1. THE SYSTEM
Hi-Fix/6 is an electronic all solid-state and mainly digital position coordinate measuring system, intended for hydrographic use. The system is designed for position-fixing and distance measuring in any oceanographic or off-shore activity in which continuous knowledge of precise location within a specified area and/or distance from one or a pair of the stations to a point in the selected area, is required. The system is not subject to any propagation or reception limitations other than those experienced with all radio position-fixing systems.
1.1. Hi-Fix/6 basic principles
The basic principles of Hi-Fix/6 are as follows. A repeated sequence of bursts of continuous-wave radio transmission is made from up to six stations. The stations transmit in turn in the sequence and the transmissions are frequency stabilized and phase locked. Position along ordinates in the transmission fields is determined by integratin continuous sequences of instantaneous phase measurements. These phase measurements are made using transmissions from selected pairs of the chain stations. Each measurement compares the received phase of one transmission of a pair with the received phase of the other.
1.1.1. Method of measuring position
The method of measurement can be explained as follows. Consider the phase-locked transmissions to be continuous and suppose a phase comparison meter to be positioned on a straight line joining two of the stations. If the phase comparison meter were able to display the differences in received phase of both transmissions in relation to a stable phase reference, the reading displayed would be a partial measure of its position on the line. This measurement would be a fraction of a 360° phase change and could be dimensionally resolved in terms of the transmission wavelength divided by two. To clarity this, if the phase-meter were moved 90° of the transmission wavelength away from a point between the stations and along the line joining them, it would indicate a 180° phase difference from its reading before it moved. This would be because it would be comparing a signal received 90° away with respect to a phase datum, with a signal received 90° later with respect to the same phase datum. That is to say, a 180° indicated reading would result from a movement equal to 90° of the wavelength of the transmission frequency.
It can be seen therefore that if the phase comparison meter integrated 360° phase changer, a measure of its phase-position in relation to the two stations would be possible in terms of a count of increments of half a wavelength at the transmission frequency, plus a displayed fraction of half a wavelength.
In practice the phase comparison meter is a special receiver and the count of 360° (half-wavelength) phase-changes is known as a whole and fraction lane count.
The Hi-Fix/6 receiver makes phase measurements between signals received from selected time-separated station transmissions. The receiver does this by storing the received phase of all of the transmissions and up-dating the stored phase data during each transmission-sequence. Using this stored phase data, the receiver cyclicly measures the difference in the received phase of signals from selected pairs of stations, and displays a resolution of these continuous measurements as an integrated whole lane count, plus a lane fraction.
1.1.2. Circular patterns lattice
If the reference phase datum is the phase of the transmission from one of the two stations involved in a phase comparison and the receiver is co-sited with this transmitter, a graph of displayed phase change for changed position in the area covered by the transmissions takes the form of a number of circles concentric with the station remote from the one at which the receiver is sited. The circles join points of equal phase and for convenience are spaced at half wavelength distances along points of 360°/0° phase-position. Each circle is a lane line and the graph is known as a circular pattern. The straight line joining the two stations forming the pattern is known as the pattern base-line. Such a pattern enables one ordinate of a position-fix to be determined. Another circular pattern generated with the aid of a third station remote from the station having the receiver, determines the other ordinate of the fix. This two-pattern combination is termed a circular lattice.
1.1.3. Hyperbolic patterns lattice
Another form of graphical representation of changed phase position is obtained if the transmitter radiating the phase datum transmission is not co-sited with the receiver. In this case a constant phase-reading is given by movement of the receiver along the right-bisector of the line joining the two stations involved in a phase comparison measurement. For a receiver on either side of the right-bisector, lines of 360°/0° phase position take the form of hyperbolae whose foci are the transmitting stations. The graphical pattern of 360°/0° phase-position lines in this case therefore takes the form of the right-bisector of the line joining the two stations, with hyperbolae on either side which are confocal with their respective station. The graph is termed a hyperbolic pattern. In the same way as for the circular pattern, position co-ordinates in two hyperbolise patterns (termed a hyperbolic lattice) are necessary for a position-fix. Often one of the stations involved in generating hyperbolic phase comparison patterns is common, making a minimum of three transmitting stations necessary for a hyperbolic lattice, With Hi-Fix/6, commoning of one station to the base lines of both patterns is not essential and the hyperbolic lattice can be formed from patterns resolved for separated station pairs, thus permitting cross base-line fixing, which is the most accurate form of hyperbolic fix.
The number of circular and hyperbolic patterns that can be formed from the time-multiplexed Hi-Fix/6 transmissions is in proportion to the number of stations comprising a chain, the number of phase memories provided in the receiver(s) and the selected mode of chain operation.
1. 1. 4. Use of lattices
For navigational and survey use a lane-boundary lattice is superimposed over or drawn upon a chart or map of the area covered by the chain transmissions. Lattice scaling and orientation, and the numbering of lane lines, are arranged so that when the phase-position (lane) readings provided by the receiver displays are plotted on the chart or map, they indicate the position of the receiver, within the defined limits of equipment accuracy and within the limitations imposed by natural phenomena.
