Omega Signals Research Articles

Statistical study on propagation characteristics of Omega signals (VLF) in magnetosphere detected by the Akebono satellite

This paper shows a statistical analysis of 10.2 kHz Omega broadcasts of an artificial signal broadcast from ground stations, propagated in the plasmasphere, and detected using an automatic detection method we developed. We study the propagation patterns of the Omega signals to understand the propagation characteristics that are strongly affected by plasmaspheric electron density and the ambient magnetic field. We show the unique propagation patterns of the Omega 10.2 kHz signal when it was broadcast from two high–middle-latitude stations. We use about eight years of data captured by the Poynting flux analyzer subsystem on board the Akebono satellite from October 1989 to September 1997. We demonstrate that the signals broadcast from almost the same latitude (in geomagnetic coordinates) propagated differently depending on the geographic latitude. We also study propagation characteristics as a function of local time, season, and solar activity. The Omega signal tended to propagate farther on the nightside than on the dayside and was more widely distributed during winter than during summer. When solar activity was at maximum, the Omega signal propagated at a lower intensity level. In contrast, when solar activity was at minimum, the Omega signal propagated at a higher intensity and farther from the transmitter station.Graphical .

Automatic Detection of Omega Signals Captured by the Poynting Flux Analyzer (PFX) on Board the Akebono Satellite

The Akebono satellite was launched in 1989 to observe the Earth’s magnetosphere and plasmasphere. Omega was a navigation system with 8 ground stations transmitter and had transmission pattern that repeats every 10 s. From 1989 to 1997, the PFX on board the Akebono satellite received signals at 10.2 kHz from these stations. Huge amounts of PFX data became valuable for studying the propagation characteristics of VLF waves in the ionosphere and plasmasphere. In this study, we introduce a method for automatic detection of Omega signals from the PFX data in a systematic way, it involves identifying a transmission station, calculating the delay time, and estimating the signal intensity. We show the reliability of the automatic detection system where we able to detect the omega signal and confirmed its propagation to the opposite hemisphere along the Earth’s magnetic field lines. For more than three years (39 months), we detected 43,734 and 111,049 signals in the magnetic and electric field, respectively, and demonstrated that the proposed method is powerful enough for the statistical analyses.

Determination of plasmaspheric electron density profile by a stochastic approach

The determination of plasmaspheric electron density profiles has been attempted using a stochastic approach, using Omega signals observed on the Akebono satellite. Since the wave normal directions and delay times of Omega signals can be theoretically calculated by ray tracing under an appropriate electron density model, the density profile can be estimated by fitting the calculated directions and times to the observed values. In the present paper, we introduce a flexible model and a novel algorithm in the fitting method, taking into account the stochastic factors of the density distribution and wave propagation. The stochastic representations of directions and times enable us to separately estimate the effects of the ionosphere and the plasmasphere. The validity of the proposed method is examined using observational data collected during the recovery phase of a magnetic storm. In another example, an estimation of the asymmetry of the plasmasphere is attempted.

Determination of global plasmaspheric electron density profile by tomographic approach using omega signals and ray tracing

It has been necessary requirements to determine the global electron density distribution in the plasmasphere with time resolutions, of less than a day. We have provided solutions to this requirement using the wave normal directions, delay time of Omega signals and the in situ electron density observed on-board the Japanese satellite Akebono (Sawada et al., Journal of Geophysical Research 98(11) (1993) 267, Kimura et al., Advance Space Research 15(2) (1995) 103, Advance Space Research 18(6) (1996) 279, Journal of Atmospheric and Solar-Terrestrial Physics 59 (1997) 1569). The present paper is intended to review our earlier studies.

