Study to investigate and evaluate means of optimizing the Ku-band communication function for the space shuttle

The forward link of the overall Ku-band communication system consists of the ground- TDRS-orbiter communication path. Because the last segment of the link is directed towards a relatively low orbiting shuttle, a PN code is used to reduce the spectral density. A method is presented for incorporating code acquisition and tracking functions into the orbiter's Ku-band receiver. Optimization of a three channel multiplexing technique is described. The importance of Costas loop parameters to provide false lock immunity for the receiver, and the advantage of using a sinusoidal subcarrier waveform, rather than square wave, are discussed

Mitigating the Effects of Ionospheric Scintillation on GPS Carrier Recovery

Ionospheric scintillation is a phenomenon caused by varying concentrations of charged particles in the upper atmosphere that induces deep fades and rapid phase rotations in satellite signals, including GPS. During periods of scintillation, carrier tracking loops often lose lock on the signal because the rapid phase rotations generate cycle slips in the PLL. One solution to mitigating this problem is by employing decision-directed carrier recovery algorithms that achieve data wipe-off using differential bit detection techniques. Other techniques involve PLLs with variable bandwidth and variable integration times. Since nearly 60% of the GPS signal repeats between frames, this thesis explores PLLs utilizing variable integration times and decision-directed algorithms that exploit the repeating data as a training sequence to aid in phase error estimation. Experiments conducted using a GPS signal generator, software radio, and MATLAB scintillation testbed compare the bit error rate of each of the receiver models. Training-based methods utilizing variable integration times show significant reductions in the likelihood of total loss of lock

Seguimiento de la señal de Galileo E1 OS para UAVs

Este proyecto está relacionado con el seguimiento de la señal E1 OS de Galileo. El objetivo es llevar a cabo el procesado en banda base de señales de Galileo usando Matlab, por lo que el alcance del proyecto está dentro del procesado digital de señales de radiofrecuencia. La implementación se basa en un toolbox existente desarrollado en el departamenteo de DTU Space para GPS, que ha sido adaptado para aceptar la señal de Galileo. Además de esto, se han recogido datos usando un receptor software y el toolbox ha sido probado. Asimismo, se ha llevado a cabo un estudio de multipath usando una estrategia multicorrelador. La tesis está dividida en cuatro bloques principales. El primero introduce la señal de Galileo, así como algo de teoría sobre receptores software como background para la implementación, que se describe justo después. Posteriormente, se muestra el setup para la recogida de datos junto con algunos resultados y la discusión de los mismos. Finalmente, se analiza el multipath en un capítulo separado, que consiste en una pequeña sección de teoría, las modificaciones en la implementación y la seccion de resultados

Software Defined Radio for processing GNSS signals

GPS satellites are fitted with atomic clocks, in which it relapses the main objective of this project, to recover some of their accuracy and stability on a ground based receiver. This project describes the fundamentals of GPS signals, the assembly of the installation implemented to process them in software and the corresponding experiments. In order to achieve the software processing, a USB DVB-T dongle is connected to an active antenna and to the computer. As mentioned, one of the purposes is also to understand how a GPS can be implemented by software as a the substitution of a big part of the hardware that makes it impenetrable, as they are black boxes of integrated circuits, and expensive. It is known that a Global Navigation Satellite System (GNSS) software-defined open source receiver has already been created by people in Barcelona in “Centre Tecnològic de Telecomunicacions de Catalunya (CTTC)”, a testbed for GNSS signal processing since it can be customized in every way. It has been used at some intermediate steps of the study while executing parallel experiments in the course of understanding how a GPS signal is digitally processed. In the meantime, some experiments have also been performed only employing hardware before implementing them in software, so that the concepts are visually reflected. When realizing software experiments, an interface called GNURadio has been used because of its enormous implementation of signal processing blocks. GNURadio can be used with external RF hardware to create software-defined radios, or without hardware in a simulation-like environment. Nevertheless, various simulations in the GNU (Octave software environment) have also been executed as processing in real time has not been considered a goal. However, to successfully accomplish the demodulation of the navigation data, which will contribute to restore the accuracy and stability of the satellites clocks that have sent it, the carrier frequency needs to be perfectly recovered, being this last point where the final aim of the project falls on.Outgoin

Engineering evaluations and studies. Volume 3: Exhibit C

High rate multiplexes asymmetry and jitter, data-dependent amplitude variations, and transition density are discussed

