Compass of the Future: Gyro, FOG or GPS
The world's entire commercial fleet has one thing in common today: Ships steer, almost without exception, according to a gyro compass. Even a hundred years after its invention, the gyro compass is still part of the standard equipment. Its need for regular maintenance is a disadvantage, however; the high-precision bearings of the precision gyro are subject to wear and must be replaced after several years of operation. Reason enough for the manufacturer to seek alternatives.
When the so-called Fibre-Optic Gyro (FOG) came on the market in the middle of the nineties, the maritime world at first had great hopes for the new technology, which operates without mechani-cally moving parts. High accuracy and low prices were expected as well as maintenance-free, more economical operation which would render the regular replacement of gyrospheres required by conventional gyro compasses unnecessary.
After a few years, however, it became evident that the purchase price of the fibre-optic compass was in no way lower than that of a gyro compass; in fact, it was more than three times as high. The reason is clear: Aside from the very costly manufacturing process, the material costs in particular for photoconductors were a burden, since their prices did not fall as drastically as expected. For an FOG-compass several kilometres of photoconductors are needed.
Regarding the maintenance costs for the FOGs, little experience has been gathered as yet, since most of the equipment has been in operation for only a few years. Experts estimate the average life span of a fibre optic compass sensor at about ten years. Then a complete overhaul by the manufacturer is necessary including an extensive calibration, which cannot be carried out on board but only at the factory. Altogether this is a procedure which is more difficult and considerably more expensive than the replacement of a conventional gyro compass sphere.
This results in a logistic problem as well: The high-quality FOG sensors must be quickly available for ships all over the world. To sum up: Longer running time but higher costs, therefore no real improvement.
As far as accuracy is concerned, the fibre-optic compass is even clearly inferior to the traditional gyro compass. FOGs achieve a dynamic accuracy of 0.7 degree and thus fulfil the minimum requirements of the IMO, but they are not a technical improvement in this regard. Conventional gyro compasses already achieved an accuracy of 0.4 degree in the 70’s1.
This is also the reason why the fibre optic compass has hardly been used on commercial vessels at all. Eight years after the introduction of this technology, less than five per cent of all ship newbuildings in the world are outfitted with FOGs. In the meantime demand is dwindling.
After the introduction of the FOG, development did not stand still and in 2002 something entirely new came on the market: the so-called “GPS compass". It is possible today to determine the heading of a ship with the help of two GPS antennas. The two antennas are mounted on a bar and measure the phase relationship of the satellite signals received. From the difference between the measurements the heading is calculated. Additional acceleration sensors and speed sensors support the heading calculation. Depending on the quality of the signal processing, the arrangement of the antennas and the calculator algorithms, heading accuracies of better than one degree can be achieved. This lies within the IMO accuracy requirements.
The navigation world was immediately interested in this new technology. GPS sounds as if it should offer reasonable purchase costs and freedom from maintenance, both qualities which had been desired from the FOG. Today there are actually GPS compasses which cost only half as much as a gyro compass, or less. An attractive alternative, then?
In principle, yes. If only there were no reliability considerations. In using a satellite compass one first accepts the fact that the course of a ship is no longer determined by an autonomous sensor on board, but is now depend¬ent on external systems and their reliability. Again and again there are reports of intentional and unintentional interruptions in GPS satellite operations and corresponding consequences for users. As yet there is no alternative of equal quality to the satellite system of the USA which could provide more reliability. The USA continues to reserve the right to cut off selected individual areas from use during times of crisis.
Only a second, additional satellite system which is independent of GPS would reduce this risk. Once hopes were pinned on the Russian GLONASS system; for some years there has also been the European project GALILEO. Theoretically the Russian system is available worldwide, but the highest accuracy is achieved only over Russia, because the operators have no control over earth stations in the West. Thus worldwide use is not in sight at present. GALILEO, on the other hand, is intended for use all over the world, but as things stand now it will not be ready for operation until 2012.
If in the future so-called hybrid receivers could make use of two satellite systems which were independent of each other, there would be real redundancy and thus the necessary reliability. Unless, of course, the operators in the USA and in Russia should want to interfere with or shut out each other for strategic reasons in times of crisis; that too would be technically feasible.
The possibility that unknown third parties as well might interfere with the GPS reception concerns the US authorities in particular and has moved more and more into the foreground recently. This process is called "Jamming“. With very little technical effort, small portable interfering transmitters can be built with which the GPS reception in a radius of over 100 kilometres can be rendered ineffective. Due to their low emission power, it is difficult to locate these microwave-jammers.
Potential causes of interference therefore need not be on board themselves, but could work from land or even lie at sea at a safe distance from the coast. Up to now this danger was considered slight, for who would be interested in creating such disturbances?
An equally great although quite everyday risk is always present on board: interferences caused by the ship itself. Shadows and reflections from masts, smokestack and deckhouses influence the GPS heading as well as irradiation from other transmitting antennas. The extremely weak microwave signals of the GPS process are almost swallowed up in the general noise of the atmosphere and are very hard to filter out.
Only if the antenna is mounted optimally without sources of interference on board can a reliable GPS heading be determined. It is well known that GPS-signals (1.575 GHz) are influenced by Inmarsat transmitters, which transmit at almost the same frequency (1.525-1.660 GHz). An S-band radar can also cause interference, since it operates at exactly double the frequency (3.050 GHz).
Ideally the antenna of a GPS compass should always be located at the highest point, a demand which is made by the antenna manufacturers of almost all radio- and communication systems. This is hardly possible. The limited space on the upper deck and the constantly increasing number of antennas are an ever-worsening headache for shipyard engineers and customer service technicians alike. The result is more or less satisfactory compromises.
For normal radios and even position receivers such disturbances might perhaps be acceptable; a few minutes without the ship's GPS position could possibly be accepted by the nautical officer. The heading, however, is a different matter: Disturbances or deviations are fatal here, because they take immediate effect. Autopilots react to every change in the compass course and always set off an immediate turn of the ship. Therefore the GPS compass is not acceptable as the main heading source for navigation. For this reason the outfitting regulations according to SOLAS and IMO do not approve this technology for use in large ships.
If the developments in the various compass technologies are compared with each other, the conclusion is in general that autonomous procedures on board still offer the greatest reliability. Satellite courses are no more accurate, and they bring considerable risks endemic to the system as well.
In the meantime, gyro compass research has not remained stationary. Like the diesel motor, gyro technology has been continually refined: “Quick settling” and “automatic error correction” are standard today, parts subject to mechanical wear have been largely replaced with robust long-lived electronics and optical transmission technology. The lifetime of the mechanical precision gyro has been further improved and instead of requiring the former annual overhauls, modern gyro compasses need no maintenance for at least three years.
Ship newbuildings today have one or two gyro compasses, with the possibility to switch over to the magnetic compass which is still obligatory. The user selects the desired heading sensor at the touch of a button; the equipment automatically allows for the corresponding correction values for speed error, deviation and variation. All indicators always show the true north heading and show in addition from which compass the heading was taken.
A fibre-optic compass could also be integrated into the system, but due to the disadvantages discussed above, FOGs play almost no role nowadays. The GPS compass is of more interest. If a GPS course is desired in addition to the gyro course and magnetic course, a GPS compass can be integrated into the system and then there are three sensors to choose from. The GPS compass however is only as "reserve“; the gyro compass remains the first, most reliable and most accurate source of the ship's heading.