Navigation Signals Research Articles

Robust autocalibrated structured low-rank EPI ghost correction.

We propose and evaluate a new structured low-rank method for echo-planar imaging (EPI) ghost correction called Robust Autocalibrated LORAKS (RAC-LORAKS). The method can be used to suppress EPI ghosts arising from the differences between different readout gradient polarities and/or the differences between different shots. It does not require conventional EPI navigator signals, and is robust to imperfect autocalibration data. Autocalibrated LORAKS is a previous structured low-rank method for EPI ghost correction that uses GRAPPA-type autocalibration data to enable high-quality ghost correction. This method works well when the autocalibration data are pristine, but performance degrades substantially when the autocalibration information is imperfect. RAC-LORAKS generalizes Autocalibrated LORAKS in two ways. First, it does not completely trust the information from autocalibration data, and instead considers the autocalibration and EPI data simultaneously when estimating low-rank matrix structure. Second, it uses complementary information from the autocalibration data to improve EPI reconstruction in a multi-contrast joint reconstruction framework. RAC-LORAKS is evaluated using simulations and in vivo data, including comparisons to state-of-the-art methods. RAC-LORAKS is demonstrated to have good ghost elimination performance compared to state-of-the-art methods in several complicated EPI acquisition scenarios (including gradient-echo brain imaging, diffusion-encoded brain imaging, and cardiac imaging). RAC-LORAKS provides effective suppression of EPI ghosts and is robust to imperfect autocalibration data.

Рекомендації щодо розробки захищеного від завад приймача глобальних навігаційних супутникових систем для вирішення завдань Збройних Сил України

Analysis of experience of local wars the armed conflicts of the last decades testifies that the mobile actions of troops play a key role in achievement of success of operations. Best in the military plan of country during creation of new and modernization of present standards of armament spare the special attention to rigging of every unit of military equipment by modern facilities of navigation.Comparison of different facilities of navigation shows substantial advantage of satellite navigation systems. At the same time the low level of signal of the indicated systems substantially simplifies their suppression facilities of radio electronic fight of opponent. Thus there is contradiction between the requirements of high reliability of apparatus of consumers of navigationals of military-oriented and its subzero antijammingness. It causes the necessity of application of methods and algorithms of defence of navigation apparatus from obstacles, search of antijamming technical decisions. Descriptions of facilities of radio electronic fight of both Russian Federation and countries of NATO are basis for the calculation of level of antijammingness of the navigation modules for the navigation providing of soldiery consumers of satellite navigation systems. The analysis of modern developments shows in industry of counteraction to facilities of radio electronic fight, that the autonomous use of the separate systems of navigation is less effective, than their complexing and integration. Among the different variants of integration of navigationals most effective integration of satellite and inertial navigation systems is considered. In the article, basic progress of modern facilities of radio electronic suppression, source of unintentional obstacles to the transceivers of signals of global satellite navigationals trends are analysed, basic correlations over, which allow on the basis of information about the parameters of navigation signals to consider the critical situations of functioning of navigation apparatus and get corresponding requirements in relation to possible correlation signal/obstacle on the entrance of takers-off, are brought, basic tasks, architecture of protected module, are reasonable.

A comprehensive assessment of four-satellite QZSS constellation: navigation signals, broadcast ephemeris, availability, SPP, interoperability with GPS, and ISB against GPS

In this study, a comprehensive assessment of four-satellite Quasi-Zenith Satellite System (QZSS) constellation is conducted, including the quality of navigation signals, the accuracy of broadcast ephemeris, the availability of satellite constellation, the performance of single point positioning (SPP), the interoperability with GPS, and the inter-system bias (ISB) against GPS. Regarding the signal quality, no significant difference between QZSS and GPS can be found. The signal-in-space ranging error (SISRE) of QZSS satellites is 0.59–0.62 m. The service rate of QZSS-only positioning is 69.8–77.8% in QZSS service areas. A positioning accuracy of 5.70, 3.20 and 6.99 m in east, north and up directions can be achieved for the QZSS-only SPP. After introducing QZSS observations into GPS-only SPP processing, the positioning accuracy can be slightly improved. The ISB with a short-term stability of 1.75 ns behaves like systematic biases, and thus cannot be ignored in the GPS/QZSS combined SPP.

