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MF Positioning Systems

Anyone who listened to or worked the 160m amateur band or 'top band' is it is universally known, or indeed the marine frequencies around 2 Mc/s will likely remember the 'jingle bells' sound of these systems. In the UK, shore-based chains of stations operated in the MF range at around 1900 kc/s for HiFix, and 1800 and 2100 kc/s for HyperFix. For many, the sound is nostalgic; for others a reminder of a nuisance we were well rid of. Whichever way you look at it, this is a piece of our history and I hope that, through this web site, I have adequately recorded this fact.

Introduction

The same Decca company that was responsible for the low frequency Decca Navigator hyperbolic navigation system saw that a market existed for smaller scale but more accurate systems that could be used for accurate survey work. During the 1960's and 70's the rise in offshore surveying and in particular the North Sea oil and gas exploration generated a need for a positioning system effective at large distances from the shore yet with an accuracy down to a few metres. Decca, who had operated the Low Frequency Navigator system since 1946 realised that a system working on the same basic principle but using a higher radio frequency would provide improved accuracy at the expense of range, but nevertheless would fulfil the requirement. Development began in 1960 and the result was Hi-Fix, launched in 1962.

Features

Hi-Fix was a precision hyperbolic positioning system with a maximum accuracy of 1.5 metres at a range of 50 kilometres from the transmitters. It was originally intended as a portable system that could be deployed wherever survey operations were taking place but many installations remained in place for a very long time. During its life it was subject to improvement and some development, resulting in the Hi-Fix-6 variant that, with the aid of more transmitters and the ability to average a number of 'fixes', offered improved accuracy. However the development potential was limited, and Decca who by that time had become part of Racal, began work on a third-generation positioning system, HyperFix. This offered better range than Hi-Fix, and could be used with typically 6 but up to 18 transmitter sites so could cover a vast area from a fixed chain. HyperFix chains began to replace Hi-Fix beginning in 1983 and continued to operate until closure in 1995.

During the 1980's the GPS satellite system started to become operational, having been long in development. By 1990 the technology was mature and largely accepted by the surveying industry. Differential (DGPS) techniques were evolved to counteract the US government's Selective Availability and other errors and this resulted in an accuracy down to metres. None of the MF radio positioning systems could offer the accuracy and repeatability of DGPS, without having to set up a chain of transmitters in the area of interest. Additionally these chains would only cover a relatively small area whereas GPS would be received almost anywhere on the earth's surface. The MF systems could not survive faced with this competition, and they started to disappear.

The UK fixed HyperFix chains were mostly decommissioned during the mid 1990's within a year or two, although a few operated by the MoD remained in service until 2000. There is still however one remaining vestige of HyperFix operation in the UK and that is the North Sea DeltaFix chain which looks set to continue for some years yet.