Decca Navigator Research Articles

Review of the Monothematic Series of Publications Concerning Research on Statistical Distributions of Navigation Positioning System Errors

This review presents the main results of the author’s study, obtained as part of the post-doctoral (habilitation) dissertation entitled “Research on Statistical Distributions of Navigation Positioning System Errors”, which constitutes a series of five thematically linked scientific publications. The main scientific aim of this series is to answer the question of what statistical distributions follow the position errors of navigation systems, such as Differential Global Positioning System (DGPS), European Geostationary Navigation Overlay Service (EGNOS), Global Positioning System (GPS), and others. All of the positioning systems under study (Decca Navigator, DGPS, EGNOS, and GPS) are characterised by the Position Random Walk (PRW), which means that latitude and longitude errors do not appear randomly, being a feature of the normal distribution. The research showed that the Gaussian distribution is not an optimal distribution for the modelling of navigation positioning system errors. A higher fit to the 1D and 2D position errors was exhibited by such distributions as beta, gamma, and lognormal. Moreover, it was proven that the Twice the Distance Root Mean Square (2DRMS(2D)) measure, which assumes a priori normal distribution of position errors in relation to latitude and latitude, was smaller by 10–14% than the position error value from which 95% fixes were smaller (it is known as the R95(2D) measure).

Statistical Distribution Analysis of Navigation Positioning System Errors-Issue of the Empirical Sample Size.

Positioning systems are used to determine position coordinates in navigation (air, land, and marine). Statistical analysis of their accuracy assumes that the position errors (latitude—δφ and longitude—δλ) are random and that their distributions are consistent with the normal distribution. However, in practice, these errors do not appear in a random way, since the position determination in navigation systems is done with an iterative method. It causes so-called “Position Random Walk”, similar to the term “Random Walk” known from statistics. It results in the empirical distribution of δφ and δλ being inconsistent with the normal distribution, even for samples of up to several thousand measurements. This phenomenon results in a significant overestimation of the accuracy of position determination calculated from such a short series of measurements, causing these tests to lose their representativeness. This paper attempts to determine the length of a measurement session (number of measurements) that is representative of the positioning system. This will be a measurement session of such a length that the position error statistics (δφ and δλ) represented by the standard deviation values are close to the real values and the calculated mean values ( and ) are also close to the real values. Special attention will also be paid to the selection of an appropriate (statistically reliable) number of measurements to be tested statistically to verify the hypothesis that the δφ and δλ distributions are consistent with the normal distribution. Empirical measurement data are taken from different positioning systems: Global Positioning System (GPS) (168′286 fixes), Differential Global Positioning System (DGPS) (864′000 fixes), European Geostationary Navigation Overlay Service (EGNOS) (928′492 fixes), and Decca Navigator system (4052 fixes). The analyses showed that all researched positioning systems (GPS, DGPS, EGNOS and Decca Navigator) are characterized by the Position Random Walk (PRW), which resulted in that the empirical distribution of δφ and δλ being inconsistent with the normal distribution. The size of the PRW depends on the nominal accuracy of position determination by the system. It was found that measurement sessions consisting of 1000 fixes (for the GPS system) overestimate the accuracy analysis results by 109.1% and cannot be considered representative. Furthermore, when analyzing the results of long measurement campaigns (GPS and DGPS), it was found that the representative length of the measurement session differs for each positioning system and should be determined for each of them individually.

The Genesis of the Decca Navigator System

The Decca Navigator System was very unusual among radio-navigation systems in that the entire system was championed and marketed by a commercial company. How it came about was also unusual – it was neither the product of market research nor a military specification. It was a solution to one of the major problems of the 1930s facing those who attempted to use radio methods for the location of vehicles, that of measuring extremely small time intervals. The problem was eventually solved by pulse methods (radar), which were kept very secret at the time, but Decca offered an alternative particularly suited to marine navigation because of its use of low frequencies. No matter how good a technical solution, many systems fail if they are not actively supported by proper marketing and in this Decca Navigator was fortunate to have the backing of a major British company, the Decca Record Company. This paper describes the inception, wartime trials and eventual acceptance into general marine use of Decca Navigator. Some previously unpublished historical material from the company archives is included.

