Ionospheric Path Research Articles

Toward Operational Compensation of Ionospheric Effects in SAR Interferograms: The Split-Spectrum Method

The differential ionospheric path delay is a major error source in L-band interferograms. It is superimposed to topography and ground deformation signals, hindering the measurement of geophysical processes. In this paper, we proceed toward the realization of an operational processor to compensate the ionospheric effects in interferograms. The processor should be robust and accurate to meet the scientific requirements for the measurement of geophysical processes, and it should be applicable on a global scale. An implementation of the split-spectrum method, which will be one element of the processor, is presented in detail, and its performance is analyzed. The method is based on the dispersive nature of the ionosphere and separates the ionospheric component of the interferometric phase from the nondispersive component related to topography, ground motion, and tropospheric path delay. We tested the method using various Advanced Land Observing Satellite Phased-Array type L-band synthetic aperture radar interferometric pairs with different characteristics: high to low coherence, moving and nonmoving terrains, with and without topography, and different ionosphere states. Ionospheric errors of almost 1 m have been corrected to a centimeter or a millimeter level. The results show how the method is able to systematically compensate the ionospheric phase in interferograms, with the expected accuracy, and can therefore be a valid element of the operational processor.

CLASSIFICATION OF FACTORS DETERMINING THE SW RADIOWAVES PROPAGATING VIA IONOSPHERIC PATHS

The paper deals with classification of factors determining propagation SW band radio waves via ionospheric paths. Regular characteristics of the ionospheric communication channel have been identified in SW band. The necessity was demonstrated for recording passive noise interferences associated with the exposure of spatially inhomogeneous, magnetically active, dispersing, absorbing, changing in time ionosphere on structure of the emitted useful signal.

Gaussian mixture probability hypothesis density filter for multipath multitarget tracking in over-the-horizon radar

Conventional multitarget tracking systems presume that each target can produce at most one measurement per scan. Due to the multiple ionospheric propagation paths in over-the-horizon radar (OTHR), this assumption is not valid. To solve this problem, this paper proposes a novel tracking algorithm based on the theory of finite set statistics (FISST) called the multipath probability hypothesis density (MP-PHD) filter in cluttered environments. First, the FISST is used to derive the update equation, and then Gaussian mixture (GM) is introduced to derive the closed-form solution of the MP-PHD filter. Moreover, the extended Kalman filter (EKF) is presented to deal with the nonlinear problem of the measurement model in OTHR. Eventually, the simulation results are provided to demonstrate the effectiveness of the proposed filter.

First preliminary results for the absolute calibration of the Chinese HY-2 altimetric mission using the CRS1 calibration facilities in West Crete, Greece

In this work, absolute calibration of the Chinese HY-2 satellite altimetry mission is carried out, employing Pass No. 280 and the calibration facility, CRS1, located in the Southwest end of the island of Crete, Greece. Satellite Pass No. 280 is descending and follows a ground track almost parallel to the west coast of Crete. It comes close to the coast, at a distance of about 9km from the CRS1 calibration site, and finally goes away south of Crete. The HY-2 sensor geophysical data records (S-GDR) have been incorporated into the calibration procedures and processing has taken place for cycles No. 54–62, at 20Hz data rate. Some peculiarities in the HY-2 satellite altimeter data, as delivered and depicted in the I-GDR and S-GDR data, have also been noticed. All calibration results have been determined using a regional, precise and detailed geoid, along with a good knowledge of local ocean circulation and site characteristics and a well-defined sea-surface calibration methodology.The first preliminary results for the HY-2 altimeter calibration have shown that the initial cycles, up to No. 51, display an erratic behavior. After those cycles, the altimeter range bias values seem to be stable and reach a value of B=−45.6cm±4.4cm, when applying the net instrument corrections as provided in the GDR. If the relativistic effects of the satellite clocks are properly applied for the net instrument corrections, then the altimeter range bias goes down to B=−27cm±3cm. Also, preliminary cross-over analysis with the SARAL/AliKa and Jason-2 satellites show a bias of B=−23cm, and B=−28.5cm, respectively. The performance of the HY-2 on-board radiometer has also been examined in terms of the wet troposphere corrections and shows a mean difference of −1cm±0.1cm with respect to in-situ GNSS-derived corrections. Finally, the ionosphere path corrections of the HY-2 satellite show a difference of +1cm±1.1cm, when compared against the GNSS-derived ionosphere values.

