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general practice with the permanent Decca Chains, to deploy a third slave station 
giving three hyperbolic patterns, whereby two patterns having a good angle of cut 
will be available for fixing at all points around the central master station. 
3.9. Probably the least familiar piece of Decca equipment to specialists in the 
photogrammetric field is the Flight Log®. Essentially, this instrument comprises a 
servo-driven pen which moves laterally across a chart in response to the rotation 
of one Decometer, the chart paper being moved vertically in response to the other 
Decometer; the pen representing the position of the aircraft, records the track made 
good. The plotting axes thus necessarily being at right angles, the charts employed 
in the Flight Log are of the ‘inverse lattice’ type in which the hyperbolic co-ordinates 
are converted to a rectilinear system. Further details of the Flight Log are given in 
paras. 5.6-5.10. 
4. USE OF MOBILE DECCA CHAINS 
4.1. Figure 6 shows schematically the main units of three representative trans- 
mitting stations of the mobile type. The total weight of the units is 1245, 2183 and 
3806 pounds per station respectively; with the addition of packing, cabling and 
test gear, together with one representative type of generator for the lightweight 
station and two for the larger types, the respective overall weights per station total 
approximately 2500, 5000 and 9000 pounds respectively. To these weights must be 
added those of the necessary domestic equipment, shelter, communication gear, fuel 
supplies and spares. The transmitting aerial system takes the form of a single base- 
insulated mast supported by stay wires together with an earth mat of wires equal 
in radius to the mast height. Masts of alloy tubular or light steel girder sections are 
used, varying in height from 70 to 150 feet depending on the range and power 
requirements. Each station requires one radio mechanic with appropriate relief for 
long operating periods and the chain as a whole should be supervised by a specialist 
engineer capable of ‘servicing’ all sections of the equipment. 
4.2. The time required to bring the complete chain into action depends on the 
manpower and communication facilities available. The time is minimised in cases 
when it is practicable to prefabricate the stations, for example by building the 
equipment into a ‘skidded’ hut and carrying it as a complete entity to the site on a 
cross-country vehicle (Fig. 8). This procedure has been used on a Decca Survey in 
North Africa. In any case, the most time-consuming single factor is generally the 
erection of the aerial system; for the lightweight station this can be done in about 
two hours by a team of three or four men familiar with the drill, pulling the mast 
up by a derrick attached at a right-angle to the base. The 150 foot steel girder mast 
is climable and is erected by hauling up the 10-foot sections in succession and pro- 
gressively attaching the staywires. In favourable conditions, a team of six men can 
erect this mast in about eight hours. 
43. A monitor station may be established at any convenient point in the coverage 
in order to maintain a check on the stability*: '* of the patterns and on the general 
operation of the chain. The master station site is a suitable location administra- 
tively, but the readings of the monitor receiver will be subject there to a fixed shift 
due to the induction field existing in the neighbourhood of the station. This means 
that when sited close to a station the monitor does not provide an absolute check 
on pattern phasing, even though it may occupy an accurately-known location, 
although it remains an effective check on stability. 
44. The power taken by a single control unit and a single transmitter unit at 
230 volts 50/60 cycles single-phase input totals about 1.2 kilowatts, this therefore 
9