Complementary Radiolocation Techniques For Improved System Performance
Authors: Barker A. Clifford , Cottrell George A. 
Published: October 2008

Complementary systems for long-distance offshore position measurement are finding increasingly widespread acceptance. The high cost of downtime compared to the small cost of employing an additional means of position measurement is an important factor leading to this acceptance. By greatly reducing interference- caused downtime, multiple systems can be used to achieve virtually around-the-clock operations in areas where operations were once limited to shorter periods.

This paper describes operations using Raydist in combination with other radiolocation systems. Other potentially useful combinations are suggested, including a redundant implementation of Raydist alone. Benefits realized by matching differing system characteristic are assessed.

INTRODUCTION

Three distinct methods of expressing accuracy are important for offshore radio-positioning. They are:Sensitivity : The minimum change of position which can be measured. This describes the system's ability to discriminate small position changes.Repeatability Accuracy : The ability to reestablish a location in terms of previously measured coordinates. Repeatability accuracy excludes errors due to inaccurate knowledge of station coordinates and other charting type errors. Also, fixed propagational errors will not contribute to repeatability error.Absolute Geographic Accuracy : The overall accuracy of determining geographic position. All sources of error, including chart projection errors are included.

Repeatability accuracy is a primary factor, being a measure of the ability to revisit an exact location of interest. In the case of multiple-sensor systems, sensitivity can also be quite important. For example, in a fine grain geophysical survey, it is .essential that the system used for primary positioning possess sufficient sensitivity to assure that no holidays are left. At the same time, tolerable geographic error may be much larger than either sensitivity or repeatability. Because of its exceptional sensitivity and high repeatability accuracy, Raydist is ideal for fine grain surveys. For around-the-clock operations at offshore distances greater than 150 miles, systems such as satellite, Loran C, Omega, inertial and Shoran can provide a useful combination with Raydist for verification of overall geographic accuracy. The discussion which follows describes several combinations of Raydist with other navigation systems and also considers a redundant configuration of Raydist alone.

RAYDIST DR AND DRS-H

Systems described in this paper employ either the DR or DRS-H form of Raydist with other radiolocation systems. These modern forms of Raydist should not be confused with older systems, some of which are still operational. During more than 25 years of use, Raydist has undergone numerous technological refinements. Hence, older Raydist systems are in no way representative of modern Raydist technology.

In the range-range mode, the DR and DRS-H Raydist systems employ two shorebased stations which operate at approximately 1.6 mHz. A mobile CW transmitter is operated at 3.2 mHz, the primary measurement frequency of the system. Base station signals received at the ship are doubled and hetrodyned with the mobile CW signal to generate audio tones at approximately 400 Hz. The base stations, in turn, receive the mobile OW signal and generate audio tones at identical frequencies.