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Simulating Satellite Solar Arrays

Wed, 08/31/2005 - 8:00pm
Bill Brandenburg and Robert Jackson

APL automates spacecraft power tests with LabVIEW

At the Applied Physics Laboratory (APL), a not-for-profit research and development division of The Johns Hopkins University, researchers contribute both scientific

 
Figure 1: TIMED begins testing at Vandenberg Air Force Base. Upon the spacecraft's arrival at the base, APL, NASA and Vandenberg teams unloaded the spacecraft and its ground support equipment, and transported it to one of Vandenberg's space launch complexes where TIMED was prepared for launch. Image courtesy NASA/APL. 

expertise and engineering design skills to the U.S. space program. The laboratory has designed spacecraft satellites from conception to launch for the U.S. National Aeronautics and Space Administration (NASA). Recent projects include the TIMED, CONTOUR, MESSENGER and STEREO spacecraft. As integral parts of the space program, these spacecraft were designed to study the outer regions of Earth's atmosphere, obtain an up-close look at comets, measure the composition of Mercury's surface, and enable scientists to understand the coronal mass ejections (CMEs) that make up the sun's solar flares. Designing and validating these complex systems requires the joint efforts of a team of experts working at universities, industries and government agencies and often calls for solutions that go beyond what traditional instrumentation can provide.

Development environment for design and validation

A critical component of the development process, satellite design and validation testing is complicated by the fact that engineers design and test the systems on the ground but, ultimately, these systems must operate in space. Back in 2001 when the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)

Solar Connections Mission 
The TIMED (Thermosphere • Ionosphere • Mesosphere • Energetics and Dynamics) mission is studying the influences of the sun and humans on the least explored and understood region of the Earth's atmosphere, as part of NASA's Solar Terrestrial Probes Program. TIMED was launched aboard a Delta II launch vehicle on Dec. 7, 2001. The Mesosphere and Lower Thermosphere/Ionosphere (MLTI), a region between Earth's environment and space, is where the sun's energy is first deposited into Earth's environment.

NASA extended the TIMED mission for another three years of operations and data analysis began in January 2004. This will be followed by an additional year of data analysis in 2007, to be conducted after orbital operations are completed. During the current phase of operations, TIMED's science team is focusing on how changes in the solar cycle affect: solar radiation; the MLTI region's composition, temperature, wind and seasonal variations; atmospheric waves; and the variance of geomagnetic disturbances within the upper atmosphere. They are also looking at how different radiative, chemical, electrodynamic and dynamic processes within the upper atmosphere vary with solar radiation inputs during times of reduced solar activity. For more information, visit www.timed.jhuapl.edu.
 

spacecraft was nearing completion, it was necessary to simulate the power from the satellite's solar panel arrays in order to test and validate its design. This was accomplished with the Solar Array Simulator (SAS), which uses a group of power supplies to simulate individual or entire solar array strings. Software is used to control each power supply output and to accurately simulate the operational conditions of the solar arrays in space.

An Earth-orbiting spacecraft that studies the outer reaches of Earth's atmosphere, TIMED was the first of NASA's Solar Terrestrial Probes Program missions, and was

 
click the image to enlarge 

Figure 2: The Contour spacecraft. Image courtesy NASA Goddard Space Flight Center. 

one of the first spacecraft developed using LabVIEW. APL used LabVIEW to create the program that would control each power supply, since its open connectivity to third-party hardware through instrument drivers allowed development of early prototyping systems using general purpose interface bus (GPIB) instrument control. Because GPIB cards are available for commercial PC architectures, the system could be implemented using a standard laptop and a PCMCIA-GPIB board and then tested with GPIB instruments controlled by LabVIEW, allowing engineers to easily simulate the solar array power fluctuations during orbit. As testing requirements increased, integration with PXI was able to create more advanced testing algorithms and to synchronize tests for improved results.

Meeting increasing complexity challenges

With GPIB-based systems, it was relatively easy to program and control multiple instruments. With GPIB control of stand-alone instruments, the SAS was configured for the open circuit voltage (VOC) and short circuit current (ISC) of each power supply. This was considered the fixed-mode operating condition.

However, APL's project team discovered that using the GPIB solution to control large arrays of power supplies was not adequate. For some spacecraft, the frequency

 
click the image to enlarge 

Figure 3: The total mass of each observatory is closely monitored throughout integration. Various parts and instruments are weighed prior to their installation. Here, engineers weigh each of the STEREO Solar Assembly panels to ensure they meet their design specifications. Image courtesy NASA 

in the changes of the VOC and ISC values required an SAS design with the flexibility to control the VOC and ISC setting via preprogrammed data. A new approach was developed using the industry-standard PXI platform in order to achieve tight synchronization and simulate complex power settings. This increased control of power supplies allowed simulation of solar array power dynamics in newer spacecraft designs, such as spinner spacecraft. These spacecraft have solar array panels mounted on their bodies. To simulate changing solar power requirements as a spinner spacecraft spins around its axis and rapidly changes orientation relative to the sun, the system must be able to handle larger simulation files and download them faster.