BDS-3 B1 signal modulation and multiplexing techniques with receiving performance

The navigation signal structure is one of the primary technologies of the satellite navigation system and also a symbol of the BeiDou global navigation satellite system (BDS-3). In this study, the design requirement and challenges of the BDS-3 B1 band signals are first analyzed. After that, the principle and development of an innovative modulation technique, namely Quadrature Multiplexed Binary Offset Carrier (QMBOC) modulation technique, and a generalized multi-frequency multi-component navigation signal constant envelope multiplexing technique, i.e., Constant Envelope Multiplexing via Intermodulation Construction (CEMIC), are introduced in detail. Also, the advanced performance of the new-generation B1 signal is verified through the reception and analysis of the signal transmitted by the BDS-3 satellites. The results from the performance analysis of the signal show that the QMBOC modulation can satisfy the requirements needed for the performance improvement of the next-generation navigation signal, diversified reception modes, as well as the compatibility and interoperability with other navigation systems. Also, the results reveal that the CEMIC multiplexing technique can solve the B1 multiplexing issue caused by the coexistence of new and legacy signals, as well as open and authorized signals. The QMBOC modulation and CEMIC multiplexing technique form the core of the BDS B1 signal scheme and, to a great extent, determine the time-frequency domain characteristics, reception performance, and the transmission efficiency of the signals, which embodies both the innovation of these techniques and their value in engineering application. Presently, these techniques have been applied to all BDS-3 satellites and provide services to the world, which becomes a major hallmark of BDS-3 surpassing the advanced level of the world.

Retrospective four-dimensional magnetic resonance imaging of liver: Method development.

Purpose of our research was to develop a four‐dimensional (4D) magnetic resonance imaging (MRI) method of liver. Requirements of the method were to create a clinical procedure with acceptable imaging time and sufficient temporal and spatial accuracy. The method should produce useful planning image sets for stereotactic body radiation therapy delivery both during breath‐hold and in free breathing. The purpose of the method was to improve the localization of liver metastasis. The method was validated with phantom tests. Imaging parameters were optimized to create a 4D dataset compressed to one respiratory cycle of the whole liver with clinically reasonable level of image contrast and artifacts. Five healthy volunteers were imaged with T2‐weighted SSFSE research sequence. The respiratory surrogate signal was observed by the linear navigator interleaved with the anatomical liver images. The navigator was set on head‐feet — direction on the superior surface of the liver to detect the edge of diaphragm. The navigator signal and 2D liver image data were retrospectively processed with a self‐developed MATLAB algorithm. A deformable phantom for 4D imaging tests was constructed by combining deformable tissue‐equivalent material and a commercial programmable motor unit of the 4D phantom with a clinically relevant range of deformation patterns. 4D Computed Tomography images were used as reference to validate the MRI protocol. The best compromise of reasonable accuracy and imaging time was found with 2D T2‐weighted SSFSE imaging sequence using parameters: TR = 500–550 ms, images/slices = 20, slice thickness = 3 mm. Then, image processing with number of respiratory phases = 8 constructed accurate 4D images of liver. We have developed the 4D‐MRI method visualizing liver motions three‐dimensionally in one representative respiratory cycle. From phantom tests it was found that the spatial agreement to 4D‐CT is within 2 mm that is considered sufficient for clinical applications.