Optimum width of harbour entry swept channel at cochin

The with is limited by the paucity of mine-sweepers that can be engaged continuously on the job. The width should be sufficient to cover the likely lateral sets in a ship's passage upto 30 fathom line. In calm weather and most of the channel length coinciding with direction of currents of any, the required channel width is less than 100yds. and may be swept again and again by the same two mine-sweepers (LL sweep). In rough weather or when direction of currents and drifts is different, in order to use the above narrow channel Decca navigation should be provided. Electric or Magnetic field of a Guide Cable can also be used; making buoys can be used but they are not entirely dependable on account of drifts or interference or use by enemy.

Mapping bathymetry using X-band marine radar data recorded from a moving vessel

Marine radars mounted on ships can provide remarkable insights into ocean behaviour from distances of several kilometres, placing other in situ observations and the environment around a ship into a wider oceanographic context. It has been known for some time that it is possible to map shallow water bathymetry and currents using radar image sequences recorded from shore based stations. However, a long standing question from military and hydrographic communities has been whether such techniques can be applied to radar data collected by moving vessels. If so, this presents the possibility of mapping large areas of shallow or coastal seas (albeit with a somewhat coarse horizontal resolution of 50–100 m) prior to the surveying vessel actually having to travel into potentially uncharted or dangerous shallow water areas. Trial sets of radar data were recorded by the Canadian Forces Auxiliary Vessel Quest using a Wamos radar digitiser connected to a Decca navigation radar during a number of deployments around Nova Scotia in 2008 and 2009. Georeferencing corrections derived from the existing ship navigation systems were sufficient to allow the application of the existing depth inversion analysis designed for static radar installations. This paper presents the results of bathymetry analyses of two datasets recorded from CFAV Quest while the vessel was travelling at speeds of up to 14 knots. The bathymetry derived from the radar data compare favourably with independent surveys and with the on-board echo sounder to depths of approximately 50 m.

Can old analogue sidescan sonar data still be useful? An example of a sonograph mosaic geo-coded by the DECCA navigation system

In the Baltic and the North Sea area the collection of sidescan sonar data began in the early 1970s. Although positioning systems at that time were much less accurate than today, such analogue data archives are a valuable source of comparison for estimating spatial and temporal changes in sediment patterns. This paper describes how geographic information systems (GIS) and remote sensing methods are being applied for more accurate positioning and geometric correction of analogue sidescan sonar profiles geo-referenced by the DECCA navigation system (DNS). A 20 km 2 area in the North Sea, where even after 25 years a complicated sediment pattern has only changed slightly, was selected for the comparison. Data from two sidescan sonar surveys, one from 1977 and the other from 2002, were available. The 2002 data, acquired with a positioning accuracy of estimated 25 m, were mosaicked using modern digital processing techniques. This dataset was used to estimate positioning errors in the analogue sidescan sonar profiles of 1977. After the application of a geo-referencing method of ‘rubber sheeting’ based on an irregular network of ground control points to generate a mosaic of the analogue profiles, positioning errors of 211 m in the E–W and 98 m in the N–S directions were obtained. The positioning error of the DNS calculated from equations provided by the DECCAN Navigator Company for this area, was as large as 200 m for the ‘purple’ station transmission, 197 m for the ‘green’, and 66 m for the ‘red’ one. Consequently, we conclude that the observed positioning errors were mainly caused by systematic deviations of the DNS rather than simplifications used in our methodology. Since such large errors can be corrected locally with some confidence, analogue sidescan profiles geo-referenced by DNS can be used as a significant source of information after digital image processing at appropriate map scales.

Decca Navigator closure ends an era

Decca Navigator closure ends an era

Digital signal processing of Decca Navigator radionavigation signals

New techniques are described for digital signal processing of Decca Navigator radionavigation signals. A simulator (a computer program) is used to investigate the effects of different digital filter characteristics of the accuracy of phase measurements and to find an optimum filter bandwidth. The performance of the optimum filter in the presence of noise and other sources of error is investigated. Methods are presented for compensating for initial errors and drift in the clock frequency of the system, and for synchronising to the pattern of transmissions.

