Space Surveillance Telescope Research Articles

Stray Light Suppression of Wide-Field Surveillance in Complicated Situations

Wide field and long exposure time can effectively improve the ability of space surveillance telescope to detect weak space targets. However, this is easily affected by stray light, resulting in the effective target submerged in the background of stray light. Based on the basic theory of Mathematical morphology, this paper proposes an accurate and highly robust method (ETH) for stray light suppression of space surveillance telescope, which is called the enhanced Top-Hat transform. Firstly, we define the Generalized Top-Hat transform according to the traditional Top-Hat transform. Secondly, we improve the background estimation of the traditional opening, introduce the closing to realize the multi-scale micro adjustment, and analyze the influence of the multi-scale micro adjustment on the stray light suppression and space weak targets segmentation. Finally, we introduce the noise suppression factor to reduce the residual noise in the stray light. In the field experiment, the method can effectively eliminate the interference of stray light, and greatly improves the local signal-to-noise ratio of space targets. It also shows that in a very low signal-to-noise ratio, this method can accurately and effectively segment space weak targets, and has strong robustness.

Stray light nonuniform background correction for a wide-field surveillance system.

Wide field and a long exposure time can effectively improve the ability of a space surveillance telescope to detect faint space targets. However, such systems are very susceptible to stray light. A stray light nonuniform background will cause great interference to subsequent target recognition, resulting in a large number of false alarms. This study presents an accurate and robust correction algorithm, called the improved new top-hat transformation (INTHT), for a stray light nonuniform background. First, we analyzed the formation mechanism and influence of the stray light nonuniform background. Then, to retrieve the lost targets, the size relationship of the two different, but related, structural operators is changed so the sizes of two structural operators are not equal. Finally, before comparing to the original image to take the minimum value, we added an expansion operation to restore the background size transformation caused by the different sizes of the structural operators in the previous step. This will ensure that there is no residual stray light nonuniform background. We believe, to the best of our knowledge, that the experimental results for the real captured image datasets demonstrate that, compared to other algorithms, the proposed INTHT algorithm has a higher accuracy and effectiveness in correcting a stray light nonuniform background.

Discovering the smallest observed near-earth objects with the space surveillance telescope

The Space Surveillance Telescope (SST) is an advanced optical sensor designed and tested by MIT Lincoln Laboratory for the Defense Advanced Research Projects Agency (DARPA), which is currently in the process of being integrated into the Space Surveillance Network. By operating the telescope in a manner normally intended for the discovery of small, artificial space objects, SST is serendipitously sensitive to the detection of very small asteroids as they traverse close to the Earth, passing rapidly through SST's search volume. This mode of operation stands in contrast to the standard approach for the search and discovery of asteroids and near-Earth objects (NEOs), in which longer revisit times restrict survey sensitivities to objects moving no faster than about 20 °/day. From data collected during SST's observation runs in New Mexico, we detail the discovery of 92 new candidate objects in heliocentric orbit whose absolute magnitudes range from H = 26.4 to 35.9 (approximately 18-m to 25-cm in size). Some of these discoveries represent the smallest natural objects ever observed in orbit. We compare the candidate objects with bolide observations (Brown et al., 2016).

Space Target Detection in Complicated Situations for Wide-Field Surveillance

Long exposure time and wide field can effectively improve the ability of a space surveillance telescope to detect faint space targets. However, complicated situations pose challenges for space target detection. Background star images usually manifest a rotated streak, and target trajectories can be crossed, discontinuous, or nonlinear. This paper presents an accurate and robust space target detection method, namely, spatiotemporal pipeline multistage hypothesis testing (SPMHT), to overcome the issues. Specifically, the method includes the following two stages: First, in the spatiotemporal pipeline filtering step, Spatiotemporal-related Intersection over Union (SrIoU) is used to calculate the IoU score instead of the traditional method. Benefiting from the differences between motion characteristics of targets and stars and the insensitivity of the SrIoU score to the noise, the spatiotemporal pipeline filtering can effectively remove the streak images of background stars and obtain candidate points. Second, a series of candidate points is further organized into a tree structure. We pruned in the tree structure combined with these candidate trajectories by using velocity and direction feature of moving objects. Furthermore, in the search step, fast adaptive sequence region search is used to reduce the computational cost. The experimental results for two datasets, simulated image datasets and real captured image datasets, demonstrate the effectiveness in addressing the difficulties of space target detection in complicated situations.

Unconfirmed Near-Earth Objects

Abstract We studied the Near-Earth Asteroid (NEA) candidates posted on the Minor Planet Center’s Near-Earth Object Confirmation Page (NEOCP) between years 2013 and 2016. Out of more than 17000 NEA candidates, while the majority became either new discoveries or were associated with previously known objects, about 11% were unable to be followed-up or confirmed. We further demonstrate that of the unconfirmed candidates, 926 ± 50 are likely to be NEAs, representing 18% of discovered NEAs in that period. Only 11% (∼93) of the unconfirmed NEA candidates were large (having absolute magnitude H < 22). To identify the reasons why these NEAs were not recovered, we analyzed those from the most prolific asteroid surveys: Pan-STARRS, the Catalina Sky Survey, the Dark Energy Survey, and the Space Surveillance Telescope. We examined the influence of plane-of-sky positions and rates of motion, brightnesses, submission delays, and computed absolute magnitudes, as well as correlations with the phase of the moon and seasonal effects. We find that delayed submission of newly discovered NEA candidate to the NEOCP drove a large fraction of the unconfirmed NEA candidates. A high rate of motion was another significant contributing factor. We suggest that prompt submission of suspected NEA discoveries and rapid response to fast-moving targets and targets with fast growing ephemeris uncertainty would allow better coordination among dedicated follow-up observers, decrease the number of unconfirmed NEA candidates, and increase the discovery rate of NEAs.