Ranging technology using signals of opportunity of non-cooperative communication satellites

AbstractThe positioning technique employing the ubiquitous signals of opportunity of non-cooperative satellites does not send special navigation signals, instead it passively receives satellite signals as noise, presenting advantages of concealment and difficulty for potential attackers. Thus, this study investigates the ranging principle and model using non-cooperative communication satellites and a time difference estimation algorithm. The technology of time difference measurement under non-cooperative observation mode was determined and simulated. A test platform for time difference measurement was built to receive the signal from an unknown geostationary Earth orbit communication satellite and verify the ranging feasibility and performance. The ranging accuracy was found to be smaller than 6 m, as demonstrated by experimental data, which shows the viability of the proposed positioning technique for ranging technology.

Initial assessment of single- and dual-frequency BDS-3 RTK positioning

The BeiDou navigation satellite system with global coverage (BDS-3) has been fully operational since July 2020 and provides comprehensive services to global users. BDS-3 transmits several new navigational signals based on the signals inherited from the BeiDou navigation satellite (regional) system (BDS-2). Previous studies focused on the positioning performance of BDS-2 plus BDS-3 and that of combining BDS-3 and other Global Navigation Satellite Systems (GNSSs), but there was no in-depth discussion on the positioning performance of the BDS-3-only. In this contribution, the BDS-3-only Real-Time Kinematic (RTK) positioning is analysed using the data collected in zero and short baselines in Wuhan, China. The RTK model based on Single-Differenced is first presented, and the BDS-3-only RTK positioning in cases of single and dual-frequencies is evaluated with the model in terms of the empirical integer ambiguity resolution success rates and positioning accuracy. Our numerical tests suggest two major findings. First, the positioning performance for the B1I and B3I retained from BDS-2 and the new frequency B1C is comparable, while that for the new frequency B2a is poorer. Second, the positioning performance of the new frequency combination of the B1C + B2a is not as good as that of the B1C only, owing to the unrealistic stochastic model used.

Analytical Computation of the Spatial Resolution in GNSS-R and Experimental Validation at L1 and L5

Global navigation satellite systems reflectometry (GNSS-R) is a relatively novel remote sensing technique, but it can be understood as a multi-static radar using satellite navigation signals as signals of opportunity. The scattered signals over sea ice, flooded areas, and even under dense vegetation show a detectable coherent component that can be separated from the incoherent component and processed accordingly. This work derives an analytical formulation of the response of a GNSS-R instrument to a step function in the reflectivity using well-known principles of electromagnetic theory. The evaluation of the spatial resolution then requires a numerical evaluation of the proposed equations, as the width of the transition depends on the reflectivity values of two regions. However, it is found that results are fairly constant over a wide range of reflectivities, and they only vary faster for very high or very low reflectivity gradients. The predicted step response is then satisfactorily compared to airborne experimental results at L1 (1575.42 MHz) and L5 (1176.45 MHz) bands, acquired over a water reservoir south of Melbourne, in terms of width and ringing, and several examples are provided when the transition occurs from land to a rough ocean surface, where the coherent scattering component is no longer dominant.

Causal cognitive architecture 1: Integration of connectionist elements into a navigation-based framework

The brain-inspired Causal Cognitive Architecture 1 (CCA1) tightly integrates the sensory processing capabilities found in neural networks with many of the causal abilities found in human cognition. Causality emerges not from a central controlling stored program but directly from the architecture. Sensory input vectors are processed by robust association circuitry and then propagated to a navigational temporary map. Instinctive and learned objects and procedures are applied to the same temporary map, with a resultant navigation signal obtained. Navigation can similarly be for the physical world as well as for a landscape of higher cognitive concepts. There is good explainability for causal decisions. A simulation of the CCA1 controlling a search and rescue robot is presented with the goal of finding and rescuing a lost hiker within a grid world. A simulation of the CCA1 controlling a repair robot is presented that can predict the movement of a series of gears.

The Research on EMI of High-Speed Railway to NDB

The electromagnetic interference caused by electrified high-speed railway (HSR) through the protection areas of nondirectional beacon (NDB) is an important issue, especially when the train passes through the neutral section (NS). In this article, based on the theory of multisection transmission line and considering complex factors, the calculation method of medium-frequency current and electromagnetic emission (EME) on the contact wire (CW) when the train passes through NS is given. The radiation properties of the CW were studied quantitatively and its validity was verified by practical measurement. Then, the influence of HSR on NDB navigation signal is analyzed, and the protection distance of NS is given. This method provides theoretical basis for EME of HSR and guidance for airport site selection.

