engagement with the atmospheric science community with regard to the impacts of aircraft and rocket-engine combustion emissions on the atmosphere. (Winter 2002/2003)
Black Box for Spacecraft
In an effort to pinpoint sites where space debris will land on Earth, The Aerospace Corporation's Center for Orbital and Reentry Debris Studies (CORDS) is working with the Air Force Space and Missile Systems Center to develop a "black box" similar to the flight-data recorders found on commercial aircraft.
"Data obtained using a black box could provide clues as to how changes in materials and construction might prevent large pieces of space debris from hitting Earth's surface," said Bill Ailor, CORDS director.
Many spacecraft, or pieces of them, return to Earth. A black box that would survive reentry may one day give researchers information about changes in material temperatures and loads on spacecraft as they reenter the atmosphere. The box may also help determine the "footprint" or area of Earth's surface where debris will fall.
Surviving pieces of varying sizes can be spread over hundreds of miles. Many factors, including atmospheric conditions and the aerodynamic characteristics of the objects, influence the footprint location. (Winter 2002/2003)
In the Spirit of Opportunity
NASA's Jet Propulsion Laboratory (JPL) recently landed mobile scientific instruments or "rovers" on the surface of Mars. The first of the two rovers left Cape Canaveral aboard a Delta II rocket and safely touched down six months later on January 3, 2004. The second arrived three weeks later.
Aerospace was involved in this historic mission at varying levels since its beginning, roughly four years ago. "It is our first example of participation on a JPL project from inception into operations," said Dave Bearden, Systems Director of Aerospace's NASA/JPL Advanced Programs Office. Aerospace was part of a team supporting diverse areas, such as requirements management, general systems engineering, selected redundancy studies, risk management, mission visualization, subsystem peer reviews, launch vehicle mission planning, mission design and operations review, analysis of surface-to-orbit communication links, test anomaly resolution, and cost and schedule evaluations.
Aerospace's Satellite Orbit Analysis Program, for example, played a role in the spacecraft trajectory design. "We ran visualizations that showed basically the launch and the travel to Mars and the entry, descent, and landing on to the surface," said Bearden. Texture maps—representations of the geologic features on the planet—made the program even more useful in targeting certain landing areas.
Communicating with an instrument on the surface of another planet is obviously tricky. As Bearden explained, the rovers have two ways of sending signals to Earth—they can use a direct low-data-rate link, or relay their signals through other satellites orbiting the planet (the Mars Global Surveyor, Odyssey, and Mars Express). The relay method achieves much higher data rates. "We did some modeling where we looked at opportunities for communications from orbiting assets to the rovers based on different places the rover might land and where the spacecraft might be," said Bearden. Based on these models, the communications team could make recommendations about nudging the satellites one way or another to optimize communications.
Aerospace work in risk and cost management for this project could have wider applications, Bearden said. "We developed some risk management processes and tools like failure modes and effects analyses for the rovers, and those are the types of things that are readily brought back and applied to national security space," he said. An Aerospace risk study found that while the Mars Rovers were reasonably well-funded, the time available for development was less than half the historical norm for success. An Aerospace tool known as CoBRA (Complexity-Based Risk Assessment) is being adapted for broader applicability to military satellite systems. (Winter 2003/2004)
Lidar Calibrates Sensor on Orbit
The Defense Meteorological Satellite Program (DMSP) has a new tool for predicting weather that could affect ground combat operations. The Special Sensor Microwave Imager/Sounder (SSMIS) is a multifrequency passive microwave sensor that is designed to enhance and extend DMSP microwave imaging and sounding capabilities. Aerospace played a key role in conceiving and developing the new instrument and is now verifying operation following launch of the first SSMIS on DMSP F-16. SSMIS aligns temperature and water-vapor readings within the same view of Earth and uses a conical scan, providing a constant angle of incidence at Earth's surface. This is expected to increase resolution and accuracy of sounding information used in weather forecasting. Aerospace lidar measurements were recently used to confirm that key temperature and water-vapor channels are responding correctly and that calibration of most sounding channels is accurate.
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"Lidar is the only profiling method capable of meeting the accuracy and altitude range requirements needed to confirm SSMIS capabilities," said John Wessel, Distinguished Scientist in the Electronics and Photonics Laboratory. The lidar methods employed are based on Rayleigh and Raman scattering of light, he explained. In lidar, a laser emits optical pulses up into the atmosphere, and light is scattered back to the receiver by atmospheric molecules. The amplitude of the elastically scattered light (Rayleigh scattering) is proportional to atmospheric density at high altitudes. The density measurement can then be converted into temperature. Light is also scattered at Raman-shifted wavelengths, corresponding to vibrational frequencies of atmospheric constituents. Raman scattering can be used to measure water vapor in the troposphere when wavelength-selective elements are used to discriminate the water-vapor signals. Round-trip times are recorded for the signals, providing range profiles for temperature and water vapor. Radiative transfer calculations are performed on the lidar profiles, providing accurate simulations of radiances expected from the SSMIS microwave channels. These can then be compared to the actual profiles derived from SSMIS.
Robert Farley and Shaun Stoller deployed the Aerospace/DMSP lidar at Barking Sands, Kauai. Wessel analyzed the lidar data to produce atmospheric water-vapor and temperature profiles, and Ye Hong applied a custom radiative transfer code to them. This code converted the measured atmospheric profiles into the brightness temperatures that SSMIS was expected to observe during overpasses of the lidar site. The results agreed well with SSMIS brightness temperatures for most channels, although two channels were found to exhibit biases that may require revision of SSMIS calibration coefficients. A second campaign is underway, said Wessel, with a goal of improving measurement statistics and extending upper atmospheric temperature profiles over the range sensed by the highest altitude temperature channels of the new upper atmospheric sounder.
Aerospace began developing lidar calibration facilities for heritage microwave sensors in 1993 and has performed sensor calibrations for five DMSP satellites. (Winter 2003/2004)
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