ESTIMATION OF ATMOSPHERIC EFFECTS OF RADARSAT-2 D-InSAR PRODUCT USING GROUNDBASEDAND SPACEBORNE METEROLOGICAL DATA

Abstract. Differential Interferometric synthetic aperture RADAR (D-InSAR) has emerged as one of the influential tools for the measurement of Earth’s topography deformation. The temporal variability of atmosphere plays a contaminative role in degrading the accuracy of Interferometric measurements as it causes the path delay in the propagating RADAR signal. The Ionosphere and Troposphere are the two atmospheric layers which are responsible for the path delay error in the Interferometric measurement. In this study ground based meteorological data integrated with spaceborne meteorological data was used to mitigate the path delay induced by the atmosphere on the Radarsat-2 data. In this study Jharia Coal Fields, Jharkhand, was selected where active mining has taken place since 1894. The D-InSAR was generated from Radarsat-2 data. In Troposephric path delay calculation ground based data like water vapor, temperature and humidity acquired during the time of the pass as well as spaceborne meteorological data were used as input. The TEC from IRI-2007 model was used as input to calculate the ionospheric path delay. These Ionospheric as well as Tropospehric delay was subtracted from a Differential Interferrogram to obtain atmospheric artifacts free differential interferrogram. The corrected Diffrential Interferrogram was used to delineate subsidence areas. We observed that D-InSAR and levelling results shows good agreement with each other. The Diffrential Interferomteric pairs with low coherence causes phase unwrapping problem. Therefore, only those pairs with good coherence were considered for the study. The used technique provides an opportunity to understand the contribution of Ionospehric and Tropospehric path delay effects on the D-InSAR results.

Correction’s method of the electron density model in ionosphere by ray tracing techniques

When applying the ray tracing in ionospheric propagation, the electron density modelling is the main input of the algorithm, since phase refractive index strongly depends on it. Also the magnetic field and frequency collision modelling have their importance, the former as responsible for the azimuth angle deviation of the vertical plane containing the radio wave, the latter for the evaluation of the absorption of the wave. Anyway, the electron density distribution is strongly dominant when one wants to evaluate the group delay time characterizing the ionospheric propagation. From the group delay time, azimuth and elevation angles it is possible to determine the point of arrival of the radio wave when it reaches the Earth surface. Moreover, the procedure to establish the target (T) position is one of the essential steps in the Over The Horizon Radar (OTHR) techniques which require the correct knowledge of the electron density distribution. The group delay time generally gives rough information of the ground range, which depends on the exact path of the radio wave in the ionosphere. This paper focuses on the lead role that is played by the variation of the electron density grid into the ray tracing algorithm, which is correlated to the change of the electron content along the ionospheric ray path, for obtaining a ray tracing as much reliable as possible. In many cases of practical interest, the group delay time depends on the geometric length and the electron content of the ray path. The issue is faced theoretically, and a simple analytical relation, between the variation of the electron content along the path and the difference in time between the group delays, calculated and measured, both in the ionosphere and in the vacuum, is obtained and discussed. An example of how an oblique radio link can be improved by varying the electron density grid is also shown and discussed.

OTHR multipath tracking using the bernoulli filter

A multipath target tracking and recognition algorithm is proposed for an over-the-horizon radar (OTHR) tracking system in the presence of detection uncertainty and clutter. This algorithm, multipath Bernoulli filter (MPBF), uses a generalization of the Bernoulli filter for joint detection and tracking of a single target. Due to multiple ionospheric propagation paths between the radar sites and the target, finite set statistics (FISST) is used to derive the likelihood function of MPBF. While the measurement models in OTHR are nonlinear, a closed-form solution in the form of Gaussian mixture is implemented by extended Kalman filter (EKF). Since multipath tracks occur in the multipath probabilistic data association (MPDA) algorithm and the original MPBF, a method is added in MPBF to identify which are the real or multipath tracks. Simulation results are presented to demonstrate that the proposed algorithm is capable of recognizing multipath tracks and has better performance over the MPDA.

Experimental Demonstration for Ionospheric Sensing and Aircraft Detection With a HF Skywave Multistatic Radar

In this letter, a new high-frequency (HF) skywave multistatic radar that is used for ionospheric sensing and aircraft detection is presented. With multiple receiving sites, this radar can detect the aircraft in much larger surveillance with high probability and can obtain the characteristics of many ionospheric paths simultaneously. In addition, this radar adopts a binary-phase-coded continuous waveform to achieve a high Doppler resolution, a large signal processing gain, and a low probability of interception. Details of the system configuration, the ambiguity function of the sounding waveform, and the signal processing algorithm are described. The experimental results show that backscatter ionograms and oblique ionograms can be simultaneously obtained and that the aircraft with different flying conditions can be clearly detected with an HF skywave multistatic experimental radar.

