Global Positioning System Array Research Articles

Robust Adaptive Beamformer Based on Constant Modulus Penalty Criteria

In this study, a robust adaptive beamformer based on constant modulus (CM) criteria is developed to improve the robustness of the array beamforming, which is a reconstructing minimal optimization for solving the mismatch problem of weight vector caused by steering vector mismatch. In the global positioning system (GPS) L1 band, firstly, a GPS array signal is modelled by designing a dual-polarized antenna array. Secondly, the distortion problem of beamforming is formulated in the traditional minimum variance distortionless response (MVDR) beamformer. For repairing the weight vector mismatch problem, a novel beamformer based on the CM envelope response is proposed to reconstruct MVDR beamforming in the array processing. Besides, min-max penalty criteria are used to enable the beamformer to allocate more degrees of freedom (DOFs) when penalizing the MVDR beamformer responses. Finally, an auxiliary two-element real variable is designed to plan the proposed beamformer. But it is still a nonconvex quadratic programming problem, so an alternating direction method of multipliers (ADMM) is employed to transform the objective function into several subproblems. Illustrative numerical simulation results are provided for validating the effectiveness of the proposed beamformer by comparing it with other existing approaches.

Morphological and Spectral Features of Ionospheric Structures at E- and F-Region Altitudes over Poker Flat Analyzed Using Modeling and Observations

Electron density irregularities in the ionosphere modify the phase and amplitude of trans-ionospheric radio signals. We aim to characterize the spectral and morphological features of E- and F-region ionospheric irregularities likely to produce these fluctuations or "scintillations". To characterize them, we use a three-dimensional radio wave propagation model-"Satellite-beacon Ionospheric scintillation Global Model of upper Atmosphere" (SIGMA), along with the scintillation measurements observed by a cluster of six Global Positioning System (GPS) receivers called Scintillation Auroral GPS Array (SAGA) at Poker Flat, AK. An inverse method is used to derive the parameters that describe the irregularities by estimating the best fit of model outputs to GPS observations. We analyze in detail one E-region and two F-region events during geomagnetically active times and determine the E- and F-region irregularity characteristics using two different spectral models as input to SIGMA. Our results from the spectral analysis show that the E-region irregularities are more elongated along the magnetic field lines with rod-shaped structures, while the F-region irregularities have wing-like structures with irregularities extending both along and across the magnetic field lines. We also found that the spectral index of the E-region event is less than the spectral index of the F-region events. Additionally, the spectral slope on the ground at higher frequencies is less than the spectral slope at irregularity height. This study describes distinctive morphological and spectral features of irregularities at E- and F-regions for a handful of cases performed using a full 3D propagation model coupled with GPS observations and inversion.

Deformation Analysis of 2012 Mw8.6 Indian Ocean Earthquake Based on GPS Data in Preseismic, Coseismic, and Postseismic Phases

The earthquake was one of the biggest natural disasters in Sumatra and dramatically affected this region and the surrounding area. Determination of surface deformation due to the earthquake is essential for disaster mitigation. The Global Navigation Satellite System (GNSS) is a commonly used method for determining surface deformation due to earthquakes. This study analyzes surface deformation during the preseismic, coseismic, and postseismic phases due to the 2012 Mw8.6 Indian Ocean earthquake. The study used Global Positioning System (GPS) data from the Sumatran GPS Array (SuGAr) network. The most significant horizontal deformation was observed at the LEWK station, which was 280.554 mm towards the northeast and experienced a subsidence of 40.830 mm in vertical deformation. Horizontal deformation is still felt by 22.453 mm to the northeast and vertical deformation of 8.810 mm (uplift) at stations that are farther (580 km) from the earthquake's epicenter. However, in the observation period of 60 days (postseismic phase), stations closer to the epicenter are still experiencing a postseismic phase. In contrast, stations far from the epicenter show that the postseismic phase is almost complete. In the preseismic phase, all stations experience almost the same horizontal deformation, ranging from 2.210 mm-3.639 mm, but with a different direction of movement, which may be caused by previous intense earthquake activity, which is still releasing energy (postseismic phase). On the other hand, the vertical deformation during the preseismic phase generally experiences an uplift except at the LEWK station. The results of this study can be additional information for earthquake mitigation in the Sumatra region

