Headlines

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.

A 2020 Vision of the Moon

The White House in early January announced an ambitious plan to establish a human presence on the moon as a stepping-stone to an eventual piloted mission to Mars and beyond. Pete Aldridge, former president of Aerospace, will chair a special commission of private- and public-sector experts to advise the President on its implementation.

(International Launch Services photo by Carlton Bailie)

According to the plan, the space shuttle will remain in service only until 2010, the deadline for completing work on the International Space Station. It will be replaced by a new type of spacecraft, the Crew Exploration Vehicle. Flight tests will begin in 2008, and the first flights with an onboard crew will begin no later than 2014. The vehicle will be capable of transporting astronauts and cargo to the space station after the shuttle is retired, but its main purpose will be to carry astronauts to other bodies in space. The vehicle will begin ferrying astronauts to the moon as early as 2015 to establish a base there for further human exploration of the cosmos by 2020. Robotic missions, beginning in 2008, will scout the lunar surface to prepare for human outposts.

The full implications of the proposal for organizations such as Aerospace are not entirely clear. John Skratt, Principal Director of Space Launch Projects, remarked that "the nature of our technical support may not change (independent assessment, testing in the labs, modeling and simulation, special technical support, requirements analysis and management, etc.) but the application will be different. The challenge will be to make the shift happen in a smooth and timely manner."

Notably absent from any description of the initiative, said Skratt, is "exactly how we will launch the payloads necessary to fulfill whatever the plan will be. The suggestion is the continued use of expendable launch vehicles, both domestic and foreign," he said, adding, "My guess is that the issue of what the launch system will look like initially and downstream will be the basis of vigorous future debate."

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.

"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.

Ultrasound Technique Clears Rocket Motors for Flight

(Boeing photo by Carlton Bailie)

A nondestructive evaluation method developed at Aerospace helped pave the way for a successful launch of the Delta II rocket that carried the GPS IIR-10 satellite into orbit. In some rockets, the nozzle exit-cone liners are manufactured by wrapping composite tape around a metal mandrel to form plies at a specific angle relative to the nozzle centerline. The ply angle determines a critical trade-off between thermal conductivity and erosion rates. If the ply angles are too high or too low, nozzle structural components will overheat, causing the exit cone to fail. Unfortunately, checking ply angles is extremely difficult in completed nozzles. So, when several of the graphite-epoxy motors on the Delta II were suspected of having aberrant ply angles, the Air Force asked Aerospace to investigate.

Eric Johnson, Associate Director of Materials Processing and Evaluation, explained that the Aerospace technique involves measurement of the ultrasonic pulse-echo delay as a function of nozzle station. The delay times are compared with a nozzle-specific ply-angle lookup table that is developed by modeling the liner material as a transverse isotropic solid with fitting parameters determined via an ultrasonic through-transmission measurement. Working on the launchpad, Aerospace acquired data for the nine GPS IIR-10 nozzles. The results showed that the ply angles were within acceptable limits. The contractor is in the process of developing an automated nozzle scanner that will incorporate this inspection method. At the Air Force's request, Aerospace recently completed a similar inspection of the nine nozzles slated for the GPS IIR-11 mission, Johnson said.

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