Omega Navigation System Research Articles

Technical Extracts from the Memoirs of Dr. J. A. Pierce

The following is the third and final part of an extract from the memoirs of Dr. J. A. Pierce who was deeply involved in the development of radionavigation aids in the USA between 1941 and 1973. These are, of course, personal reminiscences, and some issues might be challenged by others working in the same field at the time. However, they give a remarkable insight into the problems faced by the scientists of the day and how they were overcome. This final part deals almost exclusively with the development of the Omega Navigation System, which was the first truly ‘global’ navigation system to enter service. The editing principle used here has been to preserve Dr. Pierces' own wording while eliminating those parts of a purely personal nature. The entire original document without cuts may be found on www.internationalnavigation.org

The Esrange MST radar: A brief introduction and procedure for range validation using balloons

The Esrange VHF radar (ESRAD) is a relatively new mesosphere‐stratosphere‐troposphere (MST) class radar located in northern Sweden (67.88°N, 21.10°E), about 30 km east of Kiruna. The radar operates continuously and has already been included in several campaigns in which multiple profiling instruments have been used, for example, lidars, radiosondes, and scientific balloons. To intercompare the different measurements, it is essential to have precise height estimates. In this paper we begin by presenting a short technical overview of the radar. Then we describe a radar validation conducted in 1997 during a period when numerous scientific balloons were launched from Esrange. All of these balloons were tracked using one or more of the following techniques: Omega navigational system, Global Positioning System (GPS), or the Esrange C‐band tracking radar (ESR). A number of these balloons passed close enough to the ESRAD beam that they could be detected by the MST radar. This unique data set has been used to check the range estimation of ESRAD. Furthermore, the data have been used to make a rough check of the geometry of the formed beam.

A Novel Method for Extending GPS to Underwater Applications

The need for accuracy, precision, and data registration in underwater positioning and navigation should be viewed as no less stringent than that existing on the surface or in the aerospace environment. This paper proposes a system that can be used to provide navigation service to free-ranging underwater users. This system uses buoys that act as roving pseudolites and translate GPS radio-based service to an acoustic-based service under water. Using this service, an unlimited number of users can independently locate themselves and navigate. Underwater extensions of Loran, Omega, or other radio-based navigation systems are also amenable to the technique.

Omega System Performance Assessment

The system availability index is defined as the probability that an Omega user can effectively utilize three or more Omega station signals with a properly functioning Omega receiving system at any location in the user's operating area (which may include the entire earth's surface) and at any time (hour/month/year). This index (denoted as PSA) has been developed to provide a probabilistic measure of system performance from the user's viewpoint and to provide a standard for evaluating the operational performance of the Omega system. The system availability model, used to compute PSA, is structured to address four system elements: (1) signal transmission reliability, (2) signal reception reliability, (3) signal coverage, and (4) regional priorities for Omega usage. Sample calculations and results are presented for assumed alternative station power level assignments to illustrate the numerical characteristics of PSA. Although the model is applied to the Omega system, the methodology presented in this paper is general and could be applied to other radionavigation systems. The development of the model/algorithm and the calculations performed to obtain the sample results were supported by the U.S. Coast Guard Omega Navigation System Center.

Omega: system status update-1988

Activity at the US Coast Guard Omega Navigation System Center is reviewed. A prime area of emphasis has been to make further progress in the transition to a mature operating system. Among the topics discussed are operations and control, electronics equipment, civil engineering subelements, the federal radionavigation plan, and system performance. Part of the transition will involve the development of a proper mission statement for continued operations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

Omega Navigation System-A Status Report

The policy statements contained in the 1986 Federal Radionavigation Plan of the U.S. indicated that the Omega navigation system should remain operative until at least the year 2005. This called for the determination of what system elements are in need of capital investments to extend their service life. This article reports preliminary results from studies undertaken to arrive at that determination. Transmitting station equipments, including antennas, are being considered for redesign or replacement. Investigations also involve system monitors, system control software, and system propagation corrections. The studies will continue for several years and contribute to deliberations leading to biennial updates of the Federal Radionavigation Plan.

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.

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.