Dynamics of West Coast of Sumatra and Island Arc Mentawai during the Coseismic Phase of the Mentawai Mw7.8 25 October 2010 Earthquake

Sumatra is located along the Sunda megathrust where the Indo-Australian plate subducts beneath the Eurasian plate. This condition provides a favorable setting for the formation of small to large scale earthquakes which can lead to other natural disasters. On October 25, 2010, an Mw7.8 earthquake near Mentawai occurred and followed by a tsunami. The Mentawai Islands and the West Coast of Sumatra’s dynamics were observed before, during, and after the rupture using the Global Positioning System (GPS) technology, widely known as the Sumatran GPS Array (SuGAr) network. RINEX data from several SuGAr’s stations in Mentawai (BSAT, SLBU, PRKB, SMGY, KTET, PPNJ) and West Coast of Sumatra (LNNG, MKMK, LAIS, MNNA) was processed using GAMIT/GLOBK software to obtain the coseismic deformation data. No anomalies were found in the preseismic phase (29 days before the earthquake) that could be used as precursors for the earthquake. The largest deformation was observed at the SLBU station in North Pagai, Mentawai, as a 29.97 cm southwestward coseismic jump and the coseismic jump at the stations in Sumatra (LNNG, MKMK, LAIS, MNNA) was below 3 cm toward the same direction. During the 29 days after the earthquake, plate relaxation at the postseismic phase was observed. This suggest that some period of time is required to recover to its normal deformation trend. This shifted postseismic trend is indicated by a change in the direction of plate movement compared to the preseismic trend.

Multiyear Detection, Classification and Hypothesis of Ionospheric Layer Causing GNSS Scintillation

AbstractThis paper surveys six years of Global Positioning System (GPS) L1 and L2C ionospheric scintillation in the auroral zone and, with a collocated incoherent scatter radar, hypothesizes the ionospheric irregularity layer. The Scintillation Auroral GPS Array of six scintillation receivers is sited at Poker Flat Research Range, Alaska, as is the Poker Flat incoherent scatter radar (PFISR). Scintillation intervals are identified across at least four receivers of the array using S4 and sigma phi (σϕ) indices at 100 s cadence. Classification as “amplitude,” “phase,” or “both‐phase‐and‐amplitude” scintillation is performed by analyzing common time intervals of elevated S4 and σϕ. Scattering of Global Navigation Satellite System (GNSS) waves by refractive or diffractive effects is hypothesized to occur in the E or F layer, or a transition layer in between, based on the PFISR peak density altitude at the time of the scintillation event. We analyze the statistics of the irregularity layer from 2014 to 2019, spanning solar maximum to solar minimum. We find fewer scintillation events per day with the waning solar cycle, nearly all of them phase scintillations. We also find that the percentage of events hypothesized to be caused by irregularities in the E layer increases with the declining solar cycle. The local time dependence of phase scintillations is primarily at night and in the E layer. Phase scintillation events occurring during daytime occur at solar maximum and are nearly all in the F layer. The majority of the events containing amplitude scintillations are daytime F layer at solar maximum (2014).

The Properties and Origins of Corotating Plasmaspheric Irregularities: Part II—Tomography With Compact Arrays of GPS Receivers

AbstractThis paper describes insights into the nature of corotating plasmaspheric irregularities (CPIs) enabled by a newly developed method for range‐ and time‐resolved tomographic images of these structures. This was originally developed using high‐precision measurements of total electron content (TEC) gradients toward cosmic radio sources using an interferometric radio telescope, the Very Large Array (VLA). Here, the method has been adapted to work with TEC measurements from compact arrays of Global Positioning System (GPS) receivers in California and Hawaii. Because the VLA is much more sensitive to line‐of‐sight density fluctuations and the background density within the plasmasphere drops quickly with radial distance, the GPS‐based analysis has a much more limited range (maximum of ∼5,000 km with GPS versus ∼15,000 with the VLA). However, because the GPS arrays collect data continuously, they offer much more complete temporal coverage and range resolution. This enabled a thorough comparison between CPI behavior near solar maximum and that near solar minimum, which was not possible with the VLA. The solar minimum data largely agree with previously reported VLA‐based data, which were mostly confined to times near solar minimum. These indicated that the primary drivers of CPIs during these conditions are perturbations within the background electric field associated with so‐called electro‐buoyancy waves within the midlatitude ionosphere. However, the detection rates of and properties of CPIs were often markedly different near solar maximum. This points to different CPI drivers related to solar/geomagnetic activity such as substorms and/or the interchange instability at lower L‐shells (∼3–4).