During the CONTOUR (Comet Nucleus Tour) mission, a PXI-1000B DC chassis and four NI PXI-6713 analog output modules

Comet Nucleus Tour
The Comet Nucleus Tour (CONTOUR) was a NASA Discovery mission. Launched on July 3, 2002, aboard a Delta II rocket, CONTOUR was developed to visit and study at least two comets, Encke and Schwassmann-Wachmann 3, relying on the help of Earth-gravity assist maneuvers to accomplish multiple encounters with these comets. During the CONTOUR mission, communication was lost on August 15, 2002, after a solid rocket booster fired that was intended to send CONTOUR on a new path toward the distant comets. For more information, visit nssdc.gsfc.nasa.gov/planetary/nmc_sc/contour.html.

allowed engineers to download large arrays of simulated data that directly controlled the SAS power supplies, simulating in two-degree increments the rapid solar array power dynamic variations during spacecraft spin-mode operations. Using PXI and uploading data into circular buffers offered much better resolution than had ever been achieved with the GPIB-controlled instruments. The system simulated each individual solar array string to account for the cyclical solar array panel shadowing caused by various protruding optics and antennae as the spacecraft spun at its 60 rpm rate.

The transition to PXI instrumentation still enabled APL to work in the LabVIEW development environment, and the expertise gained by developing the early GPIB instrument control applications greatly contributed to the new PXI system development. As a result, APL was able to develop new solar array simulation programs in under three months.

Orbital simulations

Both the MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) and STEREO (Solar Terrestrial Relations Observatory) spacecraft required

Mercury Surface, Space Environment, Geochemistry, and Ranging Mission
Launched Aug. 3, 2004, the solar-powered Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft is on a 4.9-billion mile voyage that includes multiple loops around the sun. It will fly past Venus twice and Mercury three times before moving into orbit around its target planet. Venus flybys in October 2006 and June 2007 will use the pull of the planet’s gravity to guide MESSENGER toward Mercury’s orbit. Mercury flybys in January 2008, October 2008 and September 2009 will help MESSENGER further match that planet’s speed, setting up the maneuver in March 2011 that will start a year-long science orbit around Mercury. For more information, visit messenger.jhuapl.edu/index.html

orbital simulations. In these cases, design engineers used LabVIEW and orbital data files provided by power system engineers to establish the dynamic values for VOC and ISC as each spacecraft travels through space. Similar data files simulate the fluctuating solar array power that occurs just after launch, during the spacecraft detumble phase, as well as correct offsets generated by hardware. Offsets are the result of biasing circuitry in the ISC design; the ISC actual value exhibits a small variation due to the VOC value. Using a LabVIEW formula node function, this variation is easily corrected, and engineers can incorporate multiple correction curves based on Y=Mx+B into the LabVIEW program.

After achieving performance to simulate and download power simulation files with LabVIEW and PXI, APL added remote

Solar Terrestrial Relations Observatory Mission
STEREO (Solar Terrestrial Relations Observatory) is a two-year mission employing two nearly identical observatories that provide 3-D measurements of the sun to study the nature of coronal mass ejections. These powerful eruptions are a major source of the magnetic disruptions on Earth and a key component of space weather, which can greatly affect satellite operations, communications, power systems, the lives of humans in space and the global climate. Launch of the STEREO spacecraft is scheduled for February of 2006 aboard a Delta 7925 rocket. For more information, visit www.stereo.jhuapl.edu
 

control capabilities. With the built-in LabVIEW Web server and a dedicated Ethernet connection, engineers added autonomous control of the SAS system from a remote location. The capabilities provided flexibility and additional safety for the engineers and technicians who support the integration and launch operations for the spacecraft.

Conclusion

LabVIEW and PXI improved SAS design options for satellite power system testing. Providing a ground-based system to easily simulate operation modes, such as orbital simulation and spacecraft tumble, which previously were difficult to implement. The additional ability to control systems remotely with LabVIEW helped design engineers and launch operations to work together as a team even though they are spread over great distances.

Bill Brandenburg is an Associate Engineer in the Johns Hopkins University Applied Physics Laboratory Space Department, and Robert Jackson is a PXI product manager for National Instruments. They may be reached at sceditor@scimag.com.

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