Joint Correction of Ionosphere Noise and Orbital Error in L-Band SAR Interferometry of Interseismic Deformation in Southern California

The accuracy of L-band synthetic aperture radar (SAR) differential interferometry (InSAR) on crustal deformation studies is largely compromised by ionosphere path delays on the radar signals. The ionosphere effects cause severe ionospheric distortion such as azimuth streaking and long wavelength phase distortion similar to orbital ramp error. Effective detection and correction of ionospheric phase distortion from L-band InSAR images are necessary to measure and accurately interpret surface displacement. In this paper, we investigate the performance improvement of L-band InSAR interseismic deformation measurements in southern California through the joint correction of both ionosphere noise and orbital error. Our results show that this method can effectively remove orbit and ionosphere phase distortions. In comparison with in situ GPS measurements, the achieved InSAR measurement accuracy is improved from ~ 30 mm to ~ 10 mm by the proposed joint correction method. We show that, after the joint correction, the remaining atmosphere noise can be further mitigated through stacking, leading to an RMS error of ~ 4.7 mm/year in resultant line-of-sight velocity, as compared with ~ 11.3 mm/year before the correction. Our results demonstrate that the proposed joint correction technique provides a promising way to jointly correct orbital and ionospheric artifacts in L-band InSAR studies of crustal deformation.

Chirp Ionosonde–Radiodirection Finder as a New Tool for Studying the Ionosphere and Radio-Wave Propagation

Hardware of a chirp ionosonde–radiodirection finder, which, along with the conventional parameters, measures angular frequency characteristics on oblique ionospheric sounding paths, is briefly described. The results of the experimental studies of propagation of radio waves on paths of different length and orientation in the natural and artificially disturbed ionosphere are presented. Capabilities of the ionosonde–radiodirection finder for over-the-horizon HF radio detection and ranging of the Earth’s ionosphere on a global scale are shown.

Ionospheric Correction of SAR Interferograms by Multiple-Aperture Interferometry

Interferometric synthetic aperture radar (InSAR) is a powerful technique that precisely measures surface deformations at a fine spatial resolution over a large area. However, the accuracy of this technique is sometimes compromised by ionospheric path delays on radar signals, particularly with L- and P-band SAR systems. To avoid ionospheric effects from being misinterpreted as ground displacement, it is necessary to detect and correct their contributions to interferograms. In this paper, we propose an efficient method for ionospheric measurement and correction and validate its theoretical and experimental performance. The proposed method exploits the linear relationship between the multiple-aperture interferometry phase and the azimuth derivative of the ionospheric phase. Theoretical analysis shows that a total electron content (TEC) accuracy of less than <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$1.0 \times 10^{-4}$</tex></formula> TEC units can be achieved when more than 100 neighboring samples can be averaged (multilooked), and the coherence is 0.5. The regression analysis between the interferometric phase and the topographic height shows that the root-mean-square error can be improved by a factor of two after ionospheric correction. A 2-D Fourier spectral analysis indicates that the ionospheric wave pattern in the uncorrected power spectrum has disappeared in the power spectrum of the corrected interferogram. These results demonstrate that the proposed method can effectively remove ionospheric artifacts from an ionosphere-distorted InSAR image. Note that the method assumes that there is no appreciable surface displacement in the along-track dimension of the interferogram.

