Semiconductors Boost Space-Based Environmental Research
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For millennia, humans have studied the environment and tried to predict how it might change. At ground level, the relationship between widely scattered environmental events could be difficult to detect. Today, though, earth-observation satellites provide views that can help clarify the relationships between geology, meteorology, and ecology. Optical, radar, and infrared sensors can act alone or in concert to provide an illuminating view of our world from low Earth orbit (LEO) and above.
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“By using a combination of all these different sensors from space, we can recreate a bigger, better view of the world around us,” said Laura Mueller, director of Aerospace and Defense at Texas Instruments.
A Range of Sensor Types
The various sensor modalities have a variety of applications. Infrared imaging can track temperature changes. Optical cameras can measure clouds, or in the absence of clouds, they’re able to chart topographical features. Radar has longer wavelengths than light and can penetrate clouds, although standard radar sacrifices resolution for its penetrative ability. A very large antenna—i.e., one with a large aperture—can improve radar resolution, although such an antenna would be impractical to launch and deploy on a satellite.
Yet persistent researchers have found a workaround called synthetic aperture radar (SAR). “By taking advantage of how satellites move along their orbit, we can create a virtual or synthetic aperture that’s effectively several kilometers in length, with a physical antenna that’s actually much smaller,” said Jason Clark, systems manager for Space and Avionics at TI. “We can then make much more precise observations, no matter the weather conditions.”
RF Semiconductor Components
Implementing a satellite SAR imaging payload requires a variety of specialized RF components (Fig. 1), including attenuators, low-noise amplifiers (LNAs), power amplifiers (PAs), digital-to-analog converters (DACs), and analog-to-digital converters (ADCs). To help reduce bill-of-materials (BOM) count and system size, the TI AFE7950-SP RF-sampling transceiver incorporates several of these devices, including six 3-Gsample/s ADCs and four 12-Gsample/s DACs.
The device’s 3-dB input bandwidth is 10.6 GHz, enabling it to support RF sampling from the L-band (1 to 2 GHz) to lower end of the X-band (8 to 12 GHz). Other features include an instantaneous bandwidth of 1.2 GHz to enhance range resolution and facilitate the implementation of anti-jamming techniques.
An alternative to radar imaging is the camera, a passive system that captures photons from a light source such as the sun reflected off a target. Satellite camera payloads can study weather patterns, ice coverage, and the impact of natural disasters.
Figure 2 shows the block diagram of a camera payload, which includes an image sensor, sample/hold (S/H) amplifiers, data converters, and other components. Specific devices suitable for use in such a payload include the ADC3683-SP, a dual-channel ADC that offers 18-bit resolution at rates up to 65 Msamples/s to maximize dynamic range. The device operates on less than 100 mW to minimize heat generation, which can degrade sensor performance.
While camera payloads often capture visible light, passive systems can also detect wavelengths ranging from infrared through ultraviolet. Many capture images in single bands, but multispectral imagers combine images from three or more coarse spectral bands, while hyperspectral sensors can capture hundreds of narrow bands to help scientists determine the chemical composition of atmospheres and soils.
Packaging Considerations
Whatever sensors are chosen by scientists, the components that make up their imaging systems must withstand the rigors of space for long periods of time. Traditionally, satellite operators have relied on radiation-hardened components that adhere to the Qualified Manufacturers List (QML) Class V standard administered by the U.S. Department of Defense. Such devices, housed in hermetically sealed ceramic packages, are commonly used for satellites in geosynchronous or middle-earth orbits that can have lifespans of a decade or more.
Offering alternatives to QML Class V devices are radiation-hardened QML Class P components, which come in plastic packages. Plastic has been avoided in space applications because it’s subject to outgassing—i.e., plastic packages exposed to the fluctuating temperatures can emit chemicals that can degrade sensor arrays.
Class P devices, however, employ specialized space-enhanced plastics that minimize outgassing. In addition, they’re smaller than functionally equivalent Class V devices, enabling operators to pack more capabilities into their satellites. Class P devices often find use in LEO satellites that have comparatively short lifespans, generally less than seven years.
“At TI, we offer different device classifications that help our customers balance the needs of their system,” said Mueller. “We deliver products to help meet the system-level specifications and address reliability needs with our broad offering of radiation-hardened and radiation-tolerant devices.”
Minimizing Variations
For LEO satellite applications, TI offers devices fabricated in accordance with its Space Enhanced Plastic (Space EP) product-qualification process. TI manufactures these products using a controlled baseline flow that minimizes site-to-site variations in materials, radiation tolerance, and electrical specifications.
The devices employ gold bond wires to enhance reliability, and to prevent tin whiskering, Space EP products don’t include terminations with high tin content. TI performs extended highly accelerated stress testing and temperature cycling on the devices and ensures they comply with the NASA-driven American Society for Testing and Materials E595 outgassing specification.
Choosing Space EP components avoids the need for upscreening. Upscreening is the risky process of choosing a standard part and then performing extensive electrical and environmental tests to determine if it will operate outside of its datasheet specifications with respect to space-related parameters such as temperature range and radiation tolerance.
Conclusion
As LEO satellites take on increasingly sophisticated Earth-observation tasks, components fabricated using TI’s Space EP process provide the required resolutions and sample rates to enable comprehensive imaging while keeping system size compact. TI stands ready to work with the space community as Earth-observation technology evolves.
“The planet is billions of years old, and humanity has only been measuring it for the most infinitesimal time in comparison,” said Clark. “But by working with scientists today, we’re able to prepare for what they will need tomorrow.”