Omega Stations Research Articles

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.

Description of Requirements Analyses to Support Development of an FAA Position on Use of GPS as a Primary Means of Navigation in Oceanic and Remote Airspace

The U. S. Federal Aviation Administration (FAA) recently investigated the use of GPS as a primary means of navigation in oceanic and remote airspace prior to the implementation of the Wide-Area Augmentation System. This was motivated in part due to the possible anticipated shutdown of the Australian Omega station as well as the unreliability of the Liberia Omega station. The FAA goal was to use Global Positioning System (GPS) equipment that complies with Technical Standard Order (TSO) C-129 (for supplemental use of GPS), with minimal design changes and with minimal operational restrictions imposed. The FAA and MITRE identified a candidate set of design features essential or desirable for use of GPS as a primary means of navigation in oceanic and remote airspace as opposed to use as a supplemental means of navigation. FAA also identified candidate operational restrictions such as departure requirements that could be imposed to maintain an acceptable level of safety. Within the context of the candidate design and operational requirements, this article analyzes risks associated with various operational scenarios based on continuity and availability of navigation and receiver autonomous integrity monitoring (RAIM) functions for oceanic operations. The analyses described in this article were used by the FAA in their determination of a set of minimum performance and operational requirements that would permit use of GPS as a primary means of navigation for oceanic and remote operations. © 1995 John Wiley & Sons, Inc.

Analysis of Nonlinear Omega Receivers

A summary of ongoing research analyzing the VLF noise environment and various structures of nonlinear Omega receivers is presented. First, the noise amplitude is seen to have a probability density function that can be modeled as Gaussian for small amplitudes and as decreasing proportionally to (amplitude)−2.2 for large amplitudes. This model of probability density function suggests a maximum likelihood nonlinearity that is linear for small amplitudes and of the form f(x) ˜ 1/x for large amplitudes. Analysis of the behavior of two approximations of this nonlinearity (hole punch and clipper) is presented. Issues addressed include performance relative to a linear receiver as a function of structure, threshold for either nonlinearity, bandwidth prior to nonlinearity, and signal level. The study included both synthesized signals added to actual noise and off-the-air signals. Improvements of 7–14 dB relative to a linear receiver were observed. Finally, it is observed that the presence of a strong local Omega station within the receiver passband results in virtually no improvement for a weaker station, relative to a linear receiver.

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.

Navigation for balloon payloads: Recent experience with OMEGA in the US and a new slant range system

The replacement of OMEGA station G from Trinidad to Australia has recently caused some problems for balloon navigation in the United States. We report on recent experience with OMEGA during a balloon flight of the hard X-ray astronomy payload HEXE 1) 1) This High Energy X-ray Experiment is operated jointly by the Max-Planck-Institut für Extraterrestrische Physik (MPE) in Garching and the Astronomisches Institut Tübingen (AIT) from Palestine, Texas on September 28/29, 1981. In this flight three different OMEGA receivers were used. In view of the degraded receiving conditions for OMEGA we have developed a simple slant range system which may easily be adapted to other standard PCM systems. We show that a combination of range, azimuth and one OMEGA line of position (e.g. C-D) provides reasonably accurate balloon positions.

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.

A study of reception of Omega transmissions by the Navaid sounding instrumentation on Indian Monex ships during Monex-79

During the global weather experiment and Monex-79 upper air observations were made over the Arabian Sea and the Bay of Bengal using for the first time the FGGE Navaid sounding systems. Two such complete Navaid systems were installed on board Indian Monex ships. These ships were deployed in the central and south Arabian Sea during May-June 1979 and in the Bay of Bengal during July 1979. In addition to this a data acquisition module of the Navaid system was fitted in the, third Monex ship which was located in the south and central Bay of Bengal during May-July 1979. The three ships made 284 upper air ascents in all. The Navaid sounding technique and instrumentation proved their usefulness for upper wind measurements over sea areas from ocean platforms. A brief description of the Navaid instrumentation and the expendable Omegasonde used are given.
 
 In this report an evaluation of reception pattern of the worldwide Omega transmissions obtained at the systems installed on board the Indian Monex ships over the Arabian Sea and Bay of Bengal during Monex-79 (May-July 79) is also presented. The Navaid instrumentation could receive Omega signals from a larger number of worldwide Omega stations over the Arabian Sea than the Bay of Bengal during Monex-79. Omega signals at least from three or more Omega stations were always received enabling the monex ships to be within the great circle triangle formation of combination of three Omega stations, thereby yie1ding a reliable upper wind data set over the sea areas in Monex-79.

オメガ局からの3周波数の電波の位相変化の観測結果について

This paper describes the observed each absolute phase data of the three Omega frequencies (10.2kHz, 11.33kHz, 13.6kHz), obtained by a receiver with rubidium atomic standard, during a year (from June 1979 to May 1980) at a fix point on ground. From these results, the receiving condition of Omega wave was obtained for the each Omega stations. Among these Omega frequencies, 10.2kHz is the most stable and the most unstable frequency is 11.33kHz. In addition, we try to apply the method of modified composite wave Omega by W. Papousek to the data.