Inter-agency comparison of TanDEM-X baseline solutions

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is the first Synthetic Aperture Radar (SAR) mission using close formation flying for bistatic SAR interferometry. The primary goal of the mission is to generate a global digital elevation model (DEM) with 2m height precision and 10m ground resolution from the configurable SAR interferometer with space baselines of a few hundred meters. As a key mission requirement for the interferometric SAR processing, the relative position, or baseline vector, of the two satellites must be determined with an accuracy of 1mm (1D RMS) from GPS measurements collected by the onboard receivers. The operational baseline products for the TanDEM-X mission are routinely generated by the German Research Center for Geosciences (GFZ) and the German Space Operations Center (DLR/GSOC) using different software packages (EPOS/BSW, GHOST) and analysis strategies. For a further independent performance assessment, TanDEM-X baseline solutions are generated at the Astronomical Institute of the University of Bern (AIUB) on a best effort basis using the Bernese Software (BSW).Dual-frequency baseline solutions are compared for a 1-month test period in January 2011. Differences of reduced-dynamic baseline solutions exhibit a representative standard deviation (STD) of 1mm outside maneuver periods, while biases are below 1mm in all directions. The achieved baseline determination performance is close to the mission specification, but independent SAR calibration data takes acquired over areas with a well known DEM from previous missions will be required to fully meet the 1mm 1D RMS target. Besides the operational solutions, single-frequency baseline solutions are tested. They benefit from a more robust ambiguity fixing and show a slightly better agreement of below 1mm STD, but are potentially affected by errors caused by an incomplete compensation of differential ionospheric path delays.

Precision spacecraft navigation using a low-cost GPS receiver

Within the PROBA-2 microsatellite mission, a miniaturized single-frequency GPS receiver based on commercial-off-the-shelf (COTS) technology is employed for onboard navigation and timing. A rapid electronic fuse protects against destructive single-event latch-ups (SEL) and enables a quasi-continuous receiver operation despite the inherent sensitivity to space radiation. While limited to single-frequency C/A-code tracking with a narrow-band frontend, the receiver is able to provide precision navigation services through processing of raw GPS measurements on ground as well as a built-in real-time navigation system. In both cases, ionospheric path delays are eliminated through a combination of L1 pseudorange and carrier phase measurements, which also offers a factor-of-two noise reduction relative to code-only processing. By comparison with satellite laser ranging (SLR) measurements, a 0.3-m (3D rms) accuracy is demonstrated for the PROBA-2 reduced dynamic orbit determinations using post-processed GPS orbit and clock products. Furthermore, the experimental onboard navigation system is shown to provide real-time position information with a 3D rms accuracy of about 1 m, which notably outperforms the specification of the Standard Positioning Service (SPS). In view of their lower hardware complexity, mass budget and power requirements as well as the reduced interference susceptibility, legacy C/A-code receivers can thus provide an attractive alternative to dual-frequency receivers even for demanding navigation applications in low Earth orbit.

Reflection of the dynamics of interplanetary space parameters in the maximum observable frequencies on different ionospheric oblique-sounding paths

We study the correlation between variations in the solar–geophysical parameters and variations in the maximum observable frequencies (MOF) on several ionospheric short-wave oblique-sounding paths. The basic interplanetary space parameters, whose variations have the greatest impact on the MOF variations on specified paths, are determined. Variations in the Bx and Bz components of the interplanetary magnetic field (IMF) affect the MOFs on the Norilsk—Rostov-on-Don subauroral path. At mid-latitudes, the influence of the IMF components Bx, By, and Bz is significant on the Inskip—Rostov-on-Don and Irkutsk—Rostov-on-Don paths. On the Cyprus—Rostov-on-Don low-latitude path, the modulus of the IMF has the most pronounced effect. The density and velocity of solar wind manifest themselves in MOF variations on all paths. The technique of MOF retrieval by a neural network on the Cyprus—Rostov-on-Don and Irkutsk—Rostov-on-Don paths using the MOF data of the Inskip—Rostov-on-Don path is developed with allowance for the influence of the interplanetary space parameters. The retrieval efficiency of this technique is no less than 80%.

Ionospheric bending correction for GNSS radio occultation signals

Ionospheric propagation effects on Global Navigation Satellite Systems (GNSS) signals are the most pronounced during radio occultation due to long ionospheric travel paths of the received signal on low Earth orbiting satellites. Inhomogeneous plasma distribution and anisotropy cause higher‐order nonlinear refraction effects on GNSS signals which cannot be fully removed through a linear combination of dual‐frequency observables. In this paper, higher‐order ionospheric effects due to straight line of sight (LOS) propagation assumption such as the excess path length of the signal in addition to the LOS path and the total electron content difference between the curved path and the LOS path have been investigated for selected GPS‐CHAMP occultation events. Based on simulation studies we have derived correction formulas for computing raypath bending effects as functions of signal frequency, tangential height of the raypath, ionospheric parameters such as the maximum ionization and total electron content. If these parameters are known, the proposed correction method is able to correct on an average about 65–80% bending errors of GNSS occultation signals.

