219 research outputs found
Application of advanced technology to space automation
Automated operations in space provide the key to optimized mission design and data acquisition at minimum cost for the future. The results of this study strongly accentuate this statement and should provide further incentive for immediate development of specific automtion technology as defined herein. Essential automation technology requirements were identified for future programs. The study was undertaken to address the future role of automation in the space program, the potential benefits to be derived, and the technology efforts that should be directed toward obtaining these benefits
Three Dimensional Bistatic Tomography Using HDTV
The thesis begins with a review of the principles of diffraction and reflection tomography; starting with the analytic solution to the inhomogeneous Helmholtz equation, after linearization by the Born approximation (the weak scatterer solution), and arriving at the Filtered Back Projection (Propagation) method of reconstruction. This is followed by a heuristic derivation more directly couched in the radar imaging context, without the rigor of the general inverse problem solution and more closely resembling an imaging turntable or inverse synthetic aperture radar. The heuristic derivation leads into the concept of the line integral and projections (the Radon Transform), followed by more general geometries where the plane wave approximation is invalid. We proceed next to study of the dependency of reconstruction on the space-frequency trajectory, combining the spatial aperture and waveform. Two and three dimensional apertures, monostatic and bistatic, fully and sparsely sampled and including partial apertures, with controlled waveforms (CW and pulsed, with and without modulation) define the filling of k-space and concomitant reconstruction performance. Theoretical developments in the first half of the thesis are applied to the specific example of bistatic tomographic imaging using High Definition Television (HDTV); the United States version of DVB-T. Modeling of the HDTV waveform using pseudonoise modulation to represent the hybrid 8VSB HDTV scheme and the move-stop-move approximation established the imaging potential, employing an idealized, isotropic 18 scatterer. As the move-stop-move approximation places a limitation on integration time (in cross correlation/pulse compression) due to transmitter/receiver motion, an exact solution for compensation of Doppler distortion is derived. The concept is tested with the assembly and flight test of a bistatic radar system employing software-defined radios (SDR). A three dimensional, bistatic collection aperture, exploiting an elevated commercial HDTV transmitter, is focused to demonstrate the principle. This work, to the best of our knowledge, represents a first in the formation of three dimensional images using bistatically-exploited television transmitters
Abstracts on Radio Direction Finding (1899 - 1995)
The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography).
Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM.
The contents of these files are:
1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format];
2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format];
3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion
Space-based relative multitarget tracking
Access to space has expanded dramatically over the past decade. The growing popularity of small satellites, specifically cubesats, and the following launch initiatives have resulted in exponentially growing launch numbers into low Earth orbit. This growing congestion in space has punctuated the need for local space monitoring and autonomous satellite inspection. This work describes the development of a framework for monitoring local space and tracking multiple objects concurrently in a satellite\u27s neighborhood. The development of this multitarget tracking systems has produced collateral developments in numerical methods, relative orbital mechanics, and initial relative orbit determination.
This work belongs to a class of navigation known as angles-only navigation, in which angles representing the direction to the target are measured but no range measurements are available. A key difference between this work and traditional angles-only relative navigation research is that angle measurements are collected from two separate cameras simultaneously. Such measurements, when coupled with the known location and orientation of the stereo cameras, can be used to resolve the relative range component of a target\u27s position. This fact is exploited to form initial statistical representations of the targets\u27 relative states, which are subsequently refined in Bayesian single-target and multitarget frameworks --Abstract, page iii
Space Tug Docking Study. Volume 4: Supporting Analyses
For abstract, see N76-21245
RF signal sensing and source localisation systems using Software Defined Radios
Radio frequency (RF) source localisation is a critical technology
in numerous location-based military and civilian applications. In
this thesis, the problem of RF source localisation has been
studied from the perspective of the system implementation for
real-world applications. Commercial off-the-shelf Software
Defined Radio (SDR) devices are used to demonstrate the practical
RF source localisation systems. Compared to the conventional
localisation systems, which rely on dedicated hardware, the
SDR-based system is developed using general-purpose hardware and
software-defined components, offering great flexibility and cost
efficiency in system design and implementation.
