A Decca chain normally consisted of a master station controlling the phase of three slaves, which were situated about 120 degrees apart, at a radius of 60 to 100 miles from the master. That provided all-round coverage, and, because ground waves of this frequency band are not seriously attenuated by passing over land, the stations could advantageously be situated well inland.
Each Station in the chain would normally transmit a particular unmodulated phase stable carrier wave. These carriers were all harmonically related to an internal station reference which was about 14.2kHz, referred to as “f”.
* The Master Station normally transmitting a “6f” unmodulated carrier wave signal in the 85 kHz band,
* Red Slave Station transmitting a “8f” signal in the 112 kHz Band,
* Green Slave transmitting a “9f” signal in the 127 kHz band,
* Purple Slave transmitting a “5f” signal in the 71 kHz band.
The Slave stations received and phase locked their station reference oscillators to the Master “6f” Transmission.
The frequency ranges for the master and slave stations were:
Master: 84 - 86 kHz
Red slave: 112 - 115 kHz
Green slave: 126 - 129 kHz
Purple slave: 70 - 72 kHz
Since the signals were continuous wave (CW), 150 Hz spacing was sufficient to ensure there would be no interference.
These transmissions were received by a special receiver and frequency multiplying circuits therein produced phase comparisons of:
24f for the Master and Red
18f for Master and Green
30f for the Master and Purple
The block diagram of the basic Decca receiver for survey and marine use (not including Multipulse). The master and slaves were processed by superheterodyne stages in the left column. The resultant outputs were applied to frequency multiplying circuits in the middle column. The outputs of the frequency multipliers were applied to the discriminators in the rightmost columns. The difference in phase would be amplified and read on a decometer. It is very easy to see how all the slave signals are compared to the master in this diagram. f is some frequency around 14. 2 KHz. (Graphic courtesy Decca Navigator Co. Modified by Jerry Proc)
Although for most of the time the stations only transmitted their single carrier, during part of the transmission cycle, each station would transmit what was termed a Multi-pulse also called "Mark 10" transmission. In the Mark 10 context, it meant that Multipluse could only be received on Mk X receiver equipment and higher.
The Multi-pulse was transmitted by each of the stations in turn during the 20 second transmission cycle to provide a coarse reading, or a Zone reading, and was generated by all 5 transmitters at the given station briefly transmitting simultaneously. During transmission of the Multi-pulse by, say the Red Slave, all transmissions from the other stations in the Chain would be suppressed.
The original 'V type transmissions, prior to the introduction of Multipulse also had V-1 and V-2 variants. Both of these were being phased out by the mid-'70's when many of the chains were being updated. More on this as soon as additional information becomes available.
Decca Lattice: An example of a Decca lattice chart showing the lines of hyperbola from the read and green slaves.
The transmissions from the chain are received by a special shipborne receiver, which measures the difference in phase of signals arriving from master and slaves. All stations in a Decca chain must 'phase locked', and this has to be done over an appreciable distance separating the stations, sometimes up to 100 nautical miles, the phase difference being determined by this distance. Each slave station is fitted with equipment which receives the master signal, converts it to the slave frequency, and uses it to control the drive oscillator of the slave transmitter. Thus a constant phase relationship is maintained. To ensure that this relationship is maintained accurately, a monitoring station checks the transmissions.
|Click to enlarge. This simple and more practical red-green lattice chart relates the coloured lines to actual decometer readings. (Map courtesy Decca Navigator Company)|
The detected phase differences are displayed on phase meters called 'decometers', and the readings may be plotted onto Decca lattice charts, on which the lines of position are numbered in the same units as those shown on the decometers.
The decometer indications are continuous, and depending on the position in the coverage, readings of the two appropriate decameters can be taken simultaneously whenever a fix is required. The third decameter can give some additional information, but usually its readings are disregarded in the wide sector around the base-line extension. The lattice patterns are formed by hyperbolic position lines similar to those previously described. They are overprinted on ordinary Mercator charts. The slave stations are known as Red, Green and Purple slaves, according to the printed colour of the lattice lines derived from their transmissions.
Figure 2: A set of decometers.( Courtesy Decca Navigator Company)
A very high degree of instrumental accuracy is obtained by the use of continuous wave transmissions, the phase of which, on arrival, can be measured to within 4 degrees. Near the base-line, between a pair of stations, this may represent a distance as small as 10 yards, though it must be borne in mind that constant and variable errors due to operational causes exist in the system, which, in practice, does not normally give an accuracy as good as +/- 10 yards. It is, however, considerably more accurate than any system employing pulse transmissions ( ie Loran).
