Building Space Instruments in the Space Science Applications Laboratory

Lynn M. Friesen and Dan J. Mabry

When commercial alternatives can't be found, Aerospace steps in to manufacture hardware and instrumentation for specific research missions. This important engineering capability has helped keep Aerospace at the forefront of space science while enhancing overall support to a wide range of customers.

The environments of near-Earth space and the upper atmosphere pose unique challenges for the design and deployment of satellite systems. Of particular concern is the radiation that can disrupt critical electronic systems and affect mechanical components. Highly energetic cosmic rays, for example, can penetrate spacecraft and interfere with or damage electronic switches and memory devices. Large electrostatic potentials can build up on the surface of the spacecraft and suddenly discharge, damaging or destroying sensitive electronic instruments.

Understanding such phenomena is important for the design and operation of space systems; however, in most cases, the instruments needed to study these effects are either unavailable or incapable of achieving the highly specific measurements desired—and with limited commercial potential for such instruments, private corporations have little or no incentive to develop them. As a consequence, The Aerospace Corporation's Space Science Applications Laboratory has developed the capability to design, develop, and manufacture such instruments. This in-house end-to-end capability has helped keep Aerospace at the forefront of space research while ensuring the success of numerous space missions.

In fact, during the last 40 years, the laboratory has built more than 250 scientific instruments that have flown on satellites and sounding rockets and countless others that have been deployed on the ground, on mobile and aerial platforms, and at astronomical observatories (see sidebar, From Idea to Instrument). These instruments—usually one-of-a-kind devices—have observed phenomena as varied as galactic cosmic rays, the Leonid meteor showers, atmospheric gravity waves, and rocket exhaust plumes. The mix of scientific and engineering disciplines in the Space Science Applications Laboratory provides the staff, experience, and resources necessary to develop and realize complex instruments, all under one roof. A typical instrument comprises up to a dozen subsystems, requiring the laboratory's combined expertise in detector systems, low-noise analog electronics, command and control electronics, data-processing electronics, embedded software, input/output electronics, power systems, mechanical assembly, and harnesses and cables. The design and test phases of a project can range from a few months to a few years, with 18–24 months typical for programs involving complex hardware. Today, ten space-flight instruments built by the laboratory are returning data from orbit to support studies of Earth's magnetosphere and the effects of space radiation on satellites. Two more instruments await launch, and several others are under development. This article will first examine some specific sensing technologies, then describe the laboratory's involvement in a few recent science missions.

Monitoring the Space Environment

Aerospace has developed particular expertise in constructing environmental monitors for the in situ measurement of space radiation and electrostatic buildup on satellites. Electrostatic discharge is the most common cause of anomalies on satellites, and single-event effects are second.

To measure the total dose radiation impinging on a satellite, Aerospace created a versatile dosimeter using a silicon-detector-based sensor that sits beneath a hemispherical aluminum shield. The dosimeter packaging can be changed to address the mechanical and electronic interfaces of the host spacecraft. Data from dosimeters have direct applications for the past, present, and future of spacecraft design and operations: the information is used for resolving anomalies, for creating models of the space environment, and for developing test requirements for new systems. Shown are two dosimeters built for government and commercial customers.

Single-event effects are caused by the interaction of incoming ions with space-system electronics. By creating ionization in a sensitive electronic component, for example, an incident ion can change the state of a stored bit of information in memory, or cause a switch in a logic junction that can propagate throughout a system. Single-event effects can range in severity from minor upsets to system failure.

Electrostatic charges can build up on the dielectric surfaces of spacecraft in some regions of near-Earth space—notably, at geosynchronous orbit during magnetic storms. If the charge buildup reaches a critical point, a discharge can occur, triggering voltage surges that can cause anomalies and failure of the spacecraft's electronic circuits.

Radiation Dosimetry

To measure total dose radiation, Aerospace created a versatile dosimeter that uses a silicon-detector-based sensor under a hemispherical aluminum shield. The basic concept is simple: The amount of radiation that reaches the sensor is equal to the amount of radiation that would reach components within the vehicle under the same amount of shielding. A thick hemispherical shield gives the radiation dose deep within the craft, while a thin shield gives dosage nearer to the satellite surface. Typically, the dosimeter will employ several detector-shield combinations to represent various locations inside the satellite.

Although the concept is simple, the application is complicated. These dosimeters are never identical from one application to the next, but must be tailored to the mechanical and electrical interfaces of each host satellite. In addition, shield thickness must be relevant for a particular orbit and satellite application. Most important, each dosimeter—which includes electronic circuits—must be designed to survive the very radiation it monitors. Three such dosimeters are currently returning data from orbit, and two others are scheduled for delivery in 2002 and 2003.