Implementation of a Software Defined Spread Spectrum Communication System

The goal of this thesis is to develop a framework to prototype a software defined direct sequence spread spectrum transceiver that can be used as a node in an ad hoc network. We introduce the concept of a software radio, the current state of art, and GNU Radio and its concepts. We discuss in detail the design and development methods of GNU Radio and develop a flowgraph in Python to demonstrate the method of development. We present a mathematical analysis of (DSSS) modulation and demodulation schemes along with the transmitter and receiver design. We use this design to develop an analogous design in GNU Radio using the signal processing blocks that are present in GNU Radio and ones we develop. We perform simulations and tests to validate the algorithms, signal processing blocks and flowgraphs that we developed. We find that the signal acquistion algorithm is capable of determining the code and frequency offset in a received (DSSS) signal. We also find that the carrier tracking loop is capable of tracking the received carrier when the signal has a high (SNR). We conclude that GNU Radio as a technology can be used to prototype transceivers that are highly configurable and expandable. Finally, we identify and suggest some possible areas where this design can be developed and improved further

Investigation and evaluation of shuttle/GPS navigation system

Iterative procedures were used to analyze the performance of two preliminary shuttle/GPS navigation system configurations: an early OFT experimental system and a more sophisticated system which consolidates several separate navigation functions thus permitting net cost savings from decreased shuttle avionics weight and power consumption, and from reduced ground data processing. The GPS system can provide on-orbit navigation accuracy an order of magnitude better than the baseline system, with very adequate link margins. The worst-case link margin is 4.3 dB. This link margin accounts for shuttle RF circuit losses which were minimized under the constraints of program schedule and environmental limitations. Implicit in the link analyses are the location trade-offs for preamplifiers and antennas

The Differential Vector Phase-Locked Loop for Global Navigation Satellite System Signal Tracking

A novel differential vector phase-locked loop (DVPLL) is derived that takes GNSS code-phase and carrier-phase measurements from a base station and uses them to maintain an integer ambiguity resolved quality solution directly in the vector tracking loop of a rover receiver. The only state variables estimated and used to create the replica code and carrier signals from the base station measurements are three position and two clock states for a static test. Closing the individual loops solely through the navigation filter makes this a pure vector method. For short baselines, where differential atmospheric errors are small, the DVPLL can be used on single-frequency data. An L1-only live-sky static test was performed using the method resulting in a 3D accuracy of 5.3 mm for an 18.5 m baseline. An acquisition algorithm is also developed to initialize the DVPLL. The algorithm performs a search in the space-time domain vice the measurement domain. An upper bound on the failure rate of the algorithm can be set by the user. The algorithm was tested on 24-h dual- and single-frequency CORS data sets with close to a 100% success rate and on a 15- min data set of single-frequency IF samples with a 100% success rate

Carrier Phase Recovery for Coherent Optical Transmission Systems

This thesis presents studies concerned with the evaluation of the suitablility of an optical-phase-locked-loop (OPLL) as a component in a coherent optical transmission network. The performance of various binary optical transmission schemes, measured by the probability of error (Bit Error Rate), is presented. The linewidths of semiconductor diode lasers are, in general, not optimally suited to coherent transmission formats and therefore an additional power penalty must be paid if such sources are to be used. This power penalty is included in the analysis of a binary PSK transmission. The performance analysis of coherent transmission schemes is extended to encompass multi-level signalling formats. It is shown that, with the use of a sufficient number of signalling levels, these schemes can provide a receiver sensitivity which is better than a shot noise limited binary PSK transmission even when subject to the effects of high laser line width. A discussion on the properties of the laser diode, which are of interest in the design of a coherent optical transmission scheme (the source linewidth, modulation charateristics, wavelength tunablity and frequency and power stability), is provided. Measurements of these properties is given along with documentation of experimental attempts to reduce the linewidth of the laser emission. Linewidth reductions from between five and ten Megahertz down to tens of kilohertz were obtained. An analysis of the performance of an OPLL, taking into consideration the effect of phase instability of the laser diode source, is given. This analysis predicts that present day laser diodes can be used in the construction of the OPLL provided that the loop bandwidth is sufficiently large to account for the laser instability, while not being excessively large such that the performance is impaired by the influence of shot noise. This analysis is extended to describe the effect which the loop transport delay will have on such a loop, as the required loop bandwidth is extended. Similar calculations are performed to evaluate the effect of the loop receiver amplifier bandwidth. An evaluation of the OPLL response in the time domain, with the use of the Root Locus construction technique and a digital simulation, is presented. This analysis supports the findings of the analysis described above. The simulation of the loop dynamic response is found to be in reasonable agreement with the experimental findings of a previous worker. The components for the construction of an optical heterodyne-phase-locked-loop were assembled and attempts were made to lock both free running laser diodes and external cavity line-narrowed diodes. Reliable aquisition of lock was never achieved in any of the experiments performed despite the fact that beat notes of less than 50 kHz were achieved on a regular basis. The main factor which frustrated the succesful implenentation of the OPLL was deduced to have been mechanical disturbances, of the order of nanometers in magnitude, in the reflecting mirror of the external cavity sources. Hysteresis in the mechanical positioning equipment used to control the external reflector positioning was responsible for limiting the effectiveness of the frequency control loop. Experimentation was terminated by the deterioration of the laser sources, the suspected cause of which was the clustering of material defects within the active region of the device