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 Some previously unpublished historical material from the company archives is included.

This paper is the fourth chapter of a series on Air Navigation Systems during theperiod from the early oceanic flights and the inception of commercial aviation to the introduction of INS in civil aircraft. Comments on the content of the paper would be welcomed by the author and the editor.

1. INTRODUCTION. The American Global Positioning System (GPS/Navstar) and the USSR's Glonass are superb demonstrations of high technology but we should not forget that they were designed and implemented for a very different set of circumstances from that of the civil surveyor and navigator. Many of their features are not required for civil purposes and it is perfectly possible to get excellent results for many civil applications without the vast expenditure that went into those systems. This paper describes some of the ways in which it is proposed that satellites, not necessarily launched for the purpose, might be, and are being, exploited for civil navigation and location.

I read Captain Gyldeń's account of failure of his GPS set (Vol. 50, 328, May 1997) with some interest. A few days earlier we had had a few thunderstorms while I had my personal hand-held GPS running, using its own built-in antenna and a 12 volt battery. It was lying on a bench inside a wooden hut where I keep some of my amateur radio equipment. After one particularly close flash and bang, which produced a one-inch spark from my transceiver antenna, the transceiver locked up and stopped responding to keyboard commands. Then I noticed the GPS set had also stopped working, showing only random symbols on its readout. I feared the worst but, after switching them both off, leaving them for a few minutes, and then back on again, they worked perfectly.Microprocessors locking up in strong local electrostatic fields, perhaps? Maybe if I had simply left them alone they would have started working again after the charge had leaked away. Next time I nearly get hit by lightning I'll try it.

No navigator likes to be totally dependent on only one navaid-it is an article of faith for many that there should always be a backup system. Several systems have been put forward as possible backups for a GNSS but they seem to have originated more in a generalised feeling that there ought to be one rather than a dispassionate examination of what is involved. GPS/Galileo are radical departures from any previous concepts of radio navigation aids and a full-blown GNSS is an even more radical proposal. There is a good deal more involved than simply engineering and technical matters. There are the questions of who controls them; what the customer interface is; who certifies them for use in safety-related situations; and what legal recourse there is. On the answers to these questions depends whether a backup is needed and if so what form it should take. It is found in this paper that for many non-critical users there is no need for a backup, and that others who may be involved in safety-critical situations already have a backup in the form of their current systems. It is also found that in fact it may be extremely difficult to compose a GNSS in the form it is generally given; that is, a combination of GPS, Galileo and perhaps Glonass. The problem lies not in on the engineering side, but in matters of legality and the sovereignty of individual nations. For these reasons it is concluded that the development or implementation of a new system purely to act as a backup for a GNSS is not necessary.