Surface-Charge Analysis

To provide a complete picture of the in situ spacecraft environment, Aerospace also developed a surface-charge monitor with greater resolution and sensitivity than previous scientific instruments. The development effort was made possible in part through support from the National Aeronautics and Space Administration (NASA).

Energy spectra from a surface-charge monitor showing a satellite surface-charging event.

The sensor monitors the ambient plasma around a satellite. Evidence of charging (or lack of charging) is manifested in the ion and electron spectra constructed from the sensor data. The analyzer functions when high voltage is applied to an inner hemispherical plate. If positive voltage is applied, the analyzer will allow electrons to pass through to the detector; if negative voltage is applied, it will pass positive ions. The magnitude of the voltage determines the energy pass-band for the charged particles. Energy spectra are constructed by stepping the voltage through a broad range of steps in a short time. Angular information is derived through position-sensitive detection using an array of microchannel plate detectors. Intrinsically, the sensor has a field of view of about 10 by 360 degrees, though this can be controlled over a range of about 60 degrees through electrostatic steering. The surface-charge monitor uses three programmable high-voltage supplies to steer the field of view and to control the stepping system that controls the energy-level selection.

A prototype was flown on the NASA Geodesic sounding rocket in 2000, and an improved version is under development. The new model employs high-reliability electronic components instead of the commercial components used in the sounding-rocket version; it will also incorporate a fast and efficient high-voltage supply, currently being designed by Aerospace, instead of commercial high-voltage supplies. Although commercial power supplies are readily available and adequate for use in the rocket instrument, they do not meet the performance or reliability requirements of the space sensor. The Aerospace 0–4000-volt supply will have a 1-millisecond step rate and a settling time of 10 microseconds; the general-purpose supply can be used in the surface-charging monitor or other sensors that require high performance and reliability.

Aerospace developed an improved surface-charge monitor, which measures the ambient plasma. The analyzer functions when high voltage (+HV or -HV) is applied to an inner hemispherical plate. If positive voltage is applied, the analyzer will allow electrons to pass through to the detector; if negative voltage is applied, it will pass positive ions. The magnitude of the voltage determines the energy pass-band for the charged particles. Energy spectra are constructed by rapidly stepping the voltage through a broad range of steps. Angular information is derived through position-sensitive detection by using an array of microchannel plate detectors, shown conceptually as the detector ring.

Data from these dosimeters and surface-charge monitors provide invaluable information to builders and operators of satellites. Dosimetry and charging data are regularly used, for example, during analysis of onboard anomalies to determine whether a particular fault could have been caused by the environment. These measurements also help scientists monitor changes in the energetic particle environment across space and time; such information is useful in anticipating possible space weather trends and alerting satellite operators to conditions that enhance the likelihood of disruptive events. Dosimetry data can also help satellite designers determine, for example, how much shielding a computer processor should have and where it should be mounted to minimize the risk of radiation upset. Also, if an enclosure can be made thinner without endangering the packaged electronics, overall satellite mass (and subsequent launch costs) can be reduced. Data from these sensors are also useful in developing test requirements for new systems.

Ionospheric Effects

Closer to Earth, the ionosphere also presents specific difficulties for satellite designers, and Aerospace is actively studying the ways that atmospheric and ionospheric phenomena affect space systems. By understanding the underlying physical processes that affect remote-sensing systems, researchers hope to develop better methods for mitigating performance-degrading effects. One recent innovation uses a technique based on the occultation of GPS (Global Positioning System) satellites. Occultation refers to the disappearance of one object when another of larger apparent size passes in front of it. The technology will be tested in the Ionospheric Occultation Experiment (IOX), built by Aerospace and scheduled for launch in August 2001 as part of the Air Force Space Test Program's P97-1 PicoSat mission.

IOX uses a space-based dual-frequency GPS receiver to measure ionospheric properties. To make the IOX instrument, Aerospace designed a spacecraft-interface board and a microcontroller board and integrated these with the commercial GPS receiver. The microcontroller overrides the normal GPS algorithms, allowing IOX to select setting satellites rather than satellites that are in view.

IOX will test techniques for mitigating the impact of horizontal gradients in the electron densities on retrieval accuracy. More specifically, IOX will exploit small differences in the propagation through the ionosphere of two GPS signals to derive distribution of electron densities along the line of sight. To make the IOX instrument, Aerospace designed a microcontroller board and a spacecraft-interface board and integrated these with a commercial dual-frequency GPS receiver. The microcontroller, with its associated hardware and software, overrides the normal algorithms used by GPS to select satellites. Normally, GPS chooses satellites that are in view, whereas IOX uses setting satellites, allowing researchers to look through the atmosphere to determine timing differences between simultaneously transmitted signals. Aerospace is currently evaluating GPS receivers for a second instrument, the C/NOFS Occultation Receiver for Ionospheric Sensing and Specification, to support the Air Force Research Laboratory's Communication/Navigation Outage Forecasting Satellite. C/NOFS will attempt to predict equatorial ionospheric scintillation, which can affect the performance of satellite communications, GPS navigation systems, and space-based radar.