Automated Ionospheric Scattering Layer Hypothesis Generation for Detected and Classified Auroral Global Positioning System Scintillation Events

AbstractWe describe a method to detect and classify global positioning system (GPS) scintillation, and then hypothesize a possible ionospheric layer scattering the signal. The objective is to routinely identify events of interest to investigate in detail in future work. Scintillation types include amplitude, phase, or both amplitude and phase. A scintillation event is one for which a scintillation index remains above a threshold across a majority of closely spaced receivers viewing a single satellite. Events are categorized by signal frequency and scintillation type. An event is then hypothesized to be due to the E or F layer using an independent data source. Data from the scintillation auroral GPS array located in Poker Flat Research Range, Alaska, are used to analyze L1 and L2C frequencies in 2014 and 2015. The irregularity layer associated with each scintillation event is hypothesized to be due to the activity in the E layer of the ionosphere (below 150 km) or in the F layer (above 195 km) using collocated Poker Flat Incoherent Scatter Radar electron density measurements. Events in a transition layer (150–195 km) and inconclusive results are also recorded. We find that nearly all of the over 4,000 events are phase scintillations. The majority of the events are hypothesized to occur when the peak density is at E‐layer altitudes. This indicates that E‐layer‐related scintillation may be quite common at auroral latitudes and that GPS receivers are sensitive to the irregularities occurring both there and at F layer altitudes.

Design of Parabola-Shaped Planar Lossy Magnetic Surface for Improved Isolation Characteristic Between GPS Array Elements With Circular Polarization Property

This letter proposes the design of a parabola-shaped planar lossy magnetic (PLM) surface for improving the isolation characteristic of Global Positioning System (GPS) arrays. The proposed surface consists of multiple parabolic grooves that are filled with lossy magnetic material that allows the resulting surface to serve as an artificial magnetic conductor. A four-element GPS array with and without the proposed surface is fabricated and measured in a full-anechoic chamber to demonstrate the feasibility of the parabola-shaped PLM surface. The results confirm that the developed surface is suitable for improving the isolation characteristic for GPS arrays while minimizing array pattern gain reduction at increased steering angles.

A Combined Antijamming and Antispoofing Algorithm for GPS Arrays

The Global Positioning System (GPS), with its accurate positioning and timing information, has become a commonly used navigation instrument for many applications. However, it is susceptible to intentional interference such as jamming and spoofing. The conventional antijamming GPS receiver fails to work in a combined jamming and spoofing attack scenario. To solve the problem, a combined antijamming and antispoofing algorithm for a GPS receiver based on an antenna array is proposed. In this method, the jamming is eliminated by subspace projection, and then a compressed sensing framework is adopted to obtain the direction of arrival (DOA) of the despreading satellite navigation signal and detect the spoofing signal. According to the DOA of the authentic and spoofing signals, the receiver uses adaptive multibeamforming to concurrently achieve the undistorted reception of the authentic satellite signal and the suppression of the spoofing. We analyse three aspects of algorithmic performance: the antenna array direction diagram, the spoofing detection and the acquisition results. The simulation results and their analysis preliminarily show that the proposed method can detect and suppress GPS jamming and spoofing effectively.

Dual-polarized GPS antenna array algorithm to adaptively mitigate a large number of interference signals

Intentional or unintentional interference experienced by the Global Positioning System (GPS) is a significant concern for GPS-based critical infrastructures and services including aviation. An effective way to mitigate GPS interference is to utilize a GPS antenna array capable of electronically changing its gain pattern. Although a conventional GPS antenna array consists of single-polarized antenna elements, a dual-polarized antenna array has the potential to mitigate approximately twice the number of interference signals in the spatial domain as a single-polarized array because of the additional degrees of freedom provided. This paper proposes an adaptive beamforming algorithm using a dual-polarized GPS antenna array for mitigation of interference signals with various polarizations. In this paper, a dual-polarized antenna element specifically refers to two co-located crossed linearly polarized dipole antennas. The proposed minimum-variance-distortionless-response (MVDR)-based space–time adaptive processing (STAP) method utilizes a novel constraint vector that is specially designed for a dual-polarized GPS array. As the proposed constraint vector considers realistic radiation patterns of the antenna, the performance of the proposed method in terms of the signal-to-interference-plus-noise power ratio (SINR) is noticeably superior to that of previous methods under a representative interference scenario. The performance of the proposed method is compared with three previous methods utilizing dual-polarized antenna arrays.

