Signal Emission Time Research Articles

Active and passive sensors for diagnostics quasi-zenith ionospheric HF communication channels

Background. There is a growing need for active sensory diagnostics of partial HF channels to provide frequency support to quasi-zenith HF radio links in varying signal propagation conditions. Enhancing the efficiency of active sensor algorithms, particularly by reducing emission time, is topical. To address this, a transition from sequential to parallel (simultaneous) diagnostics is proposed. Another significant challenge in HF communication is narrowband interference, and overcoming this issue involves the method of passive sensory diagnostics. This method assesses the availability of partial channels by analyzing the spectral density of interference power within them.
 Aim. The goal of this study is to develop algorithms and software tools that implement spectral monitoring and parallel sensing of partial channels for sensory diagnostics of ionospheric channels in quasi-zenith HF communication.
 Methods. The proposed approach involves integrating dynamic diagnostic methods into the development of intelligent sensors for ionospheric HF radio links, along with the creation of data analysis methods. Specialized computer software is employed to address the defined tasks. Experimental studies are conducted using the developed devices, which include intelligent active and passive radio sensors for HF radio links, to assess the load on HF communication channels.
 Results. A sensor for orthogonal quasi-zenith ionospheric radio channels has been created, incorporating algorithms for synthesizing a group pulse with orthogonal subcarriers while minimizing the peak factor. Additionally, algorithms for separating subcarriers and calculating the correlation function at the reception have been developed. The sensor employs the OFDM-BPSK signal modulation method, enabling operation in simultaneous-sequential sounding mode across the potential frequency range for communication. This led to an 8-fold reduction in the total signal emission time.
 Conclusion. The scientific results obtained have broad practical applications, particularly in enhancing the efficiency of wideband HF communication systems using spread spectrum signals.

TDOA-Based Target Tracking Filter While Reducing NLOS Errors in Cluttered Environments.

We consider tracking a moving target in a wireless communication system that is based on the radio signal. Considering a bounded workspace with many unknown obstacles, we handle tracking a non-cooperative transmitter using multiple signal receivers. Here, a non-cooperative transmitter is a transmitter whose signal emission time is not known in advance. We consider a time difference of arrival (TDOA) location problem, which locates the transmitter by processing the signal measurement time at multiple receivers. In tracking a non-cooperative transmitter, non-line-of-sight (NLOS) errors occur if obstacles block the LOS line connecting the receiver and the moving transmitter. Our article addresses how to track a moving transmitter while decreasing the NLOS error in TDOA-only measurements. We propose an algorithm to localize a transmitter while decreasing the NLOS error in TDOA measurements. For tracking a moving transmitter in real time, we integrate the proposed localization algorithm and the interacting multiple model Kalman filter (IMM KF). As far as we know, our article is novel in tracking a moving transmitter based on TDOA-only measurements in an unknown mixed LOS/NLOS workspace. We show that the proposed filter considerably decreases the NLOS errors in TDOA-only measurements while running fast. Therefore, the proposed tracking scheme is suitable for tracking a moving transmitter in real time. Through MATLAB simulations, we show that the proposed filter outperforms other state-of-the-art TDOA filters, considering both time efficiency and tracking accuracy.

Analysis of Baseline Impact on Differential Doppler Positioning and Performance Improvement Method for LEO Opportunistic Navigation

The positioning system based on the signals of opportunity (SOPs) of low-Earth-orbit (LEO) satellites can meet the positioning requirements of the receiver in the global navigation satellite system (GNSS) denial environment. However, the received signal does not contain or cannot extract the orbit information and signal emission time of the radiation source, which has become the main error source in this positioning system. Taking the Iridium satellites as an example, a Doppler differential (DD)-LEO (DD-LEO) positioning framework is proposed in this article, and the baseline impact analysis of the differential Doppler positioning system (DDPS) and a performance improvement method are carried out. First, the workflow between a base and a rover is introduced, and the error elimination of the DDPS is analyzed. Second, the equivalent Doppler error models formed by the orbit error at the base and the rover are established, respectively, and the relationship between the baseline and the mean or variance of equivalent Doppler residual in the DDPS is derived. Then, aiming at the problem of poor positioning accuracy due to the inconsistency of space–time reference in the long-baseline DDPS, a signal emission time estimation (SETE) algorithm based on maximum likelihood estimation (MLE) is proposed to further improve the positioning accuracy. Finally, a simulation experiment of the influence of baseline length and direction on the DDPS is implemented, and several groups of field differential experiments with different baseline lengths are completed. The experimental results of long-baseline positioning show that the DDPS can reduce the 3-D positioning error by 48.0% compared with the single station without differential positioning. At the same time, the positioning accuracy of the rover station is further enhanced by 86.3% when the base station adopts the SETE algorithm, which verifies the effectiveness of the proposed algorithm.

