7,790 research outputs found

    A high-sensitivity GPS receiver carrier-tracking loop design for high-dynamic applications

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    © 2014, Springer-Verlag Berlin Heidelberg. In order to enhance the tracking performance of global positioning system (GPS) receivers for weak signal applications under high-dynamic conditions, a high-sensitivity and high-dynamic carrier-tracking loop is designed. The high-dynamic performance is achieved by aiding from a strapdown inertial navigation system (SINS). In weak signal conditions, a dynamic-division fast Fourier transform (FFT)-based tracking algorithm is proposed to improve the sensitivity of GPS receivers. To achieve the best performance, the tracking loop is designed to run either in the conventional SINS-aided phase lock loop mode (time domain) or in the frequency-domain-tracking mode according to the carrier-to-noise spectral density ratio detected in real time. In the frequency-domain-tracking mode, the proposed dynamic-division FFT algorithm is utilized to estimate and correct the error of the SINS aiding. Furthermore, the optimal values of the dynamic-division step and the FFT size are selected to maximize the signal-to-noise ratio gain. Simulation results demonstrate that the designed loop can significantly improve the tracking sensitivity and robustness for weak GPS signals without compromising the dynamic performance

    Advanced Algorithms for Satellite Communication Signal Processing

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    Dizertační práce je zaměřena na softwarově definované přijímače určené k úzkopásmové družicové komunikaci. Komunikační kanály družicových spojů zahrnujících komunikaci s hlubokým vesmírem jsou zatíženy vysokými úrovněmi šumu, typicky modelovaného AWGN, a silným Dopplerovým posuvem signálu způsobeným mimořádnou rychlostí pohybu objektu. Dizertační práce představuje možné postupy řešení výpočetně efektivní digitální downkonverze úzkopásmových signálů a systému odhadu kmitočtu nosné úzkopásmových signálů zatížených Dopplerovým posuvem v řádu násobků šířky pásma signálu. Popis navrhovaných algoritmů zahrnuje analytický postup jejich vývoje a tam, kde je to možné, i analytické hodnocení jejich chování. Algoritmy jsou modelovány v prostředí MATLAB Simulink a tyto modely jsou využity pro ověření vlastností simulacemi. Modely byly také využity k experimentálním testům na reálném signálu přijatém z družice PSAT v laboratoři experimentálních družic na ústavu radioelektroniky.The dissertation is focused on software defined receivers intended for narrowband satellite communication. The satellite communication channel including deep space communication suffers from a high level of noise, typically modeled by AWGN, and from a strong Doppler shift of a signal caused by the unprecedented speed of an object in motion. The dissertation shows possible approaches to the issues of computationally efficient digital downconversion of narrowband signals and the carrier frequency estimation of narrowband signals distorted by the Doppler shift in the order of multiples of the signal bandwidth. The description of the proposed algorithms includes an analytical approach of its development and, if possible, the analytical performance assessment. The algorithms are modeled in MATLAB Simulink and the models are used for validating the performance by the simulation. The models were also used for experimental tests on the real signal received from the PSAT satellite at the laboratory of experimental satellites at the department of radio electronics.

    Shuttle S-band communications technical concepts

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    Using the S-band communications system, shuttle orbiter can communicate directly with the Earth via the Ground Spaceflight Tracking and Data Network (GSTDN) or via the Tracking and Data Relay Satellite System (TDRSS). The S-band frequencies provide the primary links for direct Earth and TDRSS communications during all launch and entry/landing phases of shuttle missions. On orbit, S-band links are used when TDRSS Ku-band is not available, when conditions require orbiter attitudes unfavorable to Ku-band communications, or when the payload bay doors are closed. the S-band communications functional requirements, the orbiter hardware configuration, and the NASA S-band communications network are described. The requirements and implementation concepts which resulted in techniques for shuttle S-band hardware development discussed include: (1) digital voice delta modulation; (2) convolutional coding/Viterbi decoding; (3) critical modulation index for phase modulation using a Costas loop (phase-shift keying) receiver; (4) optimum digital data modulation parameters for continuous-wave frequency modulation; (5) intermodulation effects of subcarrier ranging and time-division multiplexing data channels; (6) radiofrequency coverage; and (7) despreading techniques under poor signal-to-noise conditions. Channel performance is reviewed