Field‐Aligned GPS Scintillation: Multisensor Data Fusion

AbstractThe Mahali Global Positioning System (GPS) array (9 receivers, 15–30 km baseline distance) in central Alaska has probed auroral structures in a field‐aligned direction during a geomagnetic substorm on 7 October 2015. We present results from a collaborative study of GPS phase scintillation, optical emission brightness, and ionospheric density perturbations, by virtue of data fusion procedure from the Mahali GPS array, all‐sky imager (ASI), and the Poker Flat Incoherent Scatter Radar (PFISR). First, we present observations in a traditional way using colocated GPS‐ASI sensors, giving us a principal pattern of the phase scintillation with respect to auroral brightness, free of any mapping ambiguities. Next, we use an assumption that the plasma irregularities are located at an altitude of 120 km, we map the optical data to this altitude, and we extend the GPS‐ASI study over the entire field of view of the GPS receiver array. We obtain a repeatable and persuasive pattern, revealing that GPS phase scintillation is clustered at the auroral edges. Moreover, investigation of the colinear ISR observations supports the altitude assumption of scintillation producing irregularities, and PFISR‐derived electric field estimates suggest that the source for irregularities is gradient drift instability. The phase scintillation was observed on all GPS receivers, phase scintillation exceeded once cycle during several electrojet intensifications, and several events lasted for more than a minute. Finally, phase scintillation was observed during all surge events, independent of the particular auroral morphology.

First light from a kilometer-baseline Scintillation Auroral GPS Array.

We introduce and analyze the first data from an array of closely spaced Global Positioning System (GPS) scintillation receivers established in the auroral zone in late 2013 to measure spatial and temporal variations in L band signals at 100–1000 m and subsecond scales. The seven receivers of the Scintillation Auroral GPS Array (SAGA) are sited at Poker Flat Research Range, Alaska. The receivers produce 100 s scintillation indices and 100 Hz carrier phase and raw in-phase and quadrature-phase samples. SAGA is the largest existing array with baseline lengths of the ionospheric diffractive Fresnel scale at L band. With an initial array of five receivers, we identify a period of simultaneous amplitude and phase scintillation. We compare SAGA power and phase data with collocated 630.0 nm all-sky images of an auroral arc and incoherent scatter radar electron precipitation measurements, to illustrate how SAGA can be used in multi-instrument observations for subkilometer-scale studies.Key PointsA seven-receiver Scintillation Auroral GPS Array (SAGA) is now at Poker Flat, Alaska SAGA is the largest subkilometer array to enable phase/irregularities studies Simultaneous scintillation, auroral arc, and electron precipitation are observed

Comment on “Preseismic ionospheric electron enhancements revisited” by K. Heki and Y. Enomoto

Comment on “Preseismic ionospheric electron enhancements revisited” by K. Heki and Y. Enomoto

