Hyperbolic Navigation Systems
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Hyperbolic navigation systems Hyperbolic navigation systems are created by establishing a specific hyperbolic radio pattern over a geographical area. A hyperbola is a locus of all points in a flat plane that have a costant difference of distance from two fixed points.
ACCURACY OF Loran-C Accuracy is affected by: 1-The accuracy of the trnasmitters 2-Variability in the speeds of propagation of the transmitted signals 3-Geometry of the transmitting stations 4-Use oflat/long conversions versus Loran-c LOPs 5-Receiver quality and sensivity 6-Accuracy of Loran –c charts 6
Plotting present position on chart You are using 7980 Southeast U.S. Chain and tracking the W/Z secondary pair. Your present position is displayed on the readout as.
W 13407.6 ms TD Z 58948.2 MS TD
To Determine Your Exact Position You Must Use An Enterpolator 8
LORAN (Long Range Navigation) Over the years, many ground based electronic navigation systems were developed prior to satellite based systems becoming possible. Most of these have been taken out of service, but LORAN still enjoys popularity and is still widely used.
LORAN (cont) The operating principles of the system are similar to those now used by GPS. Ground stations, at known locations, transmit signals at a predetermined time. These signals are received by vessels at sea, and the time delay is used to form a range LOP.
LORAN (cont) First established in 1957. Loran C is expected to remain operational until at least 2015.
Theory-hyperbolic LOPs based on time differences between master and secondary stations. Run by the Coast Guard Frequency: 100 kHz (LF) Range: 1200 nm Accuracy: 200-500 yds 16
1- System introduction Like the Decca Navigator system, the Loran C is a medium range electronic position fixing system based on hyperbolic principles. In general, the base line and coverage area of a chain is slightly greater than Decca. Most of the chains are operated by the US Government Chains are now being developed by Saudi Arabia, North West Europe and Japan.
A chain comprises a master and from two to four secondary stations (sometimes referred to as slaves). The difference in arrival time of pulsed transmissions is a measure of the vessel's position within the hyperbolic pattern (Figure 7.1).
Detailed information regarding the transmissions of the various chains is promulgated in the Admiralty List of Radio Signals (ALRS), Vol. 2 (NP 282), as are coverage diagrams.
Components Of The Loran System 1-transmitting stations, 2-the Loran receiver and antenna, 3-the Loran charts.
There is a wide variety of on-board Loran C receivers, but in general the information is displayed in one of two ways: 1- As latitude and longitude, in which case it can be plotted in the normal way on a standard navigational chart; or
2- As a time difference or TD (usually two are displayed simultaneously). In this case a special lattice chart is needed. Some of the lattice charts are normal navigational charts with a lattice overlay printed on them. These are kept corrected via Notices to Mariners and so can be used for navigation in the normal way . Other charts are just plotting charts where, after the position lines have been plotted, the position obtained has to be transferred to the navigational chart. 22
All Loran C stations transmit at the same frequency (100 kHz) and so, in order to distinguish the signals of one chain from those of an adjacent chain, the pulse repetition rate of stations in the one chain is different from that in the adjacent chain.
The Interval between the pulse transmissions for a particular chain or group of stations is known as the GROUP REPETITION INTERVAL (GRI) and is used to identify or designate the chain.
Within the chain, the transmission sequence is always master followed by secondaries in the order W , X, Y , Z.
Each secondary's transmission is delayed by carefully calculated amounts such that, no matter where a vessel is within the coverage area, the signals will always arrive in the order in which they are transmitted.
In order to distinguish the master transmission from that of the secondaries, the master transmits a group of nine pulses, while each secondary transmission comprises an eight pulse group. The numbering of the lattice lines is lowest on the secondary baseline extension and increases toward the master baseline extension.
2 -Setting-up procedure With there being so many manufacturers in the market it is not possible to give a procedure which will satisfy all equipments, but certain operations apply to the majority of sets.
2.1 The notch Filters These are provided on most sets to allow the operator to suppress signals at frequencies near to the 100 kHz Loran C transmissions.
Interfering signals may come from 1-equipment already on board the vessel, 2-others may be inherent in the area through which the vessel is passing. In both cases it is best if the source of the signals can be removed. For signals generated ashore this is not the task of the ship, but such interference should be reported. 31
It is important that the fılters are not set too close to 100 kHz as they could then eliminate or impair the incoming Loran signals. Interfering signals should be checked periodically and the notch filters adjusted as appropriate.