NASA Satellite Missions

Aerospace's ability to support Air Force and National Reconnaissance Office (NRO) space missions is greatly enhanced by involvement in the space science community at large—a presence maintained by active participation as hardware developers in NASA missions. In fact, since 1965 (beginning with ATS-1), Aerospace has constructed instruments for 15 NASA satellite programs, 15 sounding rockets, and two shuttle missions. Through participation in these NASA programs, scientists at the laboratory have gained a broader understanding of the space environment and have subsequently applied this knowledge in direct support of Air Force and NRO projects. Some of the more recent missions include SAMPEX, Polar, and TWINS.

SAMPEX

The first of the NASA Small Explorer satellites, SAMPEX was launched in 1992 on a Scout rocket from Vandenberg Air Force Base. SAMPEX houses a set of four high-resolution, high-sensitivity particle detectors used to conduct studies of solar, anomalous, galactic, and magnetospheric energetic particles. Aerospace designed and built the data-processing unit that supported the four sensors and also developed the ground support equipment, which was used throughout the design and testing of the data-processing unit and the testing of the instrument suite.

The Communications/Navigation Outage Forecasting System (C/NOFS), developed by the Air Force Research Laboratory, is designed to monitor and forecast global ionospheric scintillation in real time. Scintillation, which is caused by naturally occurring ionospheric irregularities, causes the signal-to-noise ratio of satellite communications to fluctuate. C/NOFS will alert users to impending satellite communication outages, GPS navigation degradations, and space-based radar tracking errors caused by equatorial ionospheric scintillation. The C/NOFS Occultation Receiver for Ionospheric Sensing and Specification, designed by Aerospace, relies on GPS satellites much as the IOX instrument does.

Housed in an assembly about the size of a shoebox, the data-processing unit is neither small nor especially fast; nonetheless, it is still performing its job today. Based on 1980s technology, the design employs the space-rated 80C85 microprocessor, a component used by laboratory engineers in earlier missions. It was also the laboratory's first flight instrument to make use of field-programmable gate-array technology, which has since become the standard. The data-processing unit controls the operations of the instrument suite, provides the command and telemetry interfaces to the spacecraft, and supplies low-voltage power to the sensors. Incorporating all the interfaces into a single unit helped simplify the satellite design by allowing the spacecraft to communicate with just one sensor-data processor instead of four.

The data-processing unit was designed with five interfaces—one for the spacecraft, and one for each of the four sensors. Separate interfaces were necessary because the sensors were modeled after units that had proven themselves in other applications. Rather than redesign the sensor interfaces, engineers decided to make the data-processing unit compatible with the existing interfaces. Aerospace participated in the sensor calibration and supported the launch and on-orbit activation of the instruments.

Another interesting feature of the data-processing unit is the flight code, which employs a dynamic technique to allocate bandwidth to the four sensors according to need. Each instrument is assigned an allocation of telemetry for a 90-minute orbit. At the end of an orbit, the program determines which instrument did not use its quota and then reallocates a portion of the unused bandwidth to a sensor that exceeded its quota for the orbit. This dynamic reallocation allows sensors that are most sensitive to changes in the magnetosphere to record more data while maintaining a minimum allocation for sensors less sensitive to the event. Designed for a three-year nominal mission, SAMPEX is in its ninth year and is still returning data.

Polar

Aerospace also had a large engineering role on the NASA Polar mission, launched in 1996 as part of the International Solar Terrestrial Physics program, which seeks to study Earth's magnetosphere from multiple satellites. More than a dozen Aerospace engineers and scientists took part, designing and building complete instruments or major subsystems for three of the 13 scientific experiments onboard.

Aerospace had a large engineering role on the NASA Polar mission, launched in 1996 as part of the International Solar Terrestrial Physics program. One project was the multiple-pinhole camera (left) used by the Polar Ionospheric X-ray Imaging Experiment (PIXIE). The electronic camera forms images of Earth's aurora by recording the X-rays generated when energetic electrons strike the upper atmosphere. By imaging these X-rays and measuring their energies, PIXIE determines the fluxes and characteristic energies of the parent electrons. This information will give insights into the high-altitude processes that cause Earth's aurora as well as the complex interactions between Earth's upper atmosphere, ionosphere, radiation belts, and magnetosphere. Aerospace also designed and built Polar's imaging proton sensor (right)—the first to make long-term high-altitude measurements of energetic neutral atoms from Earth's radiation belt. Although the instrument was designed to measure radiation-belt protons, it was also sensitive outside the radiation belts to energetic neutral atoms, and routinely observed these particles.