Overview of OMEGA Signal Coverage

Successful use of the OMEGA Navigation System requires external information on the system coverage (i.e., spatial and temporal accessibility of “usable” signals from the OMEGA system) in selecting OMEGA signals for position fix computations. This paper reviews the published OMEGA signal coverage information and describes in detail the type (and basis) of the coverage information currently disseminated by the U.S. Coast Guard OMEGA Navigation System Center. Assessments of the worldwide coverage provided by the system and the impact of OMEGA transmitting station outages on the system coverage are also given.

Analysis of Umbrella antenna at the Omega navigation station

The characteristics of the electrically very small umbrella antenna used for the Omega navigation system are examined by the numerical analysis. The transmitting antenna of height about 0.02 \lambda with 16 inclined elements at the Tsushima station is analyzed as an example of electrically very small umbrella antenna. In the analysis the ground is assumed to be a perfectly conducting plane and the Hallen-type integral equation is used. The difference between the calculated input resistance and the measured one is estimated as the earthing resistance. The approximate equation of radiation resistance is derived and the relation between the inclined elements and the antenna characteristics is discussed.

OMEGA Navigation System Position-Fix Accuracy Assessment

A baseline assessment is provided for the worldwide hyperbolic position-fix accuracy of the OMEGA Navigation System. The assessment is performed by evaluating the fix errors obtained using a single-frequency, hyperbolic fix algorithm, and the Morris/Swanson propagation correction prediction model. Position-fix statistical measures are computed for each monitor site and LOP pair available in 5 years of phase-difference data selected from the OMEGA MASTERFILE. The worldwide distributions of position-fix measures are then displayed for two selected OMEGA fix-geometry criteria: GDOP less than 10, and best-available GDOP at each site.

OMEGA Navigation System Regional Validation Program

The OMEGA Navigation System Operations Detail's (ONSOD's) regional validation program is explained by a discussion of the following topics: (1) Regional Validation Procedure, (3) Monitor Equipment Characteristics, and (4) Validation Data Implementation. A Validation Status Report presents events and findings relating to ONSOD's current validation work for the South Atlantic and Indian Ocean. Samples of the South Atlantic Validation data with analysis, and a progress report for the Indian Ocean Validation are included.

Observations on the GEOS 1 satellite of whistler mode signals transmitted by the Omega Navigation System Transmitter in northern Norway

Whistler mode signals of 0.9–1.1 s duration at 10.2, 11.33, and 13.6 kHz launched by the Omega Navigation System transmitter located in northern Norway have been observed on GEOS 1. The signals were observed both inside and outside the plasmasphere in two regions of the magnetosphere, in December 1977 at 10–20° north of the equator on the morningside and in May 1978 10° south of the equator on the nightside. For a sequence of particularly strong pulses in the December period, the ‘instantaneous’ wave normal directions were calculated and found to have angles of 120–140° to the magnetic field of the earth pointing 0–40° eastward. These pulses have amplitudes of the order of 10 μV/m and refractive indices above 30. At times the amplitude envelope of the signals is unmodulated, but often the amplitude and the wave normal direction are strongly modulated on a time scale of 0.2 s. We attribute these features to ray path mixing. The signals are often prolonged up to 1 s, which is interpreted as an overlap of the primary pulse with the echo of the pulse reflected from the topside ionosphere in the southern hemisphere. While the signals do not trigger strong emissions, we do observe ∼0.1‐s long bursts at the tail of some direct pulses.

Wave normal directions and wave distribution functions for ground‐based transmitter signals observed on GEOS 1

VLF whistler mode pulses of ∼1‐s duration emitted by the omega navigation system transmitter located in Norway have been observed on‐board the GEOS 1 satellite. The wave normal directions of 7 pulses have been determined by three different methods of analysis: the cross product method, Means' method, and the maximum entropy method of determination of the wave distribution function (WDF). The first method is based on the interpretation of measurements of the three magnetic wave field components in a 300‐Hz band, including the omega frequency and the second and the third method on the interpretation of the spectral matrix elements of the three magnetic wave field components at the Omega frequency. While the cross product method and Means' method assume that the electromagnetic wave field is that of a single plane wave propagated in a fixed direction with respect to the ambient magnetic field, the WDF method assumes the wave field to be random. The three methods and their solutions are compared and discussed in terms of the statistical properties of the signals (signal to noise ratio, stationarity in time, degree of polarization, ellipticity), and in terms of sensitivity to inconsistencies in the data. Some recommendations are made concerning the best choice of method in given situations. The wave normals are found to have angles ∼130° with the earth's magnetic field direction, pointing slightly eastward of the local magnetic meridian. They exhibit a time variation with a 0.2‐ to 0.4‐s periodicity.