2.2 Setting the GRI Since all Loran transmissions are on 100 kHz, the receivers are pretuned to that frequency . The chain selector is set with the group repetition interval (GRI), e.g. 7970; all stations in that chain transmit at intervals of 79 700 microseconds.
Loran-c GRI rate table(1997) 34
Thus the receiver can look among the welter of signals coming in at 100 kHz for signals which recur at that rate, and lock on to and extract them for further processing and finer measurement of the time difference between the master and each of the various secondaries (Figure 7 .2).
Pulsed transmission 1-lower power output requirements, 2-better signal identification properties, 3-and more precise timing of the signals.
2.3 Obtaining the TDs When the operator has switched on, set the notch filters and selected the GRI, the receiver should automatically lock to and commence tracking the Loran signals. The display of the information derived from the incoming signals can take many forms -from simultaneous lat./long. display to a single time difference with others being requested sequentially. Perhaps the most common method at present is to display the two TDs selected by the receiver on the basis of angle of cut of the position lines, signal strength, etc. In any case, it is always possible to override the automatic facilities and select other TDs, change to an alternative chain or 'extend' the range by being prepared to accept less precise matching techniques. 39
3- System faults In addition to the receiver's internal testing procedure, the overall transmission network is selftesting. When transmission timing falls outside tolerance, or other system faults are detected, a 'blink' procedure is ordered. This was so named because, on the early visual (CRT) displays used for matching signals and measuring time differences, the signals were made to blink or flash in order to warn the operator of a fault in the system. 40
4 -Error sources and accuracy Perhaps the most serious source of error is contamination of the groundwave by skywave. In all cases it is desired to match the phase of the groundwave of the master with the phase of the groundwave of the secondary (Figure 7.3), but towards the edge of the coverage area and at night, anywhere in the coverage area, the skywave can cause problems with the automatic matching and tracking of the two signals. 41
In some areas, skywave can even take over completely. Since the skywave will have travelled by a slightly longer route and therefore have taken a longer time, the measured time difference will be in error. Much has and is being done in modern receivers to eliminate skywave problems, but with automatic receivers it can be very difficult to know when skywave problems are present.
5 -Fixing position 5.1 Using an ocean chart with Loran C lattice Under normal circumstances, the TDs taken from the receiver can be plotted on the chart using an interpolating (Decca-type) ruler. The caution on the chart should be noted when working in areas where the lattice lines are pecked, indicating that skywave signals could be present and so positions become less reliable. Note There may be lines from more than one chain on the same chart.
5.2 Using conventional navigational charts In order to convert TD readings to position lines for plotting on a conventional navigational chart, a series of publications (HO pub. no.221) is needed, one volume for each master/secondary pair which is being used. The method is tedious and should be regarded as the exception rather than the rule, but may be necessary in areas for which no lattice charts have yet been produced. 45
5.3 Using Loran C plotting charts These are lattice charts having a minimum of navigational detail. After the chart has been used to plot the TDs, the position is transferred to the normal navigational chart. The chart is overlaid with a grid, some 4° by 4° in lat./long. At each grid intersection is a series of corrections tabulated for day (D) and night (N) and for ground/ skywave (G/S) comparison, which should be applied to the observed TDs before they are plotted. 46
5.4 Co-ordinate conversion Facilities in some receivers will convert the measured TDs to lat./long. and display this information. The accuracy of the displayed latitude and longitude will depend on a number of factors, primarily: 1 The ability to recognize skywave and apply the appropriate corrections 247- The geodetic datum to which the position is referred. The total correction will vary with time and position. 47
position output is usually well within the 0. 25NM accuracy .
LORAN C OPERATIONS Waypoint Navigation A Loran receiver’s major advantage is its ability to accept and store waypoints. Waypoints are sets of coordinates that describe a location of navigational interest. In keeping track of the vessel’s progress, most receivers display the following parameters to the operator:
Cross Track Error (XTE): XTE is the perpendiculardistance from the user’s present position to the intendedtrack between waypoints. Steering to maintain XTE near zero corrects for cross track current, cross track wind, andcompass error. Bearing (BRG): The BRG display, sometimes called the Course to Steer display, indicates the bearing from thevessel to the destination waypoint.
Distance to Go (DTG): The DTG display indicates thegreat circle distance between the vessel’s present locationand the destination waypoint. Course and Speed Over Ground (COG and SOG): The COG and the SOG refer to motion over ground ratherthan motion relative to the water. Thus, COG and SOG reflect the combined effects of the vessel’s progress throughthe water and the set and drift to which it is subject. 51