BDS-3 RDSS two-way time transfer with asymmetric delay compensation model

In the BeiDou navigation satellite system-3 (BDS-3) radio determination satellite service (RDSS), there exists a natural two-way link between the center station and the user site that can be used to achieve time transfer. Traditionally, RDSS two-way time transfer is realized by analogy with the two-way satellite time and frequency transfer (TWSTFT) technique. However, unlike TWSTFT, the signal propagation delays are not completely symmetrical in the RDSS two-way link because of the differences in signal emission time. In this study, a model of two-way time transfer with BDS-3 RDSS is presented. An asymmetric delay compensation method is introduced to solve the asymmetry of RDSS signals and improve the accuracy of two-way time transfer. In addition, BDS-3 RDSS common clock difference experiments are designed to test the performance of two-way time transfer with an asymmetric delay compensation model. Based on analyses of the experimental results, three conclusions can be drawn. First, the accuracy of RDSS two-way time transfer is improved when the asymmetric delay compensation model is applied. Second, BDS-3 RDSS two-way time transfers in different in-station beams and subbands do not show many discrepancies on the whole, and the standard deviation values of the BDS-3 RDSS time transfer results range from 1.7 ns to 2.6 ns for the test signals. Third, the two-way time transfer results are not generally affected by receiver coordinate errors.

Underwater passive manoeuvring target tracking with isogradient sound speed profile

Compared with the traditional tracking problem, the underwater 3-D passive manoeuvring target tracking faces the following challenging problems: (1) Since the signal propagation delay is varying and unknown, the relationship between the two adjacent target states is hard to describe; (2) limited by the underwater environment, it is more difficult to establish a target manoeuvring motion model; (3) the propagation speed of the underwater acoustic signal varies with the depth, and the propagation path will be curved instead of straight. Various parameters including signal propagation delay, target pitch angle and receiving frequency measured by the observer cannot be calculated as usual. In this paper, we improved a 3-D constant turn (3D-CT) model to match the underwater target motion. For the problems (1) and (3), this paper proposes a novel interacting multiple model underwater Gauss-Helmert unscented Kalman filter (IMM-UGHUKF) with isogradient sound velocity profile, which analytically describes the relationship between the state to be estimated and the passive measurement. This method augments the unknown signal emission time into the state with implicit equations, and transits the state by the Gauss–Helmert model. Simulation tests have been carried out, and the results show that IMM-UGHUKF can significantly reduce the estimation error compared with other approaches.

Periodic acoustic source tracking using propagation delayed measurements

There exist a large class of acoustic sources which have an underlying periodic phenomenon. Unlike the well-studied Bearings-Only Tracking (BOT) of an aperiodic acoustic source, this paper considers the problem of tracking a periodic acoustic source. For periodic acoustic tracking, the signal emission time is known. However, the true measurement reception time is unknown because it is corrupted by noise due to propagation delay. We augment the sensor’s signal reception time onto bearing measurements, and the information of the delay constraint is included in the original bearing measurements to compensate for the propagation delay. A Cubature Kalman Filter (CKF) is used for periodic acoustic source tracking, in which measurement prediction cannot be obtained directly because the sensor’s position at the true measurement reception time is unknown. We solve this problem by using the implicit Gauss-Helmert Sensor Model (GHSM) for estimating the sensor’s position, which consists of the sensor’s motion equation and the known measured sensor’s signal reception time equation related to the state. Then a CKF based on the GHSM (CF-GHSM) is developed for periodic acoustic tracking. Illustrative examples demonstrate that the CF-GHSM algorithm is better than other algorithms for periodic acoustic source tracking.