Accurate clock for remote areas

In many regions, conventional time signals like WWV are not available continuously; on the other hand the signals of the Omega navigational system can be received everywhere 24 hours a day. They are not, strictly speaking, time signals, but they can be used to synchronize clocks to an accuracy of about 0.01 s. The Observatory of Neuchâtel, Switzerland, has developed such a clock; it is battery operated (5 years autonomy), small (14×13×4 cm), light (400 g, including antenna and battery), and fixed‐tuned to one particular Omega transmitter.There are eight Omega stations evenly distributed around the earth, and no land area is further away than 5000 km from the nearest transmitter. For a distance of this order the clock described above has been measured to have an accuracy of better than 0.01 s in the temperature range 0°C to +40°C. The clock is actually driven by a quartz crystal, and the synchronization with the Omega signals is achieved in small steps of about 0.5 ms every 10 s. During times of poor reception, caused by local thunderstorms, the clock continues to run with the accuracy of its quartz crystal. The clock should prove useful for many geophysical applications and can be purchased in sample quantities. For detailed information write to Otfeervatoire Cantonal, Neuchâtel, Switzerland.

Observations of whistler-type echoes on signals of a ground VLF transmitter on board the ‘iterkosmos-19’ satellite

Signals of VLF transmitters of the Omega navigation system located in the auroral zone (66.4°N, 13.2°E, L= 5) were recorded by the VLF receiving equipment of the Interkosmos 19 satellite. Signals at frequencies between 10.2 and 13.6 kHz were received in a region above the transmitters, frequently with whistler-type echoes. An analysis of these echoes has shown their predominating occurrence in periods of low geomagnetic activity (Kp<2+). The occurrence region of these phenomena in the outer ionosphere has the dimension of about 1000 km and its position is betweenL= 2.5 and L= 4.4. The delay of echo-signals is practically the same during one satellite pass but its values for different satellite revolutions lie between 2.5 and 3.5 s. The frequency spectrum of these signals can be broadened up to 100 Hz. On the basis of calculations made, it can be shown that the experimental results are generally in accordance with the hypothesis of nonlinear ducting of VLF waves in the magnetosphere.

Ducted and non-ducted propagation of Omega signals within the plasmasphere

Signals of the transmitter of the Omega navigation system located in the Southern Hemisphere (Reunion, 21.0°S, 55.3°E, L= 1.35) recorded by Interkosmos satellites on passes over the Northern Hemisphere, have shown that these signals can be detected in the conjugate region within a range of L-values between 1.5 – 3.0. Both ducted and non-ducted propagation has been observed. The most frequent propagation is in the whistler mode along the line of forceL= 1.8. The measured signal delays are in accordance with the observed dispersion of whistlers.

オメガシステムによる測得位置について

Ships equipped with the OMEGA navigation system, have been increasing these recent years. However, the evaluation of a given position in this system, is still not fixed. The authors measured OMEGA signals of C-H, D-H and E-H pairs on the going and returning voyages between Japan and Australia from the 15th of January till the 10th of March, 1978. The results were as follows ; (1) On the received condition of OMEGA signals, the E-H pair was fine during all coverage, but the C-H and D-H pairs were worse while going southward, especially, the C-H pair was the worst at nighttime in the South Pacific Ocean. (2) The fixed position by C-H and E-H pairs have comparative stability and confidence in daytime, but they have no stability and confidence at nighttime and an hour around sunrise and sunset. (3) A user must be careful about the included angle based on the direction from OMEGA stations, because fixed positions have large errors in the area of a small included angle, even if the OMEGA signal can be received in fine condition.

The Omega navigation system--An overview

The Omega worldwide VLF radio navigation system is discussed. Following a brief historical perspective, the current and projected system configurations are described with emphasis on the operational characteristics of transmitting station equipment. The Omega positionfixing process is examined and the available user equipment is reviewed. The characteristics of VLF radio wave propagation, which have a significant impact on Omega navigation accuracy, are discussed. Finally, the various elements of the Omega user community are described along with the likely nature of future increased applications of the Omega navigation system.