Global electron density distribution in the plasmasphere deduced from Akebono wave data and the IRI model

Abstract Determination of global electron density distribution in the plasmasphere has been attempted using the wave normal directions and delay time of Omega signals, and in-situ electron density observed on the Akebono satellite. The present study is based on the assumption that the plasma model is represented by a diffusive equilibrium (DE) model with temperature gradient along a magnetic field line, and that the electron density at a reference altitude (1000km) is dependent on L-value. Relative concentrations of ion constituents H+, He+, and O+ at the reference altitude are determined by fitting as either L-independent or L-dependent. For the altitude range below the reference altitude, we have adopted a distribution which has one maximum density at the F2 peak and one minimum density at the E valley, both being referred to the IRI model. The parameters related to these models are determined by the non-linear least squares fitting method for the in-situ electron density, the wave normal direction and the delay time of Omega signals observed by Akebono. Wave normal directions and delay time at Akebono for the model are determined by ray tracing. The validity of our plasmaspheric model based on the DE model was checked by comparing it with the SUPIM (Sheffield University Plasmaspheric and Ionospheric Model), and we can conclude that our model is flexible enough to represent the general global distributions of the electron density in the plasmasphere, although it is possible that in some cases, unrealistic distributions of the plasma temperatures should be used in the model. The reliability of our fitting algorithm was also checked by computer simulations, and we have confirmed that the global electron density distribution can be determined from the satellite wave data, without any significant error. Several plasmaspheric electron density distributions are shown as a result of applying our method to Akebono data.

Electron density distribution in the plasmasphere in conjunction with IRI model, deduced from Akebono wave data

Determination of electron density distribution in the plasmasphere has been attempted using wave normal directions and delay time of Omega signals, and in-situ electron density observed on the Akebono satellite /1/, /2/. The present study is based on the assumption that the plasma model is represented by a diffusive equilibrium (DE) model with a vertical temperature gradient, and the electron density at a reference altitude (1000km) being dependent on the L value. The parameters related to these models were determined by non-linear least square fitting of the in-situ electron density, the wave normal direction and delay time of Omega signals observed by Akebono. Relative concentrations of ion constituents of H +, He +, O + at the reference altitude are determined by fitting as either L independent or L dependent. Connecting the global plasmaspheric density model with IRI in the ionospheric altitude is also tried.

Determination of electron density distributions in the plasmasphere by using wave data observed by Akebono satellite

Determination of electron density distribution in the plasmasphere has been attempted using wave normal directions and delay time of Omega signals observed on the Akebono satellite as well as the electron density along the satellite trajectory.

Time of day from Omega VLF signals

Omega VLF transmissions are synchronized to UTC and can thus be used not only for navigation but also for determining time. An inexpensive DSP-based 'clock' card is described which plugs into a personal computer and by means of suitable software phase locks on to Omega signals from a simple broadband VLF preamplifier. Within a few minutes from daytime system power-up, the clock card is able to supply time of day (UTC) to an absolute accuracy of a few microseconds. Because of the Omega repetition period this time has an ambiguity of module 10 s, but this can normally be readily resolved by the user entering approximate wristwatch time at the keyboard. The clock output is available both in machine readable form to the host computer and as a one second pulse direct from the DSP clock card.

Electron density profile in the magnetosphere deduced from in situ electron density and wave normal directions of omega signals observed by the Akebono (EXOS D) satellite

We describe a new method to construct the global plasma distribution in the plasmasphere using the electron density measured along the trajectory of the Akebono (EXOS D) satellite and the wave normal direction of Omega signals simultaneously observed by the same spacecraft. In this method the plasma density distribution is described by using the diffusive equilibrium model, with eight parameters, that is, die relative abundances and temperatures of ions, and the electron density at a reference altitude. This model is first fit in a least squares sense to the electron density observed along the spacecraft trajectory. The fit solution is not always unique. In such cases it is necessary to constrain the possible solutions further. Therefore we ray trace Omega signals from their source location and check if the wave normal directions at the spacecraft location are consistent with those observed. We present here three different examples of fitting methods in which fixed, freely fitted, and constrained fitted parameters yield reasonable plasma distributions.

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban ( L = 1.69, 29°53′ S, 31°00′ E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L ( L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds ( T i ∼ 5 ms) and large wave amplitudes ( B w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.

Winter Anomaly and Traveling Ionospheric Disturbances over Japan during DYANA Campaign.