The areas between the lines of zero phase difference in a Decca pattern are known as "lanes". The width of each lane on the base-line is approximately Red: 450 metres, Green: 590 metres and Purple: 350 metres. Lanes are grouped into Zones. Each Zone contains 24 Red lanes, 18 Green lanes, or 30 Purple lanes. For unambiguous presentation, the Zones are lettered, and the Lanes numbered outwards from the Master Station. Each group of ten Zones is lettered from A to J, and the Lanes in each zone are numbered:- Red: 0 to 23, Green: 30 to 47 and' Purple: 50 to 79. Readings at the Master Station are Red 0.00, Green A 30.00 and Purple A 50.00. The correct Zone Letter must be determined from normal navigational methods and by reference to the appropriate Decca latticed chart. As the zones are about 6 miles in width on the base lines, and this width increases away from the base-lines, the accepted position of the ship is generally not critical for this purpose.
It is essential that the signals from the two stations should be received separately in order to preserve their individual phase properties. Since the C.W. transmissions are simultaneous, this can only be achieved by transmitting on different frequencies: but these frequencies must have an exact common multiple. The transmissions are received by what are virtually four separate receivers within the Decca Navigator receiving equipment. The frequencies of these signals are then multiplied up to their lowest common multiple, the so-called "comparison frequency" on which the phase comparison is made. As the pattern is traversed by the Decca receiving equipment so the reading will be observed to alter steadily from 0 degrees to 360 degrees between the limits of each lane; the decometer, from which this reading is obtained, is therefore graduated in fractions of a lane instead of in degrees.
INTERPOLATOR: Shown above is the Interpolator for Decca Lattice Charts. When overlaid on a lattice chart, it would help the navigator resolve the distance between lattice lines. (This artifact was donated to HMCS HAIDA Historic Naval Ship by Duncan Mathieson)
The decometer is simply a phase meter whose dial is graduated in hundredths of a lane width; one revolution of the fractional pointer represents the extent of one lane. It will, therefore, indicate very accurately a receiver's position between two lattice lines, but it is unable to identify the particular lane in which it is situated. Since lane width varies from less than a mile near the base line to 3 miles or so at 300 miles range, this would cause a high degree of ambiguity, which a ship, entering the coverage area after an ocean passage, might not be able to resolve. Once the initial position has been established, however, the decometer, which is capable of continuous rotation, can integrate its movements in the lattice pattern by a set of counters geared to the fractional pointer.
LANE IDENTIFICATION METER
The ambiguity of the Decca Navigator system has been resolved in the Mark V (or QM5) receiver by the addition of a fourth dial called a "Lane Identification Meter". Its use enables the operator to set each decometer to the correct lane within a zone. He must still know which zone his ship is situated in, however, in order to set the correct letter on the decometer. Since a zone consists of about twenty or more lanes, this only requires that the dead reckoning position should be known within wide limits so that, except in unusual cases, no ambiguity should arise.
Should lane-slip have occurred, the fact will be apparent from the lane identification meter as soon as the ship enters the lane identification coverage area and the decometers can be reset accordingly. Essentially, lane identification consists of transmissions from master and slave at much lower frequency than the normal. This lower frequency, which is used as a comparison frequency in the receiver, is actually obtained as a beat frequency of the two transmissions originating from the same station. Thus a very much coarser pattern is obtained in which the 360 degree phase change corresponds to a whole zone. Since this lower frequency is a multiple of the pattern comparison frequency, a zone comprises a whole number of lanes. This is shown in figure 3. The lane identification meter, which measures phase difference in the same way as the decometer, will indicate the position within a zone.
Figure 3: Lane identification transmission.
If this meter is graduated in lanes, instead of fractions of a zone, it will then indicate directly the correct lane in which the receiver lies. In practice, the lane identification meter has three concentric scales (one for each pair of stations), coloured red, green and purple. Lane identification signals are transmitted from each pair in a fixed sequence at short intervals; and, as each one is received, a relay is closed, illuminating the appropriate coloured scale while the pointer indicates the correct lane on that scale. The indication for each colour remains on the meter for about 5 seconds, which is ample time in which to obtain a reading. This was due to the discovery that, in certain conditions of skywave interference, the accuracy of the meter responding to the coarse pattern was inadequate for the required purpose. The indicator consists, therefore, of two separate components: the sector pointer and the "vernier" pointer assembly. The former indicates the position to the nearest sixth of a zone, while the latter, working on an intermediate frequency, indicates the actual lane itself within that zone.
Figure 4: The Lane Identification meter and the vernier and sector pointers.
The vernier indicator is basically a meter with a pointer revolving once as the receiver travels across the space between two adjacent boundaries of a lane (a sixth of the width of a zone) of the particular pattern which affects it; but, for compactness of display, the action is geared down six times mechanically, and the single pointer is replaced by an assembly of six pointers, which are read against the same scale as the sector pointer.
It should be noted that the word "vernier" is used in this connection not in its true sense, but merely to indicate a finer indication. A reading is effected by noting, on the illuminated scale, the lane number indicated by the particular vernier pointer which is enclosed by the arms of the sector pointer. Thus, in Figure 4, if the red scale were illuminated on the meter, the correct lane reading would be between 7 and S. It should be noted that, beyond 100 miles from the base line, the center of the sector pointer will not always coincide with the vernier pointer, owing to skywave interference; but as the vernier pointer indicates the exact lane, this will cause no inaccuracy. Near the limits of coverage at night, however, the arms of the sector pointer may coincide exactly with two vernier pointers, or the sector pointer may even enclose the wrong one. The use of lane identification was approved by the British Ministry of Transport in 1949, subject to strict compliance with the instructions contained in the relevant Data Sheets, which were promulgated by the Decca Navigator Company from time to time.