Pattern and lattice geometry and the number of lattices possible in a deployment of stations depends upon the number of stations, their relative dispositions, and on whether one of the transmitting stations is moving.
Along the base-line of hyperbolic and circular patterns the distance between lane lines is equivalent to half the wave-length of the chain transmission frequency. This arises from the fact that the pattern is created by the receiver moving in the area of coverage, consequently, along the base-line, a movement away from one station is an equal movement towards the other, thus giving a 360° phase-change in reception of phase-locked transmissions, in half a wavelength. In the circular case this always remains true. In a hyperbolic lattice, pattern lane-lines diverge as they recede from the base-line, resulting in decreasing accuracy with distance. With Hi-Fix/6 this reduced accuracy can be minimized by utilizing a crossed base-line lattice effective over the survey area.
1. 2. Hi-Fix/6 user facilities
Any number of receivers may use the transmissions of a hyperbolic chain and thereby derive accurate position, information in the area of coverage. Each receiver can select its own pattern-combination for a lattice.
1. 2. 1. Fine pattern facilities
For high-accuracy measurement of off-shore position coordinates, Hi-Fix/6 offers provision for up to four vessels to use circular pattern (ranging) facilities. In the general case two shore-based stations work with stations on board each vessel and circular patterns are generated between each ship and each shore station. Position is displayed as two lane readings representing the straight-line distance of the receiver from each of the fixed stations.
For simple circular mode working only one fixed station is used at a time and position is defined as the locus of a point whose lane distance from the fixed station is displayed on one of the readouts.
In brief, a number of configurations are possible with the stations, depending upon the user's requirements. These are :
Hyperbolic mode : for multi-user operation, employing between three and six stations, according to the extent of the coverage required.
Two circular patterns mode : for up to four users, employing two or three secondary stations depending upon the area to be covered.
Single circular pattern mode : using between one and five fixed stations for radial measurement of speed and distance, or for sea-search by utilising the concentric lane-position circles as navigational position lines, up to the limit of the area to be covered.
Compound (hyperbolic and circular mode, together) : up to three circular facilities are available in addition to the hyperbolic lattices.
1.2.2 Coarse pattern facilities
In addition to the fine position - coordinate measuring and display facilities described above, Hi-Fix/6 simultaneously can provide a coarse position readout. This is derived by radiating from each station a second pulse of continuous wave transmission of the same width as the first and immediately following it. The frequency of this second transmission (f2) is a small percentage less than the fine pattern transmission frequency (f1). Circuits in the receiver compare the phase-angles derived from the f1 and f2 transmission patterns and the difference is displayed as a three digit coarse pattern reading. The displayed reading gives tenths, hundredths and thousandths of 360° phase difference between the f1 and f2 patterns.
Use of the coarse pattern display is made in the following manner.
The lane numerical value of receiver position in a coarse pattern can be known from a Main Chain Decca-derived position or from rough knowledge of position. The numerical value of the fine pattern lane nearest to the receiver position then can be determined from the fraction reading of the coarse pattern, thus permitting the displayed fine whole lane count to be set or reset. Finally the precise position of the receiver in the lattice is determinable from the tenths and hundredths readings of the fine pattern displays. The Hi-Fix/6 coarse pattern display therefore facilitates initial setting of the fine pattern display and permits almost immediate re-acquiring of position data should receiver lane-integration be interrupted for any reason.
Separately switched digital phase- shifting facilities are provided for both f1 and f2 transmissions from each chain station. Thus fine patterns can be aligned with geographical features if necessary, or a lane-line aligned with one station site of a base-line pair. Also, the coarse pattern independently can be aligned with a particular lane or lane sub-division of a fine pattern.
Note: fl and f2 identify transmitted frequencies.
Fl and F2 (used later in text) identify transmitted frequencies plus 100 kHz.
1.3. Special applications
Due to the increased number of stations allowed for in the design and the flexibility inherent in the thirteen-slot time-multiplex, Hi-Fix/6 offers an easier solution than hitherto possible for some surveying problems.
Mention already has been made of the increased accuracy inherent in a crossed base-line lattice. This ability to select at the receiver any three of the base-lines of a chain for measurement and display, also permits (for example) easier 'leap-frogging' during a coastline survey, when the six-station capability offers the effectiveness of two linked hyperbolic chains. Similarly, when six stations are disposed along either side of a curved waterway, continuous and precise fixing can be made available.
Another advantage offered by Hi-Fix/6 is that significant use can be made of the more stable across-water transmission path, even when the hyperbolic chain base-lines in use run across land. Referring to Figure 1, it can be seen that the phase-datum for the chain illustrated is established mainly across water. The prime station site in the illustrated example (a headland) could equally as usefully be an off-shore oil-rig.
The advantage offered by establishing the phase-datum across water can be increased when terrain between two (or more) of the shore-based stations of a hyperbolic chain, is mountainous, thus precluding a chain-lock by any other means. It is emphasised that in such conditions not only can a chain be properly established but that the patterns from the on-shore base-lines have the stability offered by the across-water phase-lock.