Influence of the Ionosphere on the Parameters of the GPS Navigation Signals during a Geomagnetic Substorm

The influence of geomagnetic activity on slips of navigation signals of the global positioning system (GPS), as well as an increase in the amplitude of jumps in the total electron content at high latitudes, has been studied in detail. It is shown that the navigation signal of the L2 frequency is subject to more frequent malfunctions than the L1 frequencies, both in calm conditions and during geomagnetic disturbances. The probability of total electron content jumps is higher than the probability of phase slips at the L1 and L2 frequencies. The maximum disruptions and the frequency of large jumps in the total electron content are observed during the recovery phase of the geomagnetic substorm. The data from the whole-sky camera, magnetometers, and the interplanetary magnetic field used in this study make it possible to monitor the time evolution of a substorm and to study it in detail. The indicated features of the signal behavior are apparently caused by the precipitation of auroral particles, which usually occur during geomagnetic substorms in high latitude regions.

Simulation modelling of autonomous navigation support for small satellite constellation

A small satellite system has many advantages related to its technical characteristics, production cycle and costs. At the same time, separate satellites are usually operating as a part of a ballistic-coupled constellation. The navigation and ballistic support tasks for the satellite group include calculation of satellite motion, of inter-satellite bases and prediction of the ballistic existence of the constellation. Currently, autonomous navigation support of the constellation is proved by onboard satellite navigation systems, the design cycle of which necessarily includes simulation of implemented algorithms. In the paper, the simulation model of small satellite navigation support is presented. The simulation procedure for the navigation support includes the following steps: formation the reference satellite constellation, estimation the current status of GPS and GLONASS systems orbital groupings, calculation the parameters of navigational satellite radio-visibility, solving the navigation problem according to global navigation satellite systems and inter-satellite navigation. The article also describes the procedure and results of modelling the process of the onboard navigation system functioning. Further development of the model associated with the software implementation of the small satellite disturbed orbital motion and modules for generating the radio frequency component of navigation and inter-satellite signals.

Accuracy improvement for GNSS -based compass

To calculate relative position of two navigation antennas with a relatively short baseline phase measurements are commonly used. This approach is associated with technical difficulties related to the synchronization of receivers and resolution of phase ambiguities. Double phase differences of eliminate these errors and therefore allow to calculate the azimuth with better accuracy. Proposed algorithm for phase ambiguities resolution consists of three steps: applying double phase difference model insensitive to full phase due to the usage of cosine function, secondly using differential evolution method to fit model to observation by varying orientation angles and finally computing phase ambiguities from roughly estimated angles. To reduce the phase centre shift influence on azimuth computation error, the correction function depending on navigation signal arrival angles was introduced. To find its numerical values, the experiment with rigidly fixed antennas was carried out. Its idea was to take into account the relative movement of navigation satellites in fixed antenna system. Development of a multi-antenna navigation receiver with an additional channel for calculating phase ambiguities is described.

Two-dimensional ionospheric radio-tomography by nanosatellite constellation receiving signals from the GLONASS navigation system

In this paper, we consider an approach to radiotomographic monitoring of the ionosphere by a constellation of nanosatellites with receiving equipment for L-band navigation signals on its satellites. We choose the constellation orbit and the angular position of nanosatellites on the orbit to obtain an acceptable latitudinal resolution of measurements equal to 7.5 degrees. The final sounding geometry makes it possible to obtain the first measurements in less than 3.5 minutes, and the full cycle of measurements is obtained in 74 minutes, which makes it possible to reconstruct the electron density of the ionosphere in the orbital plane in the altitude range from 200 to 500 km.