DESCRIPTION OF 'V' TYPE TRANSMISSIONS
Each station in a Decca chain transmits on a different frequency. If the master and slave stations all operated on the same frequency, the receiver would be unable to distinguish between the incoming signals. In order to compare the phases, however, all the transmission frequencies are related harmonically, and each signal is separately converted in the receiver to a frequency which is the lowest common multiple of the master and slave frequencies. The relative phases can then be compared at this "comparison frequency", which will be different for each pair. For example, if the Master, A, transmits on 60 kc/s and slave B transmits on 80 kc/s, the comparison frequency would be 240 kc/s, which is the lowest common multiple (LCM) of 60 and 80. As far as the receiver is concerned, apart from signal separation, the waves appear to have travelled from the transmitters at the comparison frequency, and it is upon this frequency that the number of lanes in a lattice depends.
Remembering that one lane is half-a-wavelength wide along the base-line, and taking 240 kc/s as the comparison frequency of the AB pair, we obtain the following: 240 kc/s is equivalent to a wavelength of 1,370 yards i.e., each lane will be 685 yards wide along the base-line. If the distance between A and B is 85.5 miles, then the number of lanes will be:
85.5 x 2,000/ 685 = 250
The phase differences between the slave signals and that of the master are displayed on the decometers during the whole time that the receiver. is switched on. Lane identification is provided, at intervals of one minute on each pattern, during a short break in the normal transmissions. For this purpose the transmission frequencies are grouped in a different manner in order to produce the required coarser patterns. For the lane identification of a pair, in addition to the master and a slave, two transmitters -- one at each section are put in operation at fixed times every minute. These additional transmitters work on frequencies 'borrowed' from two of the remaining slaves. While this 'frequency borrowing' is taking place, transmissions from the stations normally operating on these frequencies are suppressed for about half a second. The sequence of transmissions and suppressions is maintained to a rigid time schedule by automatic phase locking circuits, and the sequence of events appears on the lane identification meter in the following manner:
FREQUENCIESAT MASTER AT SLAVE 0 sec to 0.5 sec. RED Master Red 15 sec to 15.5 sec. GREEN and and 30sec. to 30.5 sec PURPLE Purple Green
The last transmission is followed by a 30 second interval before the sequence starts again, so that the lane in each pair is identified once every minute. Since the time intervals are unequal, (ie 15 sec., 15 sec., and then 30 sec.) it is easy to recognize which lane identification (L.I) is on, and then check whether the L.I. light sequence is correct. In order to avoid false Red, Green, and Purple readings during the "frequency borrowed" transmissions, the decameter circuits are cut off for this period; but, owing to storing elements in the circuit, the readings are maintained on the previous levels. This 'persistence' of the decometer readings is sufficiently long for them to be unaffected by the very short lane identification transmissions. The only effect on the decometers is a slight 'kick' of the pointers, which does not affect the accuracy of the reading.
For more information on the MULTIPULSE transmitting format, please refer to the Decca Transmitters document.
The presence of a sky wave component in the received signal may cause variable errors at the receiver. In the case of Decca, there is no means of distinguishing between the two waves. This leads to an inaccuracy in the decometer readings which will vary with the range from the transmitters and with the time of day. At ranges greater than 75 miles, the accuracy at night is noticeably lower than it is by day, since sky wave effect is normally only experienced during the night. Beyond a range of between 150 and 220 miles there is a serious danger of "lane-slip".
Disregarding systematic errors and transmission failures, the accuracy of a fix from the Decca system can be considered to depend upon:
(a) Instrumental errors
(b) Propagation errors caused by either Sky Wave or Coastal effects.
(c) Lane width.
(d) Angle of cut of the hyperbolae.
Thus Decca errors are subject to many variables and cannot be summarized precisely. The following figures give a guide to the accuracy that should generally be expected:
a) By DAY 0-100 miles or by NIGHT 0-75 miles: +/- 10 feet near base line; 1 mile at limits
b) By NIGHT 75-240 miles: Up to a maximum of about 5 miles, depending on sighting of slave stations.
Notification of any transmission failures, which might result in lane slipping, were promulgated to mariners by signals broadcast from certain coastal radio stations. Details of this service was contained in the Admiralty List of Radio Signals (Vol. V). Decca Charts.
Decca charts, produced by the Hydrographer, consisted of Admiralty navigational charts overprinted with Decca lattices. They were given the series letter 'L' with the word 'Decca' in brackets after the number - e.g., L 1408 (Decca) - to distinguish them from other lattice charts.
Contributors and Credits:
1) Matthew Parker <parkermat(at)hotmail.com>
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