    Global Navigation Satellite System Performance in Cislunar Space for Cubesat Form Factors

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    An increased Cislunar traffic is expected by the end of this decade stemming from NASA’s Artermis program. Given the prioritization limitations of the Deep-Space Network (DSN) for ranging and tracking of increased deep- space assets, a more viable, and cost effective, independent navigation capability is needed. NASA’s 2015 Navigator Global Positioning System (GPS) deployed on the Magnetospheric Multi-Scale (MMS) spacecraft has validated the feasibility of acquiring weak GPS signals at distances up to 25 Earth Radii (~150,000km) or about 40% of the Cislunar trajectory. NASA plans to upgrade the flight proven MSS Navigator GPS for the future Lunar Gateway. Concurrently, the European Space Agency has confirmed the feasibility of an interoperable GPS and Galileo receiver at Lunar altitudes for a low acquisition and tracking threshold “Weak HEO” receiver for a Cubesat platform. This engineering analysis sets out to explore: (1) the smallest Global Navigation Satellite Systems (GNSS) receiver antenna that can ensure a positive carrier and code link for a Lunar bound Cubesat; (2) the position dilution of precision (PDOP) profile of this Lunar bound space vehicle; and (3) the expected improvement of the PDOP during the Moon Transfer Orbit (MTO) for an interoperable GNSS receiver, specifically Beidou. For the designed carrier-to-noise acquisition and tracking threshold of 15 dBHz, the Eb/N0 link was assured for a helix antenna with a minimum diameter of 130 mm and length of 200 mm for the GPS L1 frequency at a data rate of 50 bps. The Galileo E5a, E5b would require a larger diameter antenna at 760 mm at 448 bps data rate while Beidou requires a 350 mm diameter antenna for a 100 bps data rate to close their respectively. Utilizing the 130 mm diameter, 200 mm length helix antenna on a Lunar MTO, the preliminary assessment indicated that the GNSS PDOP calculated from valid carrier links increases from 20 when the vehicle is within the GNSS service volume to several 100th or 1000th at 60.3 Earth Radii. Due to their similar constellation altitude geometry, the Galileo E5b PDOP growth profile is similar to that of the GPS L1. The Beidou system however has a much lower PDOP growth. This difference is attributed to the set of Beidou Geosynchronous space vehicles (SV)s that have greater angular separation to the SV- receiver line-of-sight (LoS). For an interoperable GNSS receiver that can track the GPS, Galileo, and Beidou lower bound and upper bound frequencies simultaneously, the increased number of valid signals reduces the PDOP growth below 200. This engineering analysis re-affirms the potential of utilizing existing GNSS infrastructure for onboard navigation in Cislunar space, in particular, a helical antenna that can be accommodated on a Cubesat form factor

    On-Orbit Validation of a Framework for Spacecraft-Initiated Communication Service Requests with NASA's SCaN Testbed

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    We design, analyze, and experimentally validate a framework for demand-based allocation of high-performance space communication service in which the user spacecraft itself initiates a request for service. Leveraging machine-to-machine communications, the automated process has potential to improve the responsiveness and efficiency of space network operations. We propose an augmented ground station architecture in which a hemispherical-pattern antenna allows for reception of service requests sent from any user spacecraft within view. A suite of ground-based automation software acts upon these direct-to-Earth requests and allocates access to high-performance service through a ground station or relay satellite in response to immediate user demand. A software-defined radio transceiver, optimized for reception of weak signals from the helical antenna, is presented. Design and testing of signal processing equipment and a software framework to handle service requests is discussed. Preliminary results from on-orbit demonstrations with a testbed onboard the International Space Station are presented to verify feasibility of the concept