Potential of Multi constellation Global Navigation Satellite System in Indian Missile Test Range Applications

In this paper, the potentials of using Global Navigation Satellite System (GNSS) techniques in the complex calibration procedure of the tracking sensors for missile test range applications have been presented. The frequently used tracking sensors in test range applications are- electro-optical tracking stations (EOTS) and tracking radars. Over the years, the EOTS are used as the reference for bias estimation of the radars. With the introduction of GPS in test range applications, especially the DGPS, the reference for bias estimation got shifted to DGPS from the EOTS. However, the achievable position solution accuracy is limited to the order of a few meters for DGPS, EOTS, and Radars. With the evolution of Multi-constellation GNSS and carrier-phase based measurement techniques in satellite navigation, achievable position solution accuracies may be improved to sub-meter level. New navigation techniques like real time kinematic (RTK) and precise point positioning have the potentials for use in the calibration procedures of the missile test ranges to the accuracies of centimeter-level. Moreover, because of the availability of a large number of navigation signals over the Indian region, multi-constellation GNSS receivers can enhance signal availability, reliability, and accuracies during the calibration of missile test ranges. Currently available compact, low-cost GNSS modules also offer the possibilities of using these for cost-effective, networked RTK for dynamic calibration of test ranges reducing cost and resource requirements.

BDS multi-frequency PPP ambiguity resolution with new B2a/B2b/B2a + b signals and legacy B1I/B3I signals

As of December 2019, the core constellation of BeiDou Navigation Satellite System with global coverage (BDS-3) has been fully deployed with 24 MEO, 3 IGSO and 1 GEO satellites. In addition to the legacy B1I and B3I signals, BDS-3 satellites are capable of transmitting several new navigation signals, including B1C, B2a, B2b and B2a + b. The multi-frequency signals of BDS-3 bring new opportunities for fast ambiguity resolution (AR) of BDS precise point positioning (PPP). In this contribution, the BDS five-frequency PPP AR method was developed and the benefits of BDS-3 multi-frequency signals for precise positioning were investigated. The multi-frequency uncalibrated phase delay (UPD) products were estimated firstly using observations from 222 Multi-GNSS Experiment stations. The extra-wide-lane, wide-lane and narrow-lane UPDs of BDS-3 exhibit high stability with standard deviations less than 0.1 cycle. With the derived UPD products, five-frequency PPP AR was conducted at 12 static stations and a moving vehicle. The time to first fix of BDS PPP AR was shortened to 25.5 min with multi-frequency observations, showing improvement of 25.4% when compared to dual-frequency PPP AR. Moreover, instantaneous PPP wide-lane ambiguity resolution (WAR) can be achieved with success rate of over 90%. A fast decimeter-level accuracy can be achieved for BDS five-frequency PPP WAR with positioning accuracy improved by 68%-82% compared to PPP float solutions. The vehicle-borne experiment further demonstrates that BDS PPP WAR is potential to maintain decimeter-level positioning accuracy even in a complex environment.

Hardware and Software Implementation for Global Navigation Satellite System Signal Processing by Temporary Accumulation Method to Monitor Radio-navigation Signals

Currently, a scope of satellite radio-navigation applications is more and more extending. Accordingly, there are also increasing requirements for the quality of determining coordinates and time. Therefore, radio-navigation signals monitoring, in particular their waveform control, waveform distortion analysis, is a challenge. However, a power of navigation signals is lower than the noise level, therefore, the special methods and equipment, which provide increasing a signal-to-noise ratio, are necessary to analyze their waveform. Usually, complexes based on the large aperture antennas are used for this purpose. But they have a number of disadvantages, namely high complexity, high price, large size, necessity to guide and track each individual satellite.The paper gives a brief description of the temporary accumulation method (TAM). This method can be applied to signals containing repetitive elements (basic elements). Radio-navigation signals are such signals. TAM allows you to increase the signal-to-noise ratio and evaluate the waveform of signal basic elements by superimposing and averaging the appropriate signal samples. Gain increases with increasing accumulation time.The paper proposes a small-scale breadboard of the software-hardware complex to monitor signals of global navigation satellite systems (GNSS) based on an omnidirectional antenna, a software-defined radio system, and a personal computer. This breadboard allows recording of digital samples of the navigation signals observed, and then their processing by the temporary accumulation method.Presents operation estimates of the proposed breadboard for monitoring GNSS signals. Gives estimates of the GLONASS navigation signal waveforms in the time domain (signal basic elements) and in the frequency domain (energy spectrum). Also based on the use of TAM, estimates the signal power and energy gain. Proves that using the hardware-software complex proposed for radio-navigation signals monitoring is possible.The proposed scheme of the small-scale breadboard for monitoring GNSS signals can be used directly for monitoring and also to test various methods and technologies of navigation signal processing based on the use of information about the waveform of the received signal.