Ionospheric path delay models for spaceborne GPS receivers flying in formation with large baselines

GPS relative navigation filters could benefit notably from an accurate modeling of the ionospheric delays, especially over large baselines (>100 km) where double difference delays can be higher than several carrier wavelengths. This paper analyzes the capability of ionospheric path delay models proposed for spaceborne GPS receivers in predicting both zero-difference and double difference ionospheric delays. We specifically refer to relatively simple ionospheric models, which are suitable for real-time filtering schemes. Specifically, two ionospheric delay models are evaluated, one assuming an isotropic electron density and the other considering the effect on the electron density of the Sun aspect angle. The prediction capability of these models is investigated by comparing predicted ionospheric delays with measured ones on real flight data from the Gravity Recovery and Climate Experiment mission, in which two satellites fly separated of more than 200 km. Results demonstrate that both models exhibit a correlation in the excess of 80% between predicted and measured double-difference ionospheric delays. Despite its higher simplicity, the isotropic model performs better than the model including the Sun effect, being able to predict double differenced delays with accuracy smaller than the carrier wavelength in most cases. The model is thus fit for supporting integer ambiguity fixing in real-time filters for relative navigation over large baselines. Concerning zero-difference ionospheric delays, results demonstrate that delays predicted by the isotropic model are highly correlated (around 90%) with those estimated using GPS measurements. However, the difference between predicted and measured delays has a root mean square error in the excess of 30 cm. Thus, the zero-difference ionospheric delays model is not likely to be an alternative to methods exploiting carrier-phase observables for cancelling out the ionosphere contribution in single-frequency absolute navigation filters.

First Geodetic Observations Using New VLBI Stations ASKAP-29 and WARK12M

AbstractWe report the results of a successful 7-hour 1.4 GHz Very Long Baseline Interferometry (VLBI) experiment using two new stations, ASKAP-29 located in Western Australia and WARK12M located on the North Island of New Zealand. This was the first geodetic VLBI observing session with the participation of these new stations. We have determined the positions of ASKAP-29 and WARK12M. Random errors on position estimates are 150–200 mm for the vertical component and 40–50 mm for the horizontal component. Systematic errors caused by the unmodeled ionosphere path delay may reach 1.3 m for the vertical component.

Midlatitude ionospheric HF channel reciprocity: Evidence from the ionospheric oblique incidence sounding experiments

[1] Ionospheric HF channel reciprocity is investigated at middle latitudes on the basis of ionospheric oblique incidence sounding experiments. Two identical Wuhan Ionospheric Oblique Incidence Sounding Systems (WIOISS), located at Wuhan (30°32′N, 114°21′E) and Wanning (18°58′N, 110°31′E), are used to carry out the campaign. Comparisons of group distance and Doppler shift between Wuhan-Wanning and Wanning-Wuhan HF ionospheric propagation paths indicate that the reciprocity of the ionospheric HF channel is satisfied at midlatitude region. A 3-D ray tracing simulation is also implemented to evaluate the group distances of the two paths. Midlatitude ionospheric HF channel reciprocity, as verified both experimentally and theoretically in the present study, can be useful for HF communication systems and sky wave over-the-horizon radars.

A Barker signal transmitted over an ionospheric path

A rectangular radio pulse formed on the basis of the Barker code and transmitted over a long-distance ionospheric path is considered. The character of signal distortions and the rate of their increase are analyzed. The determining effect of the initial duration and carrier frequency of the signal on its current characteristics is revealed. For the decameter wavelength band, it is shown, that, when the parameters of the problem vary within a wide range, the autocorrelation function of a Barker signal is deformed, its center maximum being preserved and substantially spread.

Higher order ionospheric propagation effects on GPS radio occultation signals

With the increasing number of remote sensing satellites using the GPS radio occultation technique for atmospheric sounding, the estimation of higher order ionospheric effects and their mitigation have become relevant and important. Due to long ionospheric limb paths, GPS signals are strongly affected by ionospheric refraction during radio occultation. Standard dual-frequency GPS measurements may be used to estimate the first order term of the refractive index. However, non-linear terms such as the second and third order ionospheric terms and ray path bending effects are not considered in occultation measurements so far. Analysing selected CHAMP–GPS occultation events different higher order ionospheric terms are estimated and their effects on dual-frequency range estimation and total electron content (TEC) estimation are discussed. We have found that the separation between the GPS L1 and L2 ray paths exceeds the kilometer level during occultation for a vertical TEC level of more than 160 TEC units. Corresponding errors in the GPS dual-frequency range estimation and TEC estimation are found to exceed the meter and 10 TEC units level, respectively.