The winter anomaly and medium-scale traveling Ionospheric disturbances (MSTIDs) detected during the DYANA campaign in 1990 are analyzed to investigate their characteristics and possible relationship to meteorological phenomena in the lower atmosphere. Anomalous f-min enhancements (FMEs), an indication of the winter anomaly, occurred around January 15 and between January 20 and 29, 1990. For the latter period, the FME region extended down to 31°N. D-region electron density enhancements associated with the FMEs were also recognized from phase measurements of the 10.2kHz Omega signals. The FMEs are not due to solar flares or geomagnetic disturbances, but are related to meteorological processes in the lower atmosphere. Comparison of the FMEs with neutral wind flows observed by the DYANA rockets and MU radar indicates that the FMEs are largest for a few days before the reversal of the stratospheric wind direction from westward to eastward caused by planetary wave activity. There is a 5-day delay between the onsets of the FME and the lower stratospheric (20-hPa) temperature increase. A minor stratospheric sudden warming that appeared around 60°N from February 1 to 14 did not affect the D-region (no FMEs) at least over Japan.A large number of MSTIDs were identified from ionospheric total electron content data. Although a few of these MSTIDs were caused by geomagnetic activities, the majority have sources in the lower atmosphere. We find that daily and diurnal variations of the MSTID occurrence are controlled by F-region electron density (foF2).

The Soviet VLF Navigation System

The VLF navigation system operated by the Soviet Union for the past 20 years is described. Issues discussed include grid geometry, signal frequencies, format, transmitted power, receiver structures, on air/off air schedule, phase stability relative to a local cesium time standard, and the regular, sudden phase shifts at 2100 Z daily. The reason for pursuing the study is now primarily of historical interest. However, with recent statements from Soviet officials indicating that the system may be operated in the future for worldwide civil use, its potential use in conjunction with the existing Omega system is of renewed interest. In more technical areas, it may be possible to use propagation data on Soviet transmissions to derive information both on the propagation of Omega signals into the Soviet Union and in support of research into the measurement of phase velocities and diurnal phase variations throughout the VLF band.

Propagation characteristics of Omega signals and their triggered emissions observed by EXOS‐D satellite

Omega signals have been frequently observed by the Japanese satellite EXOS‐D (Akebono) in the magnetosphere. We have statistically analyzed VLF‐WBA (wide‐band analyzer) data in a period of 6 months, which were acquired by Kagoshima (KSC) and Prince Albert (PA) satellite tracking stations. About a quarter of the data obtained at KSC contain Omega signals from Australia and about a sixth of the data at PA contain those from North Dakota. Ray tracing calculons for the Australian Omega signals are consistent with the results of observations. 10 cases of triggered emissions are also found in the analyzed data. Amplitude and phase characteristics along an Omega pulse are investigated for the triggering event and an apparent phase lead, corresponding to a frequency increase by about 10 Hz was found at the center frequency of signals after the triggering starts.

Phase and amplitude perturbations on subionospheric signals produced by a moving patch of artificially heated ionosphere

The Ionospheric Heating Facility (1‐MW HF transmitter and steerable array) at Ramfjordmoen, Norway, was used to generate a localized and steerable discontinuity in the Earth‐ionosphere waveguide for scattering Omega signals. The phase of the scattered signal (“echo”) relative to the direct signal varied with position of the heated patch of ionosphere (the discontinuity) in proportion to the change in echo path length. Previous results by Barr et al. (1985), using the same technique, Omega signals and path, show very similar behavior when looked at in this way. In this respect, both experiments simulated the phase and amplitude perturbations (“trimpis”) produced by lightning‐induced (via electron precipitation) changes in the waveguide and show that measurements of the varying echo phase of trimpis can be used to track lightning‐induced perturbations. The steerable discontinuity can also be used as a waveguide probe to measure the phase velocity of the strongest waveguide mode received on the ground by such scattering.

Omega Compass Applications

The primary use of Omega signals is to extract phase information as a means of fixing an observer's position. This paper considers the feasibility and potential applications of extracting directional information from Omega or other VLF signals. This directional information could be used as input to a gyro-magnetic compass as an alternative to the magnetic input. It could also be used to assist the tracking and thus improve the position fixing capability of the Omega receiver itself.

