This article discusses that starting in 2001, scientists will be able to use the Geoscience Laser Altimeter System (GLAS) to obtain data on the effects of global warming at the north and south poles. GLAS includes a laser system to measure distance, a device to receive signals from Global Positioning System satellites, and a star-tracker altitude-determination system. The laser will transmit four-nanosecond pulses of infrared light at a wavelength of 1064 nanometers and visible (green) light at 532 nm. FEMAP's graphic capabilities allow scientists to provide general views of our finite-element models, animated mode shapes, and 3D assembly views of projects we are working on, such as GLAS.
Among the places where the effects of global warming are expected to be most clearly visible are the regions near Earth’s north and south poles. To help illuminate this problem, NASA Goddard Space Flight Center in Greenbelt, Md., has undertaken to develop and launch a satellite, the Geoscience Laser Altimeter System (GLAS), that will use a laser to obtain data about the effects of global warming at the poles.
GLAS, scheduled for launch in 2001 as part of the Earth Observing System program, is a satellite laser altimeter designed to measure ice-sheet topography and associated temperature changes as well / as cloud and atmospheric properties. Operation of GLAS over land and water will provide along-track topography data.
GLAS includes a laser system to measure distance, a device to receive signals from Global Positioning System satellites, and a star-tracker altitude-determination system. The laser will transmit four-nanosecond pulses of infrared light at a wavelength of 1,064 nanometers and visible (green) light at 532 nm. In addition, photons reflected back to the spacecraft from Earth’s surface and the atmosphere, including the inside of clouds, will be collected in a one-meter-diameter telescope. Forty times a second, laser pulses will illuminate spots (footprints) 70 meters in diameter spaced at 175-meter intervals along Earth’s surface.
The GLAS development process is quite complex. The instrument itself will comprise thousands of parts and weigh about 600 lbs. It will use one of three lasers to send a beam to the Earth’s surface, so the craft’s telescope can spot the laser reflection. Altimetry and lidar detectors are to gather the scientific data, while a star camera and gyroscope will maintain pointing accuracy. According to aerospace engineer Ryan Simmons of the Mechanical System Analysis and Simulation Branch at Goddard, “Positional error between the various components must be extremely low to obtain accurate results. The allowable rotational error of all the optical components combined is on the order of 80 microradians—approximately 0.005 degrees.”
Simmons and his team use a variety of computer-aided- engineering (CAE) tools to conduct structural, dynamic, and static analyses that will support the design of this complex structure. The workhorse solver is UAI/NAS- TRAN from Universal Analytics Inc. (UAI) in Playa del Rey, Calif., while FEMAP, from Enterprise Software Products Inc. in Exton, Pa., serves as the pre- and post-processor. “We use FEMAP because it’s easy to learn and use,” Simmons said. The group receives computer-aided- design (CAD) data from designers as IGES files or as two-dimensional drawings.
To obtain accurate analysis results, high-fidelity finite- element models are necessary. This is especially true with GLAS because the allowable errors are so tiny. FEMAP makes this process relatively easy. Using FEMAP’s organizational features, such as layers and groups, sections of a larger model can be created and worked on separately, rather than with the entire model in view. This also assists in viewing results.
“We’ve employed a novel method of Flexures modeling a honey-comb panel that serves as the GLAS main bench,” Simmons said. “The traditional method of modeling honeycomb panels in NASTRAN is to use two-dimensional plate elements with modified bending and shear properties. We have this type of model in use for dynamics analysis. However, this approach was not acceptable for thermal and static analysis, because our honeycomb bench is four inches thick.” Accordingly, engineers modeled the honeycomb core using solid elements and the face sheets as plate elements. “We ran into many problems doing this,” Simmons noted, “because the mechanical properties of a honeycomb core vary dramatically, depending on the direction.
“This needed to be incorporated into the finite-element model,” he added. The process was made more difficult because honeycomb-core material properties are not generally available in the form needed to make the model work. “We had to do some research on our own and even make some educated assumptions. FEMAP made the process easier because it allows material properties to be varied by direction,” Simmons said. “Then we needed to correlate our model with our dynamics model and with any other information we had available. In the end, we have a model that we are confident will give us very good analytical results. And when testing begins, we will correlate the model even further with those results and pass that information onto future projects.”
Because GLAS is a rather large project, its six mechanical analysts are generally working on their own subsystems. At times, these subsystem models need to be integrated into a larger model. FEMAP allows for easy integration of other FEMAP models and even of NASTRAN bulk data In general, Simmons and his team all work from the same origin on the same coordinate system. But occasionally they need to rotate and translate separate parts-a process that's easy to accomplish using FEMAP.
“When most people hear ‘NASA,’ they think of the Space Shuttle and astronauts,” Simmons said. “However, the NASA organization is ultimately about education. After all, what is the point of conducting all the experiments and collecting all the data if we don’t provide it for learning? That same philosophy applies to the engineering community. Here, in the Mechanical Systems Analysis and Simulation Branch, we like to convey the results of our analyses both internally and externally. Part of education is effective communication.”
Simmons uses FEMAP to communicate among the various groups at Goddard as well as with those seeking information on the Internet. “A picture is truly worth a thousand words,” he observed. “FEMAP’s graphic capabilities allow us to provide general views of our finite-element models, animated mode shapes, and 3-D assembly views of projects we are working on, such as GLAS. I can show the various parts in exploded views, so anyone looking at the picture can ascertain that there are many parts that comprise the final product. For presentations, these assembly views are quite powerful. Audiences can obtain a better understanding of the product and determine exactly how parts fit together in the real world.”