Omega Navigation 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

A Time Source for a Data Acquisition System Designed for Phase Propagation Study of Magnetic Pulsations.

A time source for a data acquisition system was designed to use the fluxgate magnetometer for the measurement of long-distance phase propagation of magnetic pulsations at very low latitudes. The time source consists of a very low frequency (VLF) signal receiver, antenna, time code generator with crystal oscillator, and microcomputer chip. Omega navigation signals received by the VLF receiver are utilized for the time base. The computer-aided system can choose the Omega station with the greatest signal strength at the observation site and can decode the transmitted signals to utilize them as the time source of the internal clock. The prominent feature of this system is software-aided fully automatic operation at any location in the world. The overall accuracy of the clock is expected to be ±30 ms.

A simple theoretical model for computing omega navigation errors near Antarctica

The propagation of VLF signals around a model icecap has been computed using Kirchhoff diffraction theory. The Antarctic continent has been modelled as a spherical cap, whose pole is coincident with that of the South Pole, which totally absorbs VLF radio waves propagating over it. Using this simple model, the range errors expected, whilst travelling between Antarctica and New Zealand, on signals from Omega La Reunion and Argentina have been calculated and compared with recently derived measurements. It has been found that in order to model the measured range errors accurately it has been necessary to modify the simple spherical cap model to that of a semi-circular spherical cap, producing what is effectively knife edge diffraction. To ‘best fit’ the data, the northernmost limit of the spherical cap has been found to be 66.1°S for propagation from La Reunion and 75°S for propagation from Argentina. These model icecaps agree well with the northernmost boundary of the Antarctic continent where signals from Omega La Reunion and Argentina graze it tangentially.

The Use of a GPS-Equipped Aircraft as a Mobile Platform to Study the Propagation of Omega Navigation Signals

The almost complete constellation of GPS satellites now provides excellent positional information for aircraft, with typical worst-case errors of the order of 100 metres. The low frequency waves used for Omega navigation have wavelengths of the order of 30 kilometres and thus, even in an aircraft moving at varying speeds up to around 500 km per hour, it is now a simple matter to measure VLF phase with a significance at the 1 degree level. This paper describes the use of GPS to provide accurate positional information to VLF receivers located on aircraft of the Royal New Zealand Airforce (RNZAF) en-route to Antarctica. The marked phase irregularities on signals from La Reunion and Argentina, observed north of the icecap in earlier studies using aircraft fitted with inertial navigation systems, have been confirmed and described with much greater precision. Smaller phase anomalies have also been observed on signals from Omega Australia and Omega Hawaii, whilst flying directly over the Antarctic icecap. The latter signal was previously regarded as unperturbed and used in earlier studies as a phase reference. Small-scale periodic variations of phase and amplitude have also been recorded on signals propagating from Omega Australia. These variations are considered to be caused by the reflection of the VLF waves at discontinuities in the height (e.g. mountains) or the lower boundary conductivity of the Earth-ionosphere waveguide. Reflections of smaller amplitude have also been observed flying over the open ocean before reaching Antarctica.

A simple theoretical model of the diffraction of VLF electromagnetic waves by the Antarctic icecap

The Antarctic continent has been modelled as a spherical cap whose pole is coincident with that of the South Pole, which totally absorbs VLF radio waves attempting to propagate over it. The propagation of Omega navigation signals around this model icecap has then been computed using Kirchhoff diffraction theory. Spherical caps extending to 66.5 and 75.5°S have been found to accurately model the signals from Omega La Reunion and Argentina, respectively, received on flights between Christchurch, New Zealand and Scott Base in Antarctica, up to the boundary of the theoretical icecap. These model icecaps were found to be good fits to the boundary of the Antarctic continent, when measured at the 1–1.5 km contour of ice thickness, in the region where the VLF waves diffracted around the icecap. The good agreement obtained between the experimental field strength data and those computed theoretically, using only simple diffraction theory, suggests that coastal refraction plays at most only a secondary role in circumpolar propagation.

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.

Past, Present and Future Applications of OMEGA Navigation Technology*

Airborne OMEGA navigation technology is now entering its third generation. The first generation “OMEGA” offered affordable, worldwide navigation, principally in the North Atlantic and other oceanic areas requiring Sole Means of navigation certification. The second generation “OMEGA/VLF” featured maturing software and improved signal reception and handling characteristics which enabled it to meet the more stringent RNAV certification requirements. The third generation “OMEGA/VLF NAVIGATION MANAGEMENT (NMS)” now combines OMEGA/VLF navigation information with a variety of position sensor inputs, including; VOR/DME, DME/DME and GPS. The combination of advanced technology, mature OMEGA software and multiple position inputs now offers a cost effective, reliable and exceptionally flexible navigation system which satisfies a wide range of current and future navigation needs.

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.

High‐amplitude VLF transmitter signals and associated sidebands observed near the magnetic equatorial plane on the ISEE 1 satellite

On February 16, 1983, during a period of strong magnetic disturbance (Kp ∼ 5), as the ISEE 1 satellite passed near the magnetic meridian of the Omega navigation transmitter (98°W, 46°N geographic, L ∼ 3.4), the Stanford University VLF wave receiver detected the presence of Omega transmitter signals over the range 2 ≤ L ≤ 3.7 at magnetic latitudes (λm) within 10° of the magnetic equator. Over a 500‐km orbital segment near L ∼ 3.4 and λm ∼ 5°S, the electric field amplitude of the Omega signals reached ∼ 0.6 mV/m, a value &gt; 40dB higher than the signal amplitude both preceding (L ∼ 3.7–3.5) and following (L ≤ 3.2) the high‐amplitude period. Over the range 3.3 ≤ L ≤ 3.7 the transmitter pulses at 13.1 and 13.6 kHz were associated with sideband signals spaced in frequency roughly symmetrically about the carrier and generally reduced in amplitude ∼5 dB with respect to the carrier. The strongest sidebands were generally offset in frequency from the carrier by ∼ ±55 Hz. Simultaneous data from the University of Iowa Plasma Wave experiment indicated that the region of high wave amplitude was located just beyond the outer edge of the plasmapause where low values of cold plasma density (∼40 el/cm³) prevailed. High amplitudes were observed only during the time in which the signal frequency lay within the range of a natural noise band of rising center frequency in which bursts of VLF emissions were occasionally triggered by “knee‐trace” whistlers. On the basis of the group time delay and amplitude distribution of the Omega signals and the presence of knee‐trace whistlers, it is concluded that the high amplitude signals originally propagated along the base of the plasmapause surface into the southern hemisphere and subsequently reflected or scattered back up to the satellite. The high‐wave amplitude was presumably produced through the coherent whistler mode instability as the input waves interacted with gyroresonant energetic electrons near the magnetic equatorial plane, and the presence of sideband signals indicates that this interaction had reached a nonlinear stage. The wave magnetic field in the interaction region is estimated to be approximately 20 mγ. The high amplitudes reached by the signals indicates that particle trapping effects could be responsible for the sideband generation.

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.