In this thesis, the theoretical results of source localisation
are evaluated and put into practice. To be specific, the
practical localisation systems using different measurement
techniques, including received-signal-strength-indication (RSSI)
measurements, time-difference-of-arrival (TDOA) measurements and
joint TDOA and frequency-difference-of-arrival (FDOA)
measurements, are demonstrated to localise the stationary RF
signal sources using the SDRs. The RSSI-based localisation system
is demonstrated in small indoor and outdoor areas with a range of
several metres using the SDR-based transceivers. Furthermore,
interests from the defence area motivated us to implement the
time-based localisation systems. The TDOA-based source
localisation system is implemented using multiple spatially
distributed SDRs in a large outdoor area with the sensor-target
range of several kilometres. Moreover, they are implemented in a
fully passive way without prior knowledge of the signal emitter,
so the solutions can be applied in the localisation of
non-cooperative signal sources provided that emitters are
distant. To further reduce the system cost, and more importantly,
to deal with the situation when the deployment of multiple SDRs,
due to geographical restrictions, is not feasible, a joint TDOA
and FDOA-based localisation system is also demonstrated using
only one stationary SDR and one mobile SDR.
To improve the localisation accuracy, the methods that can reduce
measurement error and obtain accurate location estimates are
studied. Firstly, to obtain a better understanding of the
measurement error, the error sources that affect the measurement
accuracy are systematically analysed from three aspects: the
hardware precision, the accuracy of signal processing methods,
and the environmental impact. Furthermore, the approaches to
reduce the measurement error are proposed and verified in the
experiments. Secondly, during the process of the location
estimation, the theoretical results on the pre-existing
localisation algorithms which can achieve a good trade-off
between the accuracy of location estimation and the computational
cost are evaluated, including the weight least-squares
(WLS)-based solution and the Extended Kalman Filter (EKF)-based
solution. In order to use the pre-existing algorithms in the
practical source localisation, the proper adjustments are
implemented.
Overall, the SDR-based platforms are able to achieve low-cost and
universal localisation solutions in the real-world environment.
The RSSI-based localisation system shows tens of centimetres of
accuracy in a range of several metres, which provides a useful
tool for the verification of the range-based localisation
algorithms. The localisation accuracy of the TDOA-based
localisation system and the joint TDOA and FDOA-based
localisation system is several tens of metres in a range of
several kilometres, which offers potential in the low-cost
localisation solutions in the defence area
Pre-phase A Study for a Analysis of a Reusable Space Tug. Volume 5 - Subsystems Final Report
Reusable space tug subsystems for operational and performance requirement
Interference Mitigation and Localization Based on Time-Frequency Analysis for Navigation Satellite Systems
Interference Mitigation and Localization
Based on Time-Frequency Analysis for
Navigation Satellite SystemsNowadays, the operation of global navigation satellite systems (GNSS) is imperative across a multitude of applications worldwide. The increasing reliance on accurate positioning and timing information has made more serious than ever the consequences of possible service outages in the satellite navigation systems. Among others, interference is regarded as the primary threat to their operation. Due the recent proliferation of portable interferers, notably jammers, it has now become common for GNSS receivers to endure simultaneous attacks from multiple sources of interference, which are likely spatially distributed and transmit different modulations.
To the best knowledge of the author, the present dissertation is the first publication to investigate the use of the S-transform (ST) to devise countermeasures to interference. The original contributions in this context are mainly:
• the formulation of a complexity-scalable ST implementable in real time as a
bank of filters;
• a method for characterizing and localizing multiple in-car jammers through
interference snapshots that are collected by separate receivers and analysed
with a clever use of the ST;
• a preliminary assessment of novel methods for mitigating generic interference
at the receiver end by means the ST and more computationally efficient variants of the transform.
Besides GNSSs, the countermeasures to interference proposed are equivalently applicable to protect any direct-sequence
spread spectrum (DS-SS) communication
Antennas and Propagation
This Special Issue gathers topics of utmost interest in the field of antennas and propagation, such as: new directions and challenges in antenna design and propagation; innovative antenna technologies for space applications; metamaterial, metasurface and other periodic structures; antennas for 5G; electromagnetic field measurements and remote sensing applications
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