Omega Navigation in the Shadow of Antarctica

1. INTRODUCTION. Very-low-frequency radio waves were used to implement the Omega navigation system because of their low attenuation (2–4 dB/1000 km) when propagating globally in the waveguide formed between the Earth and the ionosphere. However, it became apparent in the early seventies, throughout the period when the majority of the stations of the Omega network were commissioned, that VLF signals propagating over permafrost or glacial ice could suffer anomalously large attenuations, of greater than 20 dB/1000 km, especially during the daytime. In the Northern Hemisphere problems have arisen with the heavy attenuation of Omega signals propagating over the Greenland ice sheet. In particular a very bad region for Omega coverage occurs around Winnipeg in Canada (the ‘Winnipeg Hole’). In this area Omega North Dakota suffers from ‘near field’ effects, Omega Liberia is contaminated by trans-equatorial modal effects and Omega Norway is removed by the attenuation of its signals when crossing the Greenland ice-cap. There have even been discussions on the feasibility of constructing extra VLF transmitters in Canada to alleviate this problem.

Timing of portable seismographs from omega navigation signals

Abstract The continuous, worldwide availability of very low-frequency signals from the Omega navigation system makes them useful for application to timing of portable seismographs, particularly in remote parts of the world where other radio sources may be difficult or impossible to receive. In our seismograph system, Omega phase data, which are sampled relative to a temperature-compensated quartz crystal oscillator, are digitized and recorded along with seismic data, and used to correct chronometer errors in postexperiment processing. An analysis procedure that statistically optimizes the correction process has been developed. Once time is known to ±5 sec, corrections are determined using a combination of two processes: (1) synchronizing the received Omega pattern to the propagation-corrected transmitted Omega pattern, and (2) measuring the drift rate from short-term (10-sec) changes in Omega phase. The Omega pattern synchronization yields the absolute time reference, and the cumulative drift rate gives the relative time correction. Standard errors of less than 1 msec are obtained under most circumstances. Absolute timing accuracies as high as 4 μsec are obtainable, however, through continuous phase tracking of Omega signals and application of very low-frequency, phase-velocity models. This suggests Omega9s suitability for earth-imaging experiments using dense arrays of portable seismographs. Although it is probable that Omega will eventually be replaced by satellite-based systems such as the Global Positioning System, it will most certainly be years before such a system can compete in terms of cost, reliability, and worldwide applicability.

Some new results in improving position accuracy with OMEGA

Abstract Some of the factors affecting the accuracy of positioning with the OMEGA radionavigation system are discussed. A simplified algorithm of computing the diurnal corrections for OMEGA is developed. It is based on the calculation of the mean velocity of propagation of VLF radiowaves as a function of the effective height of a spherical waveguide. The formulae to compute the latter are given. A composite signal method proposed by Pierce and its modification by Papousek and Reder are discussed; it is shown that the accuracy of the modified version is not better than that of the original. Both in Pierce's method and its modifications the waveguide is treated as a plane. Its sphericity is taken into account in this article, which results in better compensation of the phase variations of OMEGA signals. It is shown that fractionally rational transformations of the phase count may result in an additional error. To reduce that error, a high reliability of the ambiguity resolution is required at all used frequ...

An Omega/Transponder Display System

A new application of the Omega navigation system is described in which a ground station is enabled to display multiple vehicle locations. Pulses from the vehicles are triggered by Omega signals and the times of arrival of the pulses at the ground station are processed to provide a ‘radar-like’ display of each vehicle's range and bearing.This paper was presented at the eleventh Annual Meeting of the International Omega Association held in Quebec in August 1986.

Two Improvements in Omega Windfinding Techniques

Abstract A one-dimensional local spline smoothing technique is applied to Omega navigational signals for the purpose of windfinding. Wind profiles so produced depend largely on two parameters of the smoothing procedure: the nodal spacing, which determines the smallest resolvable scale, and a filtering wavelength, which produces the necessary smoothing of the phase data, and prevents representational distortion of any power from the unresolved scales. Phase “noise” from stationary test sondes is superimposed on synthetic Omega signals to compare wind profiles obtained with this new procedure with profiles computed using other techniques. Is it shown that the effect of aircraft maneuvers on Omega wind accuracy is not completely removed by the normal practice of evaluating all phase derivatives at a common time. Additional improvements in accuracy of 2–3 m s−1 can be obtained by a “rate-aiding” technique using aircraft navigational data.