Unequal a priori probability multiple hypothesis testing in space domain awareness with the space surveillance telescope.

This paper investigates the ability to improve Space Domain Awareness (SDA) by increasing the number of detectable Resident Space Objects (RSOs) from space surveillance sensors. With matched filter based techniques, the expected impulse response, or Point Spread Function (PSF), is compared against the received data. In the situation where the images are spatially undersampled, the modeled PSF may not match the received data if the RSO does not fall in the center of the pixel. This aliasing can be accounted for with a Multiple Hypothesis Test (MHT). Previously, proposed MHTs have implemented a test with an equal a priori prior probability assumption. This paper investigates using an unequal a priori probability MHT. To determine accurate a priori probabilities, three metrics are computed; they are correlation, physical distance, and empirical. Using the calculated a priori probabilities, a new algorithm is developed, and images from the Space Surveillance Telescope (SST) are analyzed. The number of detected objects by both an equal and unequal prior probabilities are compared while keeping the false alarm rate constant. Any additional number of detected objects will help improve SDA capabilities.

Improving space domain awareness through unequal-cost multiple hypothesis testing in the space surveillance telescope.

This paper investigates algorithms to improve the detection of space objects with the space surveillance telescope (SST) system. These space objects include natural objects such as asteroids and artificial satellites in Earth orbit. Using a proposed multiple hypothesis test (MHT), the detection performance is compared to the currently used algorithm as well as a matched filter and an equal-cost MHT algorithm. To compare these algorithms, a data set collected by the SST of a geosynchronous Earth orbit satellite, ANIK-F1 entering the Earth's eclipse, is utilized. It is found that an unequal-cost MHT gives increased performance over a point detector, a matched filter, and equal-cost MHT over a large range of potential intensities. Results are presented as probability of detection and receiver operating characteristic curves. In addition, the performance of the algorithm as a function of number of hypotheses used is investigated.

Development and application of spherically curved charge-coupled device imagers.

Operation of a CCD imager on a curved focal surface offers advantages to flat focal planes, especially for lightweight, relatively simple optical systems. The first advantage is that the modulation transfer function can approach diffraction-limited performance for a spherical focal surface employed in large field-of-view or large-format imagers. The second advantage is that a curved focal surface maintains more uniform illumination as a function of radius from the field center. Examples of applications of curved imagers, described here, include a small compact imager and the large curved array used in the Space Surveillance Telescope. The operational characteristics and mechanical limits of an imager deformed to a 15 mm radius are also described.

Detecting small asteroids with the Space Surveillance Telescope

The ability of the Space Surveillance Telescope (SST) to find small (2–15m diameter) NEAs suitable for the NASA asteroid retrieval mission is investigated. Orbits from a simulated population of targetable small asteroids were propagated and observations with the SST were simulated. Different search patterns and telescope time allocation cases were considered, as well as losses due to FOV gaps and weather. It is concluded that a full-time, dedicated survey at the SST is likely necessary to find a useful population of these NEAs within the mission launch timeframe, especially if an object must be observed on >1 night at SST to qualify as a detection. The simulations were also performed for an identical telescope in the southern hemisphere, which is found to produce results very similar to the SST in New Mexico due to significant (∼80%) overlap in the population of objects detected at each site.In addition to considering the SST’s ability to detect small NEAs, a parallel study was performed focusing on >100m diameter objects. This work shows that even with limited telescope time (3 nights per month) a substantial number of these larger objects would be detected.

IMPROVING THE SPACE SURVEILLANCE TELESCOPE'S PERFORMANCE USING MULTI-HYPOTHESIS TESTING

The Space Surveillance Telescope (SST) is a Defense Advanced Research Projects Agency program designed to detect objects in space like near Earth asteroids and space debris in the geosynchronous Earth orbit (GEO) belt. Binary hypothesis test (BHT) methods have historically been used to facilitate the detection of new objects in space. In this paper a multi-hypothesis detection strategy is introduced to improve the detection performance of SST. In this context, the multi-hypothesis testing (MHT) determines if an unresolvable point source is in either the center, a corner, or a side of a pixel in contrast to BHT, which only tests whether an object is in the pixel or not. The images recorded by SST are undersampled such as to cause aliasing, which degrades the performance of traditional detection schemes. The equations for the MHT are derived in terms of signal-to-noise ratio (S/N), which is computed by subtracting the background light level around the pixel being tested and dividing by the standard deviation of the noise. A new method for determining the local noise statistics that rejects outliers is introduced in combination with the MHT. An experiment using observations of a known GEO satellite are used to demonstrate the improved detection performance of the new algorithm over algorithms previously reported in the literature. The results show a significant improvement in the probability of detection by as much as 50% over existing algorithms. In addition to detection, the S/N results prove to be linearly related to the least-squares estimates of point source irradiance, thus improving photometric accuracy.

Space Surveillance Telescope: focus and alignment of a three mirror telescope

The Space Surveillance Telescope (SST) is a three-mirror Mersenne-Schmidt telescope with a 3.5 m primary mirror. It is designed to rapidly scan for space objects, particularly along the geosynchronous belt, approximately 36,000 km above the Earth. The SST has an unusually short focal ratio of F/1.0 and employs a camera composed of curved charge-coupled devices to match the telescope’s inherent field curvature. The field-of-view of the system is 6 square degrees. While the unique system design is advantageous for space surveillance capabilities, it presents a challenge to alignment due to an inherently small depth of focus and the additional degrees of freedom introduced with a powered tertiary mirror. The alignment procedure developed for the SST at zenith pointing is discussed, as well as the maintenance of focus and alignment of the system across a range of elevation and temperature conditions. Quantitative performance metrics demonstrate the success of the system alignment during the telescope’s first year of operation.