One of the innovative mechanical designs devised by the laboratory was a type of multiple-pinhole camera used by the Polar Ionospheric X-ray Imaging Experiment (PIXIE). The camera features two moveable aperture plates, each of which allows any of four collections of pinholes to open. This freedom enables the instrument to make optimal use of the detector area regardless of spacecraft altitude. The plates also permit spatial resolution to be traded against counting rate when selecting pinhole size.

Another Polar instrument tested the utility of applying compression schemes to raw data in flight to produce coefficients representative of the image data. Flight software performed spherical and linear fits to the sensor data onboard in addition to the normal operations of command, control, and telemetry processing. Because the data-processing unit used a slow 80C86 processor, coding the complex algorithms without introducing delays in recording new events presented a significant onboard scheduling problem. Although the coefficients based on the real data have not proved as useful as the raw data, the exercise yielded useful insights into the design of efficient flight software.

Aerospace is part of a collaborative effort to create TWINS (the Two Wide-angle Imaging Neutral-atom Spectrometers). TWINS uses a pair of instruments mounted on rotating platforms on two widely spaced high-altitude, high-inclination spacecraft to record energetic neutral atoms over a broad energy range. The stereoscopic technique permits 3-D visualization and resolution of large-scale structures and dynamics within the magnetosphere.

Also noteworthy, Polar's imaging proton sensor—designed and built at Aerospace —became the first instrument to make long-term high-altitude measurements of energetic neutral atoms from Earth's radiation belt. Energetic neutral atoms— currently a subject of intense scientific inquiry—result from the exchange of charges between cold geocoronal neutral hydrogen and the local energetic ion populations. Unaffected by Earth's magnetic field, energetic neutrals travel in straight lines from the point of charge exchange. Remote detection of these particles provides important information on the global distribution and properties of both the geocorona and the magnetospheric ion population. Although the imaging proton sensor was primarily designed to measure radiation-belt protons, it routinely observes energetic neutral atoms outside the sensor background caused by the radiation belts. The instrument allows scientists to generate images of the energetic neutral atom emission regions and to track the waxing and waning of the ion fluxes in the radiation belts. In particular, the technology helps visualize how ion fluxes are injected into the radiation belts by large storms. Such observations give scientists a global view of the changing ion populations that cannot be obtained by in situ point measurements.

TWINS

The newest in the family of instruments developed to study magnetospheric effects on space weather is the Two Wide-angle Imaging Neutral-Atom Spectrometers (TWINS), currently scheduled for delivery in April 2002. One of the first instruments funded by NASA for a "mission of opportunity," TWINS is actually a pair of identical instruments that will be launched on two orbiting spacecraft to provide the first stereoscopic image of Earth's magnetosphere. TWINS is a typical collaborative NASA project. Participants include members from the Southwest Research Institute, Los Alamos National Laboratory, the University of Southern California, West Virginia University, Aerophysics Laboratory, and the University of Bonn, as well as The Aerospace Corporation.

The sensor control board for the TWINS instrument controls the rotation platform, power supplies, and housekeeping monitors. The actual board is 4 X 5 inches and has tens of thousands of interconnects.

One of the primary engineering challenges for the Aerospace contingent is to ensure that the TWINS sensors will not interfere with other sensors onboard the space vehicles. For example, because TWINS imaging is accomplished by scanning the instrument 180 degrees on a rotating actuator, the jitter induced by the rotation required careful study to mitigate unnecessary or unacceptable disturbances. In this case, the analysis was complex, and required assistance from structural-dynamics and control-analysis experts. The availability of the collective resources of Aerospace to address issues ranging from thermal design to contamination effects to control systems is a rare advantage for engineers in designing these complex systems.

Conclusion

These projects, though diverse, are typical of the laboratory's hardware development process, which applies to space-based, airborne, and ground-based systems alike. The unique instruments developed at Aerospace serve specific scientific missions, enabling researchers to perform studies that might not be possible otherwise, given the absence of commercial alternatives. The challenge is in designing systems that are reliable and affordable. Science instruments are not market-driven, high-profit items, and their development budgets cannot be burdened by expensive leading-edge technology.

The hardware development program contributes significantly to the overall mission of Aerospace, providing direct and indirect benefits for clients—particularly the Air Force and NRO. Data obtained through Aerospace instruments help designers create more robust, efficient, and inexpensive space systems. Such information also serves to facilitate anomaly resolution, improve space models, and enhance testing protocols. Moreover, the ability to develop mission-specific instruments in house helps to reduce development time and overall mission cost.

Whether for prototype instrument or evolved sensor, the capacity to design and build custom hardware is an important capability that helps keep Aerospace at the forefront of the space-science field.

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