While survey sheet numbers 14a and 14b say what Sea-Fix is and does, and how it does it, the object here is to indicate the accuracy and operational coverage which can be expected of the System. Both these requirements influence choice of position for the Chain Stations.
Selection of Station Sites
The Sea-Fix System is not critical as regards the selection of suitable sites for the Chain Stations, although certain precautions should be observed if the best results are to be obtained. In general, similar considerations apply to Sea-Fix as to the operation of communications equipment working on ground-wave propagation at frequencies in the region od 2 to 3 MHz, but owing to its desgin features Sea-Fix operates successfully in areas of noise which preclude the use of communications transmissions in this frequency band.
Intended as they are for hydrographic or oceanographic use the stations of teh Sea-Fix chain may be installed on shore or afloat. When sited ashore a station should be installed ideally on level ground, preferably on moist soil rather than on rock or sand, and should be as close to the water's edge as possible. Clear avoidance of tall natural features such as large rock faces and trees is advisable, while contstructions like pylons, buildings, cranes, overhead traction wires, high tension cables and telephone lines should be given a wide berth. When sited at sea, in a buoy for example, the aerial should be fitted a few feet above water to avoid too much sea spray. Dependoing on the amount of l;ateral drift allowed, the type and length of mooring should be considered.
Position-fixing accuracy - Repeatability
Sites for stations (in both hyperbolic and ranging chains) are chosens to give a good angle of cut of the position lines (as near as possible to 90 degrees) in the operations area. It is useful to note that in hyperbolic chains accuracy can be improved by lengthening the base lines, since thi snarrows the lane width in front of the chain, a characteristic which applies to three-slave chains aslo.
For theoretical purposes it is convenient to consider Sea-Fix errors in two categories: first, those inherent in the nature of the system, known as random errors, and secondly terrain or 'fixed' errors. Random errors arise from fluctuations in the propagation medium, small variations in the equipment, the human reading of the lane counters and so on. They apply to each pattern position line, and can conveniently be lumped into a figure known as the 'Standard Deviation' of that position line. In Sea-Fix chains with stations situated on the shore and operating from short baselines the Standard Deviation is normally taken as 0.01 of a lane, but when the base lines are longer and operational areas larger, it may be advisable to use a larger figure. When the angle of cut of position lines, the lane width, the standard deviation and a probability factor are mathematically expressed together, a family of 'accuracy contours' for the chain is obtained. The figure noted on a contour would expect a repeat of the ofrmer erading within the limiting figure given on the contour; this is known as the repeatability at that point.
Position-fixing accuracy - Absolute
Radio transmissions travel in free space with the velocity of light. For chains with signal paths predominantly over the sea the propagation speed is slightly less, but the salinity of the surface water and the amount of land intruding into the signal paths influence this speed. Decca Survey Ltd has great experience in the selection of the correct propagation velocity and this experience is put to good use in the calibration of Seaf-Fix chains.
In a shore-based system, when teh stations have been surveyed into the geodetic net, the propagation speed has been established and the chain has been calibrated, the Sea-Fix readings can be converted into geographical or grid co-ordinates as required. However, certain residual or 'fixed' errors will remain. Should the project so demand, a table of these errors can be compiled so that the degree of positioning accuracy of which the chain is capable can be acheived.
In an oceanographic chain where the stations are waterborne, the station positions are located by astro-fix, Decca, Loran, satellite or whatever method is available at the time and distances between them are checked by lane counts. The ships course is plotted in geographicals at the desired scale. The absolute accuracy therefore depends upon the initial fix of the chain's position; the repeatibility accuracy is greatly influenced by the lateral drift of each station on its mooring.
Operational Coverage
The extent of the coverage depends largely upon the relative positions of the station sites, teh type of antennae used, and the amount of land intruding into the signal paths. A certain amount of chain planning is therefore necessary, and Decca Survey Limited offers a full consultancy service in these matters.
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