Omega Station 10.2 kHz Signal Coverage Prediction Diagrams

New Omega station 10.2 kHz signal coverage diagrams for eight month-hour combinations (February, May, August and November at 0600 and 1800 GMT) have recently been developed. The methodology of diagram development, coverage prediction models and a representative subset of the coverage diagrams are presented. Each diagram shows the predicted global accessibility of “usable” signals at a fixed global time for two signal access criteria: Criterion I, requiring signal-to-noise ratio to be greater than –20 dB (in a 100 Hz receiver noise bandwidth), and Criterion II, requiring signal-to-noise ratio to be greater than –30 dB. Both signal access criteria require that the actual deviation of the signal phase from that of the reference (“Mode 1”) signal phase be less than 20 centicycles. These diagrams show the seasonal/diurnal variations in the predicted coverage as compared to previously-published diagrams which display the predicted coverage for the two possible extreme cases of local temporal propagation conditions. These new coverage diagrams are expected to be useful to navigators and to those engaged in Omega navigational receiver design, chart construction, and other disciplines requiring information on signal coverage.

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.

コンポジット波オメガの検討とそれを利用した3距離(3-ρ)測位計算法

Composite Wave Omega Techniqes was proposed by J.A. Pierce and modified by W. Papousek and F.H. Reder in reference papers (1) and (3). These techniques hypothesize a composite wave whose propagation velocity is not affected by heights of ionospheric layers and their propagation time from Omega station can be calculated from propagation times of 10.2 and 13.6kHz Omega basic frequencies. This paper describes a evaluation result of these techniques and position determination calculation method using these techniques. Evaluation was carried by data obtained by a reciever with rubidium atomic standard as its local osillator and PPC tables from several Omega stations to Tokyo area. The calculation resolved three unknown parameters (longitude, latitude and frequency drift of reciever's local oscillator) by use of phase measurement data from three Omega stations. The results of positioning calculations are shown in Figures 7, 8, 11 and 12.

Optimum Position Estimation Using Multiple-Frequency Omega

The maximum-likelihood estimator of the line of position (LOP) resulting from K frequencies received from each of two Omega stations for high SNR is derived. The estimator takes advantage of interfrequency correlation of phase uncertainties by modeling them as resulting from phase velocity uncertainties which are jointly Gaussian with known covariance. The utilization of Omega for search and rescue systems is examined. For this application, the result of increasing the size of the unambiguous lanes of Omega by addition of a fourth frequency to the present three-frequency format is examined. The results of Monte Carlo simulation, performed for several easily implemented fourth frequencies, are presented. The simulation results illustrate the extent to which lane errors can be expected as a function of various fourth frequencies, phase velocity uncertainties, and SNR.

Wind Determination using Omega Signals

A procedure for determining vertical wind profiles using Omega signal phase information from any number of Omega stations (at least three) is given. A statistical method of smoothing the phase data is presented and weighted least-square solutions are derived to estimate the position of the Omegasonde, and the U (east-west) and V (north-south) components of the wind. The technique is then applied to the data collected at Wallops Island by the National Oceanic and Atmospheric Administration in December 1972.

Ambiguity Resolution in Systems Using Omega for Position Location

The lane ambiguity problem prevents the utilization of the Omega system for many applications such as locating buoys and balloons. The method of multiple lines of position introduced herein uses signals from four or more Omega stations for ambiguity resolution. The coordinates of the candidate points are determined first through the use of the Newton iterative procedure. Subsequently, a likelihood function is generated for each point, and the ambiguity is resolved by selecting the most likely point. The method was tested through simulation.

Potential Operational Advantages of Low Cost VLF/Omega Navigation Equipment

Evaluation flights of the Global Navigation Co. low cost, Digital Navigation System using existing Very Low Frequency (12kHz-30kHz) Communication and Omega stations were initiated by the Western Region of FAA early in 1970. The developers of the system, Dr. Karl Frudenfeld and Mr. F. P. Wipff, felt that the system would have an impact on navigation in remote areas of the world as well as in the National Aviation System, and were desirous of appraising FAA of its development status at an early date. More than forty FAA managers and specialists have flown or observed the system in indoctrination flights varying from 10 to 400 miles in length and including many approaches under VFR conditions. Direct contact with personnel involved in company funded development and survey flights of this equipment over most of the North American continent has provided additional data and confidence in the system's capability. Continually refined systems have been installed in over twenty types of airplanes and helicopters to investigate a wide range of variables of configuration, atmospheric variables, geographical considerations and other known or suspected factors in a VLF navigation system. The results indicate an excellent capability for accurate point-to-point navigation for flights up to 800 nautical miles in length (or greater with waypoint updating) as well as a capability and potential for use in conducting approaches and specialized navigation and survey functions, as well as compatability with other navigation systems and techniques. A formal FAA funded flight evaluation program is now being conducted by the Western Region under the sponsorship of the System Research and Development Service. The results of this program should be available early in 1972.