    Adaptive Interference Mitigation in GPS Receivers

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    Satellite navigation systems (GNSS) are among the most complex radio-navigation systems, providing positioning, navigation, and timing (PNT) information. A growing number of public sector and commercial applications rely on the GNSS PNT service to support business growth, technical development, and the day-to-day operation of technology and socioeconomic systems. As GNSS signals have inherent limitations, they are highly vulnerable to intentional and unintentional interference. GNSS signals have spectral power densities far below ambient thermal noise. Consequently, GNSS receivers must meet high standards of reliability and integrity to be used within a broad spectrum of applications. GNSS receivers must employ effective interference mitigation techniques to ensure robust, accurate, and reliable PNT service. This research aims to evaluate the effectiveness of the Adaptive Notch Filter (ANF), a precorrelation mitigation technique that can be used to excise Continuous Wave Interference (CWI), hop-frequency and chirp-type interferences from GPS L1 signals. To mitigate unwanted interference, state-of-the-art ANFs typically adjust a single parameter, the notch centre frequency, and zeros are constrained extremely close to unity. Because of this, the notch centre frequency converges slowly to the target frequency. During this slow converge period, interference leaks into the acquisition block, thus sabotaging the operation of the acquisition block. Furthermore, if the CWI continuously hops within the GPS L1 in-band region, the subsequent interference frequency is locked onto after a delay, which means constant interference occurs in the receiver throughout the delay period. This research contributes to the field of interference mitigation at GNSS's receiver end using adaptive signal processing, predominately for GPS. This research can be divided into three stages. I first designed, modelled and developed a Simulink-based GPS L1 signal simulator, providing a homogenous test signal for existing and proposed interference mitigation algorithms. Simulink-based GPS L1 signal simulator provided great flexibility to change various parameters to generate GPS L1 signal under different conditions, e.g. Doppler Shift, code phase delay and amount of propagation degradation. Furthermore, I modelled three acquisition schemes for GPS signals and tested GPS L1 signals acquisition via coherent and non-coherent integration methods. As a next step, I modelled different types of interference signals precisely and implemented and evaluated existing adaptive notch filters in MATLAB in terms of Carrier to Noise Density (\u1d436/\u1d4410), Signal to Noise Ratio (SNR), Peak Degradation Metric, and Mean Square Error (MSE) at the output of the acquisition module in order to create benchmarks. Finally, I designed, developed and implemented a novel algorithm that simultaneously adapts both coefficients in lattice-based ANF. Mathematically, I derived the full-gradient term for the notch's bandwidth parameter adaptation and developed a framework for simultaneously adapting both coefficients of a lattice-based adaptive notch filter. I evaluated the performance of existing and proposed interference mitigation techniques under different types of interference signals. Moreover, I critically analysed different internal signals within the ANF structure in order to develop a new threshold parameter that resets the notch bandwidth at the start of each subsequent interference frequency. As a result, I further reduce the complexity of the structural implementation of lattice-based ANF, allowing for efficient hardware realisation and lower computational costs. It is concluded from extensive simulation results that the proposed fully adaptive lattice-based provides better interference mitigation performance and superior convergence properties to target frequency compared to traditional ANF algorithms. It is demonstrated that by employing the proposed algorithm, a receiver is able to operate with a higher dynamic range of JNR than is possible with existing methods. This research also presents the design and MATLAB implementation of a parameterisable Complex Adaptive Notch Filer (CANF). Present analysis on higher order CANF for detecting and mitigating various types of interference for complex baseband GPS L1 signals. In the end, further research was conducted to suppress interference in the GPS L1 signal by exploiting autocorrelation properties and discarding some portion of the main lobe of the GPS L1 signal. It is shown that by removing 30% spectrum of the main lobe, either from left, right, or centre, the GPS L1 signal is still acquirable

    Application of GPS tracking techniques to orbit determination for TDRS

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    In this paper, we evaluate two fundamentally different approaches to TDRS orbit determination utilizing Global Positioning System (GPS) technology and GPS-related techniques. In the first, a GPS flight receiver is deployed on the TDRSS spacecraft. The TDRS ephemerides are determined using direct ranging to the GPS spacecraft, and no ground network is required. In the second approach, the TDRSS spacecraft broadcast a suitable beacon signal, permitting the simultaneous tracking of GPS and TDRSS satellites from a small ground network. Both strategies can be designed to meet future operational requirements for TDRS-2 orbit determination
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