이중 대역 GPS 배열 안테나 설계

본 논문에서는 하이브리드 급전 구조를 가지는 이중 대역 GPS 패치 안테나를 설계하였다. 설계된 개별 안테나는 GPS 배열 안테나로 사용되며, 배열에서 개별 안테나의 방향을 조절하여 패턴 왜곡 및 간섭을 최소화하였다. 제안된 안테나는 GPS <TEX>$L_1$</TEX>(1.5754 GHz), <TEX>$L_2$</TEX>(1.2276 GHz) 대역에서 각각 공진하는 2개의 방사 패치를 가지고 있으며, 광대역 및 원형 편파 특성을 구현하기 위해 칩 커플러를 이용한 2-포트 급전 방법을 사용하였다. 설계된 개별 안테나를 각각의 장착 위치에서 네 방향(<TEX>${\phi}=0^{\circ}$</TEX>, <TEX>${\phi}=90^{\circ}$</TEX>, <TEX>${\phi}=180^{\circ}$</TEX>, <TEX>${\phi}=270^{\circ}$</TEX>)으로 돌려가며 중앙 안테나와 가장 적은 패턴 왜곡과 간섭을 가지는 방향으로 안테나를 장착하였다. 배열 안테나 측정 결과, 중앙 안테나의 전면 방향 이득은 <TEX>$L_1$</TEX>과 <TEX>$L_2$</TEX> 대역에서 0.3 dBic, -1.0 dBic, 측면 안테나의 전면 방향 이득은 <TEX>$L_1$</TEX>과 <TEX>$L_2$</TEX> 대역에서 1.6 dBic, 1.0 dBic를 가지며, 이를 통해 제안된 배열 안테나가 이중 대역 GPS 배열 안테나로 사용 가능함을 확인하였다. In this paper, we propose a design of dual-band patch antennas for Global Positioning System(GPS) applications, and the designed antenna is used as an individual element of GPS arrays. A low distortion and a high isolation of the array are achieved by adjusting rotating angles of each array element. The antenna consists of two radiating patches that operate in the GPS <TEX>$L_1$</TEX> and <TEX>$L_2$</TEX> bands, and the two ports feeding network with a hybrid chip coupler is adopted to achieve a broad circular polarization(CP) bandwidth. The rotating angles of each antenna element are varied with four directions(<TEX>${\phi}=0^{\circ}$</TEX>, <TEX>${\phi}=90^{\circ}$</TEX>, <TEX>${\phi}=180^{\circ}$</TEX>, <TEX>${\phi}=270^{\circ}$</TEX>) in order to minimize the pattern distortion and maximize the isolation among array elements. The measurement shows bore-sight gains of 0.3 dBic(<TEX>$L_1$</TEX>) and -1.0 dBic(<TEX>$L_2$</TEX>) for the center element. Bore-sight gains of 1.6 dBic(<TEX>$L_1$</TEX>) and 1.0 dBic(<TEX>$L_2$</TEX>) are observed for the edge element. This results demonstrate that the proposed antenna is suitable for GPS array applications.

Observation of Earth's free oscillation by dense GPS array: After the 2011 Tohoku megathrust earthquake

Day-scale Earth's free oscillation after large earthquakes has been detected by underground instruments such as strainmeters, gravimeters and seismometers, to investigate Earth's internal structure, geodynamics, and source properties of earthquakes. Here we show that Global Positioning System (GPS) can also detect the signals of the Earth's free oscillation. A dense GPS array in Japan (GEONET) recorded the surface deformation following the 2011 Tohoku megathrust earthquake. A simple array analysis over 300 stations reduces local noise in GPS time series. We find that the dense GPS array truly detected both spheroidal and toroidal fundamental modes in three-direction displacement. This new tool has a strong potential to investigate the free oscillations particularly in low-frequency bands.

Analysis of tidal signals in surface displacement measured by a dense continuous GPS array

We analyze the tidal displacement signals, for the eight major diurnal and semidiurnal tides, from Global Positioning System (GPS) measurements made at the Western United States over the past 16years with 1075 independent stations. By careful examinations among inland versus coastal GPS data we are able to demonstrate that estimated precisions down to the level of ∼0.1mm (horizontal) and ∼0.3mm (vertical) have been reached for the tidal signals, and that at such precisions coherent spatial patterns are revealed in the residual tidal amplitudes and phases after the removal of a priori modeled effects of body tides and ocean tidal loading (OTL). We demonstrate the facility of modern precise GPS data in not only constraining the ocean tide models but, more significantly, in providing source data in terms of regional, complex Love numbers for geophysical inference of the heterogeneities of elastic and inelastic structures in the solid Earth's deep interior on tidal timescales.