The Decca Navigator's contribution to D-day

The Decca Navigator's contribution to D-day

Operating LORAN-C Position Fix with Nonhyperbolic Position Lines

Hyperbolic navigation systems ( LORAN-C, DECCA Navigator and OMEGA ) represent a widely used class of position fixingsyst.ems.Most of their incorporated traditional receivers display position fihyperbolic position lines and geometrical factors relevant to these lines. Avionics applications as well as automatic vehicle location systems entails low-cost receivers which operate wholly or largely automatically and which display position measurements in geographical coordinates (latitude -longitude) or as headings and distances to given waypoints and/or integrated with readings from dead-reckoning sensors.Solution issues to some of these problems have been addressed in two published papers by the authors.The underlying idea is to involve modifications in the processing algorithms of on-board receivers.In essence ,that modification was based on generating non-hyperbolic position lines :circles and/or ellipses out of the originally designed hyperbolic systems. Theoretical evaluation of that proposal was the subject of one the authors papers. In the present paper ,the proposed modification is applied to a real ON--EARTH LORAN-C station to confirm the obtained results. Moreover,an algorithm as well as an implementation proposals are presented .The case of short base-length as in the DECCA chains is presented with its algorithm and the proposal for implementation is highlighted.

New algorithms for hyperbolic radionavigation

Hardware and software aspects of the design of a microprocessor-based radionavigation receiver are discussed. The receiver uses the orange channel of the Decca Navigator transmissions, and is based on a design published by J.D. Last. Unlike Last's design, it computes a position fix by directly measuring the phase of the signal, using phase-locked-loop techniques, with software correction for interrupt latency. A phasedetermination algorithm is presented which compensates for local oscillator drift and for skew error due to movement of the vehicle during the measurement cycle, and which appears to be particularly resistant to errors due to skywave propagation effects. A further algorithm, based on number-theoretic techniques, is presented for performing zone identification by phase comparison using the master (6f) and orange (8.2f) transmissions only.

Effect of skywave interference on the coverage of loran-c

Receivers for the Loran-C terrestrial radio-navigation system are designed to distinguish pulses received by groundwave propagation from skywave interference components which arrive later. The technique, however, has significant limitations when implemented in receivers of finite bandwidth. The International Electrotechnical Commission and other authorities specify minimum standards of receiver performance in this respect. Drawing data from a range of sources, including Decca Navigator records, the paper proposes methods of predicting skywave-togroundwave ratio and skywave delay, the key elements of these specifications. This information is used to calculate the range limits due to skywave interference at various times and seasons, considering individual transmitters and both present and proposed Loran-C chains. The results demonstrate that the use of high transmitter powers is generally not justified. The paper points out ambiguities in the current minimum performance standards for receivers. It demonstrates that they are inadequate to protect users against the effects of skywave interference and identifies areas in which improved specifications are required.

The Accuracy and Coverage of Loran-C and of the Decca Navigator System — and the Fallacy of Fixed Errors

The proposal to develop an extensive North-West European Loran-C system, replacing many existing chains of the Decca Navigator System (DNS), has led to an intensive debate on the merits of the two navigation aids, especially in the United Kingdom. The paper reviews the principal sources of random and systematic position errors in the two systems. The wide range of DNS random errors, predominantly due to skywave interference, are compared with the Loran-C random errors, and typical coverage limits of acceptable repeatable accuracy are presented. The paper also identifies the factors which control the magnitudes of Loran-C and DNS systematic effects due to land paths. It demonstrates that differences between the two systems are substantially less than are predicted by simple models. Loran-C and DNS techniques for dealing with land paths are compared and the errors experienced by Loran users are shown to be reduced by modelling and publishing ASF values.This paper is based on a presentation made by the author at a technical symposium of the Wild Goose Association.

Position-logging Drifting Buoys Using Decca Navigator and Argos for High-Resolution Spatial Sampling

Abstract The need for a precision current-tracking system that could he deployed for up to 12 months as part of the U.K. Natural Environment Research Council's North Sea Project led to the development of a position-logging drifting buoy, which employs both the Decca Navigator and Argos systems. A specially designed Decca Navigator receiver within the buoy computes and logs position fixes, which are accurate to within tens of meters under optimal conditions. After recovery of the buoy, these fixes are transferred to an external computer for analysis and plotting. Real-time position monitoring (to a lower accuracy) is provided by an on-board Argos platform transmitter terminal. This also enables the buoy to be recovered on completion of the trials. The technical capabilities of the drifting buoy are described, and preliminary results, obtained during deployments of nine buoys in the vicinity of a tidal mixing front in the North Sea are presented.

