GPS Carrier Tracking Loop Performance in the presence of Ionospheric Scintillations

The performance of several GPS carrier tracking loops is evaluated using wideband GPS data recorded during strong ionospheric scintillations. The aim of this study is to determine the loop structures and parameters that enable good phase tracking during the power fades and phase dynamics induced by scintillations. Constant-bandwidth and variable-bandwidth loops are studied using theoretical models, simulation, and tests with actual GPS signals. Constant-bandwidth loops with loop bandwidths near 15 Hz are shown to lose phase lock during scintillations. Use of the decision-directed discriminator reduces the carrier lock threshold by ∼1 dB relative to the arctangent and conventional Costas discriminators. A proposed variablebandwidth loop based on a Kalman filter reduces the carrier lock threshold by more than 7 dB compared to a 15-Hz constant-bandwidth loop. The Kalman filter-based strategy employs a soft-decision discriminator, explicitly models the effects of receiver clock noise, and optimally adapts the loop bandwidth to the carrier-to-noise ratio. In extensive simulation and in tests using actual wideband GPS data, the Kalman filter PLL demonstrates improved cycle slip immunity relative to constant bandwidth PLLs.Aerospace Engineering and Engineering Mechanic

Free-Space Measurements of Dielectrics and Three-Dimensional Periodic Metamaterials

This thesis presents the free-space measurements of a periodic metamaterial structure. The metamaterial unit cell consists of two dielectric sheets intersecting at 90 degrees. The dielectric is a polyetherimide-based material 0.001” thick. Each sheet has a copper capacitively-loaded loop (CLL) structure on the front and a cut-wire structure on the back. Foam material is used to support the unit cells. The unit cell repeats 40 times in the x-direction, 58 times in the y-direction and 5 times in the z-direction. The sample measures 12” × 12” × 1” in total. We use a free-space broadband system comprised of a pair of dielectric-lens horn antennas with bandwidth from 5.8 GHz to 110 GHz, which are connected to a HP PNA series network analyzer. The dielectric lenses focus the incident beam to a footprint measuring 1 wavelength by 1 wavelength. The sample holder is positioned at the focal point between the two antennas. In this work, the coefficients of transmission and reflection (the S-parameters S21 and S11) are measured at frequencies from 12.4 GHz up to 30 GHz. Simulations are used to validate the measurements, using the Ansys HFSS commercial software package on the Arkansas High Performance Computing Center cluster. The simulation results successfully validate the S-parameters measurements, in particular the amplitudes. An algorithm based on the Nicolson-Ross-Weir (NRW) method is implemented to extract the permittivity and permeability values of the metamaterial under test. The results show epsilon-negative, mu-negative and double-negative parameters within the measured frequency range

An observation of LHR noise with banded structure by the sounding rocket S29 Barium-GEOS

The measurement of electrostatic and obviously locally produced noise near the lower hybrid frequency made by the sounding rocket S29 Barium-GEOS is reported. The noise is strongly related to the spin of the rocket and reaches well below the local lower hybrid resonance frequency. Above the altitude of 300 km the noise shows banded structure roughly organized by the hydrogen cyclotron frequency. Simultaneously with the banded structure, a signal near the hydrogen cyclotron frequency is detected. This signal is also spin related. The characteristics of the noise suggest that it is locally generated by the rocket payload disturbing the plasma. If this interpretation is correct we expect plasma wave experiments on other spacecrafts, e.g., the space shuttle to observe similar phenomena

Analysis of Ionospheric Scintillations using Wideband GPS L1 C/A Signal Data

A non-real-time GPS receiver has been developed and tested for use in scintillation analysis. The receiver consists of a digital storage receiver and non-real-time software acquisition and tracking algorithms. The goal of this work is to shed light on the behavior of strongly scintillating signals: signals which cause conventional GPS receivers to lose carrier lock. The receiver collects wideband GPS L1 digital data sampled at 5.7 MHz using an RF front-end and stores it on disk for post-processing. It processes the data off-line to determine carrier signal amplitude and phase variations during scintillations. The main processing algorithms are traditional code delay and carrier frequency acquisition algorithms and special signal processing algorithms that effectively function as a delay-locked loop and phase-locked loop. The tracking algorithms use non-causal smoothing techniques in order to optimally reconstruct the phase and amplitude variations of a scintillating signal. These techniques are robust against the deep power fades and strong phase fluctuations characteristic of scintillating signals. To test the receiver, scintillation data were collected in Cauchoeira Paulista, Brazil, from December 4 to 6, 2003. The data set spans several hours and includes times when one or more satellite signals are scintillating. The smoothing algorithm has been used to determine the carrier amplitude and phase time histories of the scintillating signals along with the distortion of the pseudorandom noise (PRN) code’s autocorrelation function. These quantities provide a characterization of scintillation that can be used to study the physics of scintillations or to provide off-line test cases to evaluate a tracking algorithm’s ability to maintain signal lock during scintillations.Aerospace Engineering and Engineering Mechanic

Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer

A portable civilian GPS spoofer is implemented on a digital signal processor and used to characterize spoofing effects and develop defenses against civilian spoofing. This work is intended to equip GNSS users and receiver manufacturers with authentication methods that are effective against unsophisticated spoofing attacks. The work also serves to refine the civilian spoofing threat assessment by demonstrating the challenges involved in mounting a spoofing attack.Aerospace Engineering and Engineering Mechanic

The resilience in illness model, part 1: exploratory evaluation in adolescents and young adults with cancer

BACKGROUND: Resilience is a positive health outcome identified by the Committee on Future Direction for Behavioral and Social Sciences as a research priority for the National Institutes of Health. The Resilience in Illness Model (RIM) was developed from a series of qualitative and quantitative studies, to increase understanding of how positive health protective factors (ie, social integration, family environment, courageous coping, and derived meaning) may influence resilience outcomes. The RIM also includes 2 risk factors: illness-related distress and defensive coping. OBJECTIVE: The purpose of this 2-part article was to report on evaluation of the RIM for adolescents/young adults (AYAs) with cancer. Here, in part 1, our purpose was to describe the exploratory RIM evaluation, and in part 2 we describe the confirmatory RIM evaluation. METHODS: An exploratory evaluation of RIM was done using exploratory latent variable structural equation modeling with a combined sample from 2 studies of preadolescents and AYAs with cancer aged 10 to 26 years (n = 202). RESULTS: Results, including goodness-of-fit indices, support the RIM as a theory with a high level of explained variance for outcomes of resilience (67%) and self-transcendence (63%). Variance explained for proximal outcomes ranged from 18% to 76%. CONCLUSIONS: Findings indicate that, following confirmatory testing, the RIM may be a useful guide to developing targeted interventions that are grounded in the experiences of the AYAs. IMPLICATIONS FOR PRACTICE: Understanding of the AYA cancer experience to improve holistic care is increased

Multi-payload measurement of transverse velocity shears in the topside ionosphere

Using a multi-payload sounding rocket mission, we present the first direct measurement of velocity shear in the topside auroral ionosphere. In regions of large, ∼200 mV/m, transient electric fields we directly measure differences in the plasma drift velocity. From these differences, shear frequencies reaching ±6Hz are measured. These directly measured shears are compared with the shear inferred from single payload measurements. It is shown this traditional measurement of shear overestimates the shear frequency by a factor of two for this event, highlighting the importance of the temporal component of near-DC electric field structures. Coincident with these strong fields and shears are enhanced emissions of broadband, extremely low frequency (BB-ELF) plasma waves, and a narrowband wave emission near the H+-O+bi-ion resonant frequency

Trigger, an active release experiment that stimulated auroral particle precipitation and wave emissions

The experiment design, including a description of the diagnostic and chemical release payload, and the general results are given for an auroral process simulation experiment. A drastic increase of the field aligned charged particle flux was observed over the approximate energy range 10 eV to more than 300 keV, starting about 150 ms after the release and lasting about one second. The is evidence of a second particle burst, starting one second after the release and lasting for tens of seconds, and evidence for a periodic train of particle bursts occurring with a 7.7 second period from 40 to 130 seconds after the release. A transient electric field pulse of 200 mv/m appeared just before the particle flux increase started. Electrostatic wave emissions around 2 kHz, as well as a delayed perturbation of the E-region below the plasma cloud were also observed. Some of the particle observations are interpreted in terms of field aligned electrostatic acceleration a few hundred kilometers above the injected plasma cloud. It is suggested that the acceleration electric field was created by an instability driven by field aligned currents originating in the plasma cloud

Multipayload interferometric wave vector determination of auroral hiss

We extend traditional, single payload, interferometric techniques to a multiple payload sounding rocket mission, and apply these techniques to measure the parallel and perpendicular wavelength of auroral VLF hiss from 8 kHz–20 kHz. We model the wavelength distribution of auroral hiss as a cone at a fixed angle with respect to the magnetic field that is isotropically distributed in the perpendicular plane. We apply this model to calculate the interferometric observables, coherency and phase, for a sounding rocket mission whose wave electric field receivers are on payloads that are separated 2–3 km along the magnetic field and 55–200 m across the magnetic field. Using an interferometer formed by comparing the collinear sphere-to-skin electric field antennas on a single payload, we estimate a lower limit on the perpendicular wavelength of VLF hiss of ∼60 m. Analysis of coherency and phase due to this conical wave vector distribution for a multipayload interferometer reveals the existence of a spin dependent coherency pattern. From this coherency pattern we generate an upper limit perpendicular wavelength estimate for VLF hiss of ∼350 m. The inter-payload phase gives an accurate estimate of the parallel wavelength of ∼6000–8000 m. This parallel wavelength is combined with the lower (upper) limit perpendicular wavelength estimates to generate upper (lower) limits on wave-normal angle. These limits are each within one degree of the predicted electrostatic whistler wave resonance cone angle verifying that VLF hiss propagates on this resonance cone