Positioning Systems

This was an underwater positioning system made by Decca Survey where beacons were deployed to the seabed and then their positions were measured according to accepted survey techniques. The mobile unit(s) interrogated the fixed beacons in turn measuring the round time for getting an response back. The resulting range measurements were used to compute a surface position in X,Y,Z, and provide this to navigating packages. It was used in the 70s and into the 80s, it has since been supplanted by improved but similar equipments from Sonadyne, and from Simrad, to name two.

Also known as RSDN-20, Alpha is a Russian long range Radio Navigation System, which operates in the Very Low Frequency (VLF) band. It is thought to date from the 1950's and there are five known transmitters, located at Novosibirsk (master station), Krasnodar, Khabarovsk, Revda and Seyda. The transmission format is six time slots each 400mS wide and followed by a 200mS gap, making a total cycle duration of 3.6 seconds. Each site radiates a CW carrier in its appointed time slot, on one of the frequencies 11.905 kHz, 12.045, 12.091, 12.649 kHz and 14.881 kHz. Determination of position is by comparing the phase at some common base frequency, however it is obscure how this is derived. The system is still operational at the time of writing and can easily be heard late at night using a VLF receiver of the 'whistler receiver' type with a short whip antenna feeding a very high impedance amplifier. More in depth information about Alpha may be found here, and details of VLF receivers of the type mentioned above here.

Artemis is a short range positioning system developed by Christian Huygens Laboratorium (CHL) of the Netherlands and taken in by the Nederlands Survey Apparaturen BV (NeSA). Thus it came into the Racal-Decca sphere in stages. Operating at 9.7-9.8GHz, the system offered exceptional accuracy (0.5m range and 0.1degree azimuth) and a range of up to 30Km. At short ranges out to about 1.5km this related to +/- ? m beyond this the two elements tended to separate so that range accuracy meant one kept a good distance measurement from the FIXed unit but the angular measurement degraded relative to the distance. Its an easy mathematical relationship – the range in metres is multiplied by the sine of 0.1 deg to give an error value.

The Mobile and Fixed installations consisted each of two Units, an Antenna Unit and a Control Unit; MAU, MCDU, FAU, and FCDU respectively.

Readings were of transmission time from the MAU to the FAU and back again, recomputed to metre ranges; and of an angular measurement made at the FIX-ed station relative to a known (surveyed) point. These were then transferred to the Mobile for use there. The transmitters had an output power of 100mW in the high setting and 3mW at low power/short range. These were the CW ratings so as the transmissions were interrupted at the modulation rates effective power was less. Related to a torch bulb one would have barely seen the glimmer!

It was originally designed in Holland for use in the harbours and waterways there and later typically used with the fixed stations installed on oil production platforms and on other fixed sites, and used to provide positioning to either supply vessels and diving tenders for dynamic control of vessel position. Or as a survey package for a specific survey vessel working in the in the immediate area.

As it was a one-to-one package multiple users created problems for the technicians in avoiding mutual interference. These were to a large extent counteracted by extending the number of frequency pairs accessible and by the selective addressing facility.

There have been many well known versions used since the late 70’s, the Mk3 which has provided the basis for these comments, the Mk4, a development with microprocessors and slightly more user-friendly packaging, and the Mk 5 which is still in current production (2015)

The benefits, features, technical specifications and a downloadable pdf brochure are available from the web site of Guidance Marine Limited who currently own the Artemis brand.