Experiences with the GPS in Unstabilized CubeSat

The paper summarizes the experiences with the operation of the piNAV GPS receiver in a 1U unstabilized CubeSat operated on LEO orbit. piNAV L1 is a GPS receiver developed by an author for small satellite missions. The receiver is equipped with the 15 GPS L1 C/A channels and acquisition accelerator that shortens the cold start of the receiver on LEO to 65 s. The typical power consumption is 120 mW. Lucky-7 is a private 1U technological CubeSat with power budget 1 W that operates on the quasisynchronous polar orbit at altitude 520 km. One of its scientific missions is to test the operation of GPS. The space experiments proved the successful operation of the GPS receiver. The position information was available for approximately 80% of the time, where the position outage was caused by a satellite rotation and relatively long navigation signal reacquisition. The experimental data proved that the position availability can be improved by a higher-performance signal acquisition engine.

Reference trajectory-based coverage analysis method in three-dimensional space for multi-radio integrated navigation systems

ABSTRACT GPS can be integrated with other radio navigation systems when GPS signals are not available due to navigation warfare. Before deploying navigation signal sources, the coverage analysis can be performed in order to check a required navigation performance is satisfied in 2-dimensional space. Usually, the coverage analysis is performed for the area in which a vehicle is operated. When an air vehicle is operated and the method in 2-dimensional is directly extended, the computational load can be excessively heavy. In this paper, a reference trajectory-based coverage analysis method for multi-radio integrated navigation systems is proposed using CRLB in order to alleviate computational burden in 3-dimensional space. The performance of the proposed method is evaluated for 2 trajectories and 6 arrangements of navigation signal sources. The results show that the proposed method can be used in the coverage analysis in 3-dimensional space.

GNSS spoofing detection using a maximum likelihood-based sliding window method.

The Global Navigation Satellite System is vulnerable to interference signals that can potentially disable the system, because the signal strength tends to be very weak. Interference such as jamming, which disables the receiver via excessively high signal strength in the satellite navigation frequency band, and spoofing, which induces the receiver to output erroneous position and time data via signals similar to actual navigation signals, disrupt satellite navigation systems. As the threat of interference is increasing, considerable research effort has been expended in an attempt to deal with it in various ways. Spoofing attacks are especially difficult to detect. This paper deals with a technique to detect a spoofing signal and to mitigate attacks on satellite navigation systems. The satellite navigation signal is influenced by the navigation satellite itself and errors due to environmental factors, and spoofing signal detection should be well reflected in the navigation signal. Especially, in the case of mobile receivers, it is not easy to detect a spoofing signal because the exact position of the receiver cannot be known. To detect a spoofing signal, additional hardware may be required; in some cases, heterogeneous sensors, such as inertial sensors, may be used. The technique introduced in this paper effectively discriminates spoofing signals based only on receiver measurements, without the need for additional devices. It generates test statistics based on the pseudorange, which is the measured value of the receiver position, and detects spoofing signals by setting the monitoring interval according to a “sliding window”. Because the proposed method uses output data and measurements obtained from the receiver, it can be applied to general receivers at low cost.