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.

Argo was a product of the Cubic Corporation of the USA. It was also known as DM54. It was a positioning system using phase comparison techniques and operating in the 1.6-2.0 MHz band. It offered accuracies of better than 20 metres at the maximum range of 400km. The way in which Argo operates is different to that of Hi-Fix or similar systems; it operates primarily in the range-range mode with up to seven mobile stations being able to use the same shore stations by means of time sharing. There was also a hyperbolic mode that allowed an unlimited number of users to share the system.

An Argo chain would consist of a master station and up to four slave stations. The transmission cycle is two seconds long, and is divided into seven 'sub-cycles', each of which is further divided into 44mS time slots in which a station may radiate a 36mS long RF pulse- except for the first time slot in a sub-cycle, in which the master station transmits a triggering pulse which is 120mS long.

The cycle commenced when the master station radiates a trigger pulse that, because of its length, is uniquely identifiable by all stations. On receipt of the trigger pulse, the first ship-borne (mobile) station would transmit an interrogation pulse in the next time slot. This pulse was received by all of the slave stations, which measured the phase difference between the interrogation pulse and their own synthesised master oscillator. Each of the slaves then replied in turn in one of the four following time slots, with an RF pulse whose phase was adjusted to replicate that of the received phase. The mobile receiver received all of these replies, and computed its postion by comparing the round-trip phase delay to each slave station. Following the next trigger pulse, the next mobile would transmit and so on until all seven will have obtained positions. There then followed a period of silence before the cycle repeated. Note, the position was still only determined to within one ‘lane’.

Lane identification could be used and required a second radio frequency approximately 10% higher than the main. This facility was not continuously active, the mobile operator having to initiate such a lane-check. Transmissions were then made alternately on the prime and on the lane identification frequencies until the control unit had computed the lane error, which was presented to the operator as a number of lanes. There is the possibility that this was made continuous in a late modification.

Argo stations comprised a Range Processing Unit (RPU) and an antenna coupling unit. The arrangement was the same for all stations, except that the mobile required an additional Control and Display Unit. The transmitter output was up to 100 watts. The units were pre-programmed with up to 16 frequency pairs, and no retuning was required over a 200kHz switching bandwidth. The transmitter incorporated SWR monitoring and would reduce the output power to safe levels when working into a poorly matched load. Antennas could be of between 8 and 30 metres length.

One special feature of an Argo transmitter installation was the phase measuring loop antenna attached to the transmitter antenna. This was used to check the phase of a transmitted signal so that it did have the required phase. This was probably the best solution to the problem of variations around the antenna site, e.g. different moisture levels in the soil of a shore site.

Another feature of the system was the ability to integrate with a variety of navigation or processing computer packages due to several flexible interfaces; GPIB/IEE488 parallel, and RS232 serial out, both being provided as standard.

Some Argo chains were deployed around the N. Sea in the mid 80’s.

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.:

http://www.guidance.eu.com/artemis

BRAS and RS-10 are similar hyperbolic position fixing systems which continue to be used in the countries of the former Soviet Union. Their transmissions, thought to originate from chains located around the Baltic coast, may be heard on frequencies around 1810 kHz. The BRAS system appears to comprise a master and two slave stations that transmit sequentially, with a cycle time of about 0.9 seconds. Unusually, instead of a continuous-wave (CW) transmission the chain stations are modulated by a series of pulses at 820 Hz repetition rate, so that the transmission sounds like a series of carriers 820Hz apart when tuned on a conventional receiver. RS-10 uses the same transmission format but allows for a master and up to five slave stations.

[more info on this site: link to follow]