Underwater 3D Doppler-Angle Target Tracking with Signal Time Delay.

The traditional target tracking is a process of estimating the state of a moving target using measurement information obtained by sensors. However, underwater passive acoustic target tracking will confront further challenges, among which the system incomplete observability and time delay caused by the signal propagation create a great impact on tracking performance. Passive acoustic sensors cannot accurately obtain the target range information. The introduction of Doppler frequency measurement can improve the system observability performance; signal time delay cannot be ignored in underwater environments. It varies with time, which has a continuous negative impact on the tracking accuracy. In this paper, the Gauss–Helmert model is introduced to solve this problem by expanding the unknown signal emission time as an unknown variable to the state. This model allows the existence of the previous state and current state at the same time, while handling the implicit equations. To improve the algorithm accuracy, this paper further takes advantage of the estimated state and covariance for the second stage iteration and propose the Gauss–Helmert iterated Unscented Kalman filter under a three-dimensional environment. The simulation shows that the proposed method in this paper shows superior estimation accuracy and more stable performance compared with other filtering algorithms in underwater environments.

Sagnac Effect and SET Error Based Pseudorange Modeling for GPS Applications

Radio Frequency (RF) signals broadcast by Global Positioning System (GPS) satellites have become an indispensable part of military and civil applications, right from missile guidance and civil aviation to farming and surveying. But the critical services provided by the GPS demand millimeter level positional accuracy. Over the years, research has been conducted and it is realized that the positional inaccuracy is mainly due to error in the pseudorange. The pseudorange is derived from the signal emission time (SET) of the satellite (known location) and signal reception time at the receiver (unknown location). Hence the better receiver position accuracy can be achieved by precise estimation of SET and accurate satellite position. If these errors are not corrected, they lead to inaccurate satellite position, which will propagate directly in to the range measurements. In this paper, an algorithm is proposed to estimate the SET and correct the satellite position for sagnac effect. Here the error in SET is modeled by taking into account the satellite clock aging parameters and the elliptical trajectory of the satellite, similarly the sagnac effect is corrected considering the earth's rotation during the signal propagation time. The implementation of the proposed algorithm for the ephemeris data collected from the Dual Frequency (DF) GPS receiver located in Indian subcontinent proved better positional accuracy. The SET and sagnac impact are assessed for all the tracked satellites, but the impact analysis pertaining to only SV PRN07 (Space Vehicle Pseudorandom Noise) are presented in this paper. The proposed algorithm can be implemented to achieve higher accuracy for navigation applications like civil aviation, category I/II Precision Approach (PA) aircraft's landing and Real Time Kinematic (RTK) positioning.

UGHF for acoustic tracking with state-dependent propagation delay

Acoustic tracking with propagation delay is a challenging problem for the following reasons: 1) It is difficult to perform an accurate state prediction, as the time interval between the current state and the previous state is varying and unknown due to the propagation delay; and 2) the target time (signal emission time) needs to be estimated in addition to the target position and velocity. With the state augmented with the target time, the state transition cannot be described by the commonly used explicit Gauss–Markov model. In this paper, we propose a new approach to solving this difficult problem by using the Gauss–Helmert model for the state transition, which consists of an implicit equation between two consecutive states. An unscented Gauss–Helmert filter is then developed based on this formulation. The new approach is applied to the bearings-only tracking problem with state-dependent propagation delay. Simulation tests are conducted to demonstrate the performance of the unscented Gauss–Helmert filter, which is shown to outperform other approaches in terms of estimation accuracy, especially when a target is moving at high speed.

