Current Issue : January - March Volume : 2019 Issue Number : 1 Articles : 5 Articles
In many incidents involving amateur drones (ADr), the big challenge is to quickly deploy\na surveillance system that countermeasures the threat and keeps track of the intruders. Depending on\nthe area under concern, launching a single surveillance drone (SDr) to hunt the intruder is not\nefficient, but employing multiple ones can cope with the problem. However, in order to make this\napproach feasible, an easy to use mission setup and control station for multiple SDr is required, which\nby its turn, requires a communication infrastructure able to handle the connection of multiple SDr\namong themselves and their ground control and payload visualization station. Concerning this Issue, this paper presents a proposal of a network infrastructure to support the operation of multiple SDr and its practical deployment. This infrastructure extends the existing Micro Air Vehicle Link\n(MAVLink) protocol to support multiple connections among the SDrs and between them and a\nground control station. Encouraging results are obtained, showing the viability of this proposed\nprotocol extension....
Tiangong-2, the second Chinese manned spacecraft, was launched into low Earth orbit on\n15 September 2016. The dual-frequency geodetic GNSS receiver equipped on it is supporting a number\nof scientific experiments in orbit. This paper uses the onboard GNSS data from 3â??31 December 2016\n(in the attitude mode of three-axis Earth-pointing stabilization) to analyze the data quantity, as well as the\ncode multipath error. Then, the dynamic and reduced-dynamic methods are adopted to perform the post\nPrecise Orbit Determination (POD) based on the carrier phase measurements, respectively. After that,\nthe orbit accuracy is evaluated using a number of tests, which include the analysis of observation residuals,\nOverlapping Orbit Differences (OODs), orbit comparison between dynamic and reduced-dynamic and\nSatellite Laser Ranging (SLR) validation. The results show that: (1) the average Root Mean Square (RMS)\nof the on-board GNSS phase fitting residuals is 8.8 mm; (2) regarding the OODs determined by the\nreduced-dynamic method, the average RMS in radial (R), along-track (T) and cross-track (N) directions is\n0.43 cm, 1.34 cm and 0.39 cm, respectively, and there are no obvious system errors; (3) the orbit accuracy\nof TG02 determined by the reduced-dynamic method is comparable to that of the dynamic method,\nand the average RMS of their differences in R, T, N and 3D directions is 3.05 cm, 3.60 cm, 2.52 cm and\n5.40 cm, respectively; (4) SLR data are used to validate the reduced-dynamic orbits, and the average RMS\nalong the station-satellite direction is 1.94 cm. It can be seen that both of these two methods can meet the\ndemands of 3D centimeter-level orbit determination for TG02....
In order to satisfy the real-time requirement of spacecraft autonomous navigation using natural landmarks, a novel algorithm called\nCSA-SURF (chessboard segmentation algorithm and speeded up robust features) is proposed to improve the speed without loss of\nrepeatability performance of image registration progress. It is a combination of chessboard segmentation algorithm and SURF.\nHere, SURF is used to extract the features from satellite images because of its scale- and rotation-invariant properties and low\ncomputational cost. CSA is based on image segmentation technology, aiming to find representative blocks, which will be\nallocated to different tasks to speed up the image registration progress. To illustrate the advantages of the proposed algorithm,\nPCA-SURF, which is the combination of principle component analysis and SURF, is also analyzed in this paper for comparison.\nFurthermore, random sample consensus (RANSAC) algorithm is applied to eliminate the false matches for further accuracy\nimprovement. The simulation results show that the proposed strategy obtains good results, especially in scaling and rotation\nvariation. Besides, CSA-SURF decreased 50% of the time in extraction and 90% of the time in matching without losing the\nrepeatability performance by comparing with SURF algorithm. The proposed method has been demonstrated as an alternative\nway for image registration of spacecraft autonomous navigation using natural landmarks....
In Global navigation satellite system (GNSS) data processing, integer ambiguity acceptance\ntest is considered as a challenging problem. A number of ambiguity acceptance tests have been\nproposed from different perspective and then unified into the integer aperture estimation (IA)\nframework. Among all the IA estimators, the optimal integer aperture (OIA) achieves the highest\nsuccess rate with the fixed failure rate tolerance. However, the OIA is of less practical appealing due to\nits high computation complexity. On the other hand, the popular discrimination tests employ only two\ninteger candidates, which are the essential reason for their sub-optimality. In this study, a generalized\ndifference test (GDT) is proposed to exploit the benefit of including three or more integer candidates to\nimprove their performance from theoretical perspective. The simulation results indicate that the\nthird best integer candidates contribute to more than 70% success rate improvement for integer\nbootstrapping success rate higher than 0.8 case. Therefore, the GDT with three integer candidates\n(GDT3) achieves a good trade-off between the performance and computation burden. The threshold\nfunction is also applied for rapid determination of the fixed failure rate (FF)-threshold for GDT3.\nThe performance improvement of GDT3 is validated with real GNSS data set. The numerical results\nindicate that GDT3 achieves higher empirical success rate while the empirical failure rate remains\ncomparable. In a 20 km baseline test, the success rate GDT3 increase 7% with almost the same\nempirical failure rate....
Near-nadir interferometric imaging SAR (Synthetic Aperture Radar) techniques are\npromising in measuring global water extent and surface height at fine spatial and temporal resolutions.\nThe concept of near-nadir interferometric measurements was implemented in the experimental\nInterferometric Imaging Radar Altimeters (InIRA) mounted on Chinese Tian Gong 2 (TG-2) space\nlaboratory. This study is focused on mapping the extent of high mountain lakes in the remote\nQinghaiâ??Tibet Plateau (QTP) areas using the InIRA observations. Theoretical simulations were first\nconducted to understand the scattering mechanisms under near-nadir observation geometry. It was\nfound that water and surrounding land pixels are generally distinguishable depending on the degree\nof their difference in dielectric properties and surface roughness. The observed radar backscatter is\nalso greatly influenced by incidence angles. A dynamic threshold method was then developed to\ndetect water pixels based on the theoretical analysis and ancillary data. As assessed by the LandSat\nresults, the overall classification accuracy is higher than 90%, though the classifications are affected\nby low backscatter possibly from very smooth water surface. The algorithms developed from this\nstudy can be extended to all InIRA land measurements and provide support for the similar space\nmissions in the future....
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