Detection and modeling of Rayleigh wave induced patterns in the ionosphere

[1] Global Positioning System (GPS) allows the detection of ionospheric disturbances associated with the vertical displacements of most of the major shallow seismic events. We describe a method to model the time and space distributions of Rayleigh wave induced total electron content (TEC) patterns detected by a dense GPS array. We highlight the conditions for which a part of the ionospheric pattern can be directly measured, at teleseismic distance and above the epicenter. In particular, a satellite elevation angle lower than 40° is a favorable condition to detect Rayleigh wave induced ionospheric waves. The coupling between the solid Earth and its atmosphere is modeled by computing the normal modes of the solid Earth–atmosphere system. We show the dependency of the coupling efficiency on various atmospheric conditions. By summation of the normal modes we model the atmospheric perturbation triggered by a given earthquake. This shows that a part of the observation is a Rayleigh-induced radiation pattern and therefore characteristic of the seismic rupture. Through atmosphere-ionosphere coupling, we model the ionospheric perturbation. After the description of the local geomagnetic field anisotropic effects, we show how the observation geometry is strongly affecting the radiation pattern. This study deals with the related data for two earthquakes with far-field and near-field observations using the Japanese GPS network GEONET: after the 12 May 2008 Wenchuan earthquake (China) and after the 25 September 2003 Tokachi-Oki earthquake (Japan), respectively. Waveforms and patterns are compared with the observed TEC perturbations, providing a new step toward the use of ionospheric data in seismological applications.

Direction of arrival estimation using virtual array search

This study explores the application of the advanced array interpolation technique for direction of arrival estimation. It uses an array of Global Positioning System (GPS) sonobuoys, as a method for enhancing the bearing estimation accuracy, improving target tracking capabilities. Currently, the bearing estimation of underwater targets using sonobuoys is based on fusion of bearings, obtained through individual element processing. This preserves the advantage of having a widely spread field of sonobuoys to detect underwater targets. However, the low levels of signal-to-noise ratio (SNR) jeopardise the bearing estimation accuracy. This can be improved using array processing of a directional frequency analysis and recording (DIFAR) sonobuoy field. The environmental conditions and deployment strategies impose limitations on the array processing of a DIFAR sonobuoy field. These factors mainly cause the array of sonobuoys to be a non-uniform sparse array. Advanced array interpolation is proposed to mitigate the current processing limitations degrading the system performance. This method is tested using acoustic level simulation. Simulation results show that the proposed method improves the processing of non-uniform sparse arrays based on the SNR and the inter-element distances of the array.

Ionospheric holes made by ballistic missiles from North Korea detected with a Japanese dense GPS array

A dense array of global positioning system (GPS) receivers is a useful tool to study ionospheric disturbances. Here we report observations by a Japanese GPS array of ionospheric holes, i.e., localized electron depletion. They were made by neutral molecules in exhaust plumes (e.g., water) of ballistic missiles from North Korea, Taepodong‐1 and ‐2, launched on 31 August, 1998, and 5 April, 2009, respectively. Negative anomaly of electron density emerged ∼6 min after the launches in the middle of the Japan Sea, and extended eastward along the missile tracks. By comparing the numerical simulation of electron depletion and the observed change in ionospheric total electron content, we suggest that the exhaust plumes from the Taepodong‐2 second stage effused up to ∼1.5 × 1026 water molecules per second. The ionospheric hole signature was used to constrain the Taepodong‐2 trajectory together with other information, e.g., coordinates of the launch pad, time and coordinates of the first stage splashdown, and height and time of the second stage passage over Japan. The Taepodong‐2 is considered to have reached the ionospheric F region in ∼6 min, flown above northeastern Japan ∼7 min after the launch, and crashed to the Pacific Ocean without attaining the first astronautical velocity. The ionospheric hole in the 1998 Taepodong‐1 launch was much less in size, but it is difficult to compare directly the thrusts of the two missiles due to uncertainty of the Taepodong‐1 trajectory.

Ionospheric hole behind an ascending rocket observed with a dense GPS array

Abstract An ascending liquid-fuel rocket is known to make a hole in the ionosphere, or localized electron depletion, by leaving behind large amounts of neutral molecules (e.g. water) in the exhaust plume. Such a hole was made by the January 24, 2006 launch of an H-IIA rocket from Tanegashima, Southwestern Japan, and here we report its observation with a dense array of Global Positioning System receivers as a sudden and temporary decrease of total electron content. The observed disturbances have been compared with a simple numerical model incorporating the water diffusion and chemical reactions in the ionosphere. The substantial vanishing of the ionosphere lasted more than one hour, suggesting its application as a window for ground-based radio astronomical observations at low frequencies.