Satellite Clock Error and Orbital Solution Error Estimation for Precise Navigation Applications

Global Positioning System (GPS) is a satellite-based navigation system that provides a three-dimensional user position (x,y,z), velocity and time anywhere on or above the earth surface. The satellite-based position accuracy is affected by several factors such as satellite clock error, propagation path delays and receiver noise due to which the GPS does not meet the requirements of critical navigation applications such as missile navigation and category I/II/III aircraft landings. This paper emphasizes on modelling the satellite clock error and orbital solution (satellite position) error considering the signal emission time. The transmission time sent by each satellite in broadcast ephemerides is not accurate. This has to be corrected in order to obtain correct satellite position and in turn a precise receiver position. Signal transmission time or broadcast time from satellite antenna phase center is computed at the receiver using several parameters such as signal reception time, propagation time, pseudorange observed and satellite clock error correction parameters. This corrected time of transmission and broadcast orbital parameters are used for estimation of the orbital solution. The estimated orbital solution was validated with the precise ephemerides which are estimated by Jet Propulsion Laboratory (JPL), USA. The errors are estimated for a typical day data collected on 11th March 2011 from dual frequency GPS receiver located at Department of Electronics and Communication Engineering, Andhra University College of Engineering, Visakhapatnam (17.73°N/83.319°E).

A Quick Location Method for High Dynamic GNSS Receiver Based on Time Assistance

Traditional A-GPS positioning method when quickly calculate a position, need a condition that the approximate position must not exceed 150km, otherwise the calculation will be very complex. This paper proposes a time-assisted fast positioning method for high dynamic GNSS receiver, effectively solving the problem of large search calculation in traditional method, even if exact position is unknown after the signal is recaptured. According to the known auxiliary time information and implied  elevation information, this paper put forwards a custom coordinate system for building twodimensional search space, which could reduce the number of search-dimensions. It proposes a search method based on receiver clock calculated by analyzing the influence of time auxiliary accuracy. By using GPS ephemeris data provided by the IGS, it builds a simulation environment and analyzes the influence of different preferred satellites based on the custom coordinate system on the calculation, and thus puts forward a principle for choosing the preferred satellites. Simulation examples show that through the rational combination of satellites to create a custom coordinate system, and when time auxiliary accuracy is less than 60us, the calculation can 100% guarantee to restore a complete satellite signal emission time and obtain an accurate position.

New Insights into Response Functions of Liquids by Electric Field-Resolved Polarization Emission Time Measurements

We resolve information about the dynamics of simple liquids that has been obscured in prior frequency and time-domain measurements by way of full field-resolved polarization emission time (FR-PET) measurements of carbon disulfide. The amplitude and phase of the field-resolved transient-grating signal is used to calculate a spectrogram of the signal field at each delay between the transient-grating (TG) pump and probe pulses. The temporal maximum of the spectrogram, defined to be the signal emission time, varies with pump-probe delay; it follows the convolution of the TG pulses while the pulses overlap and exhibits recurrences at times when the nuclear dynamics are the main component of the liquid material response. Since this is a third-order nonlinear spectroscopic method, the isotropic and anisotropic signals are constructed from the polarization tensor components. The frequency-integrated anisotropic component of the signal is equivalent to the signal measured in optical Kerr Effect (OKE) experiments. The FR-PET determination of the signal emission times is a direct measurement of the third-order nonlinear (polarizability) polarization and, hence provides new strong constraints on appropriate models of the liquid dynamics. Models for the material response function are used to calculate the signal emission times. In particular, we show that the proper treatment of the time-correlation function for orientational motion gives the best fit to the FR-PET data for rotational diffusional motion. We also establish that librational motion is not a short-time (coherent) motion that leads to rotational diffusion. Finally, we find that the Bucaro-Litovitz form for interaction-induced dynamics is not entirely correct for the CS(2) liquid we study. We suggest that the failing may result from the implicit assumption of two-body interactions, which is only appropriate for gases.

Heterodyne detection of the complete electric field of femtosecond four-wave mixing signals

A novel optimized heterodyne method that recovers the complete electric field of any four-wave mixing signal at its point of origin is demonstrated. A tracer pulse is sent along the signal path and characterized at the sample by frequency-resolved optical gating. Spectral interferometry is used to determine the phase difference between the tracer and a reference pulse, the absorptive change in tracer phase in the sample, and the reference-signal phase difference. Together, these measurements allow calculation of the signal phase at the sample. The phase of three pulse-scattering signals from solutions of the infrared dye IR 144 in methanol determines the absolute signal-emission time within 0.5 fs.

Silence as a Defense against Predatory Bats in Two Species of Calling Insects

Silence as a Defense against Predatory Bats in Two Species of Calling Insects