The Space-Based Visible Program: Technology Demonstrations
FIGURE 2. Major technology demonstrations on the SBV sensor include (a) high stray-light rejection optics, (b) the focal-plane array, and (c) the onboard signal processor. The high stray-light rejection capability allows the SBV sensor to track satellites near the sunlit earth limb. The four 420 x 422-pixel charge-coupled-devices (CCD) in the focal-plane array were fabricated in the Solid State division at Lincoln Laboratory in the late 1980s, and are used by the SBV sensor to detect photons from stars, satellites, and man-made debris. The onboard signal processor processes the focal-plane images to yield star and streak reports needed for routine space surveillance of resident space objects (RSO).
The degree to which stray light is rejected can be quantified by the minimum detectable object that can be seen in the presence of stressing backgrounds. At the SBV Critical Design Review, the goal was set to establish the capability of detecting a 68-cm-diameter specular sphere with a reflectivity of 0.8 at a range of three thousand kilometers at a tangent height of a hundred kilometers above the sunlit earth. The detection-sensitivity results shown in Figure 3 illustrate that this goal was substantially exceeded; minimum detection capability is currently equivalent to a 22-cm-diameter sphere under the conditions described above. Figure 3 also shows that performance of the sensor has not degraded since delivery of the telescope to Lincoln Laboratory [4, 5].
FIGURE 3. SBV detection sensitivity for space-borne targets. The on-orbit performance of the SBV sensor allows a 22-cm-diameter specular sphere (with a reflectivity of 0.8) to be tracked at a range of three thousand kilometers against an earth-limb background at a tangent height of one hundred kilometers. This capability exceeds the minimum detectable target diameter of 68 cm in the original design goal by a factor of three.
The second important technology incorporated into the design of the SBV sensor is low-noise charge-coupled-device (CCD) focal-plane arrays, shown in Figure 2(b). These four abutting 420 x 422-pixel arrays, each with a frame-store region for rapid readout, were designed and fabricated by the Semiconductor division at Lincoln Laboratory in the late 1980s.
CCD focal planes can be characterized by the dark current and its nonuniformity, along with the read noise, the charge-transfer efficiency, the well depth, and the percentage of damaged pixels. All of these are affected by on-orbit radiation. The SBV focal plane has exceeded performance expectations with respect to all these measures. The dark current and its nonuniformity appear to be increasing slowly because of radiation damage after almost three years in orbit. Even if the trend continues, however, the detection thresholds set at the SBV Critical Design Review will not be exceeded for another eight years. The focal-plane noise has also been affected slightly on orbit but will not be significant for more than ten years. There has also been no detectable change in the charge-transfer efficiency.
The third important technology demonstrated by the SBV program is that of the signal processor, shown in Figure 2(c). During routine space-surveillance operations, SBV sensor data are gathered by staring at a chosen location in the sky and collecting the image data over a sequence of frames, which is referred to as a frameset. A typical frameset includes as many as sixteen frames, resulting in almost three million pixels of information. The quantity of raw data generated by this process is far too large to be downloaded on the 1-Mbit/sec communications link periodically available to the MSX. The signal processor analyzes these three million pixels of information per field area and retains only the information most vital for space surveillance, thus reducing the data volume by as much as a factor of a thousand. The retained information consists of a selection of stars needed to determine the pointing of the SBV sensor and any streak signatures left by RSOs moving through the field of view. Figure 4 illustrates the effect of the signal processor by showing the superposition of sixteen raw frames (left) and the signal-processed image of these sixteen frames (right). In these images the stationary point sources are star detections and the streaks indicate detections of satellites [6-8].
FIGURE 4. (a) SBV raw full-frame CCD exposure and (b) an associated signal processor image. The onboard signal processor reduces the volume of raw data by as much as a factor of a thousand, thus allowing for more effective downloading of image data across narrow-bandwidth telemetry links.
The appendices entitled "Space-Based Visible Hardware" and "Calibration and Testing" provide a more detailed description of the SBV hardware- including the telescope, the focal-plane array, and the signal processor-and its construction, testing, and calibration.
In summary, the technology demonstrations on the SBV sensor have been extremely successful, paving the way for the design of future operational space-based space-surveillance systems.