Accurate current measurement using drogued buoys employing Decca Navigator and Argos

Free-floating drogued buoys have been developed for detailed studies of tidal processes, frontal circulation, eddies, and mixing in the North Sea. They are equipped with two independent tracking systems-Decca Navigator and Argos. Decca Navigator, a hyperbolic radio navigation system with extensive coverage in Western Europe, provides accurate and frequent fixes, and the Argos satellite system is used to monitor progress in real time and to assist recovery. The low power consumption microprocessor-based Decca Navigator receiver designed specifically for this application synchronizes to the precisely timed Decca transmission and then goes into sleep mode, waking briefly at 10-min intervals to take position fixes. The position data are stored in a 2-Mb memory card that has a three-month capacity at the 10-min rate. The recovery procedure, using both Argos and a small VHF beacon, is described, together with other recent developments. A typical buoy track from a study in the North Sea is included.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Loran-C Measurement Trials in Ireland and U.K.: Interference, Noise, and Field Strength Results

A program of measurements is reported which has confirmed that carrier-wave interference is the key factor controlling Loran-C signal-to-noise ratio in Northwest Europe. The tests, at eight sites in Ireland and the U.K., identified 68 interfering signals, 17 of which exceeded the worst-case atmospheric noise. Many interferers were synchronous with Loran-C spectral lines. Especially prominent among the most serious interferers were Decca Navigator signals. The paper discusses the need for efficient filtering techniques and, where notch filters are used, the development of strategies to balance the conflicting claims of high-powered, wide-area interferers with those of the many low-powered Decca stations.

Radionavigation in a time of change

The dramatic reductions in the cost, size, weight and power consumption of electronic equipment during the last decade have had an especially profound effect on the practice of radionavigation. Traditional users of radio aids now have equipment of smaller size and increased reliability and performance which provide many additional functions by virtue of their greatly enhanced computing power. At the same time a completely new consumer market has been created which has come to dominate the entire radionavigation industry. This process of change is illustrated by examining the development of low-cost receivers for three radionavigation systems: direction-finding, Loran-C and Decca Navigator. The paper then shows how the use of other marine and aeronautical navigation aids has changed and describes novel applications of radio techniques made possible by these technical developments.

Position determination at sea using low-frequency radio waves--The effect of a nearby mountain

Measurements of direction-finding (DF) error in position determination based on low-frequency (LF) radio waves were made at sea employing the Decca navigation system. The measured DF error is found to be large when a mountain is nearby. Assuming the mountain to be a hemisphere with complex dielectric constant, we evaluate DF error using electrostatic theory. Analytical predictions of the expected position error caused by the mountain's presence are qualitatively in fairly good agreement with measured values. In connection with this, fluctuations of the electric field intensities are also described.

Coordinate Conversion Techniques in Microprocessor-based Receivers for Hyperbolic Radio-navigation Systems

Traditional receivers for the low-frequency, hyperbolic radio-navigation systems – Loran-C, Decca Navigator and Omega – are expensive and are operated manually. They either display position fixes in hyperbolic coordinates for plotting by hand on navigation charts printed with hyperbolic overlays or they plot tracks automatically, often on distorted charts. Recently, low-cost receivers have been developed for marine navigation which operate wholly or largely automatically and which display position measurements in geographical coordinates – latitude and longitude – or as headings and distances to waypoints which are entered by the navigator in geographical coordinates.

Reception of Decca Navigator multipulse and zone identification transmissions by single-channel receivers

Receivers for the Decca Navigator system have recently been developed in which a single, frequency-agile receiving channel, operating under microprocessor control, replaces the five single-frequency channels of a conventional receiver. The paper describes techniques which allow such receivers to utilise the multipulse lane-identification and the zone-identification transmissions of the Decca chain which were designed for receivers with multiple channels. An algorithm is described by means of which the microprocessor calculates zones and zone fractions from samples of the phases of the transmissions received. Operational results, measured under conditions of night-time skywave propagation, are presented which demonstrate that the technique works successfully and that it has certain advantages over conventional methods of multipulse and zone-identification reception.