TELCOMMUNICATIONS WORK PROGRAMME

Optical Measurements in glass fibre:

  1. Calometric Method for Determining Absorption Loss in Bulk Glass.

    The increase in temperature when light was absorbed in a glass sample was measured by using sensitive thermometers. The absorption loss was then deduced independently of scattering losses. The method had the additional advantage that very small samples were used.

  2. Measurements of Angular Scattering in Fibres.

    Light scattered from the fibre in a particular direction was collected by a telescope and detected at a photomultiplier. Measurements using a range of different directions were made and the polar diagram of the scattered light was plotted. Since the amount of light collected was very small the photomultiplier was cooled with liquid nitrogen and photocounting techniques were used.

  3. The Far-Field Pattern of Light Emitted by a Fibre.

    The far-field pattern at the end of the fibre was observed on a screen. Quantitative measurements were made by replacing the screen by a photodetector. The imperfections in the fibre which caused changes in the angular or mode distribution was deduced from this pattern. With a sufficiently long fibre an equalibrium distribution was obtained since the changes in the distributon were compensated by differential attenuation. The channel capacity of a fibre was also deduced from the far-field patterns. The values obtained were in agreement with direct observation of the time dispersion of light pulses propagated through the fibres.


Waveform and spectral analysis programme

SRDE developed computer techniques for predicting the performance of communication systems by assembling a mathematical model of the proposed equipment from a library of standard building blocks. The basic units consisted of amplifiers, filters, modulators, demodulators, differentiators, integrators and other more specialised modules. The system was stimulated by a suitable generator and the computer program enabled the spectrum and the waveform of the signal at each point in the chain to be evaluated. In addition measurements were made of power, voltage and the bandwith occupied by the spectrum. The combination of high speed and flexibility in the programme gave the designer a tool of considerable power.

Software design

SRDE developed techniques for simplifying the design of the complex software required to control the computers used to switch and control in a military telecommunication network.

  1. The use of modular multiprocessor techniques which are only partially interconnected to reduce the complexity of the overall control software.
  2. The use of shared schedules with dynamic priorities to simplify error recovery in modular multiprocessor systems.
  3. The adaptation and use of a high level language Coral 66 for use in communications switching systems, for both control and applications software.

Library of in-house computer programs


Propagation

Research was carried out on studying the propagation of radio waves by scattering in the troposphere and ionosphere and anomolous UHF propagation.

In conjunction with GEC Ltd (contract) a one-way tropospheric scatter link at a frequency of 2600 Mhz between Hemsby (Norfolk) and Wembley, a range of about 100 miles, was established. The signal was recorded at Wembley and the results processed at SRDE. The main variables analysed were siting effects, diversity and digital error as well as the general behaviour of the signal over this short range path.

Another contract with GEC Ltd. investigated the fading in UHF (750 Mhz ) tropospheric propagation. Equipment was constructed for plotting out the probability distribution of the signals and this facility enabled the identification of the onset of a tropospheric scatter component. Equipment was constructed to investigate the factors which determine the losses at UHF frequencies over distances appropiate to the new HOBART communication system. A 2000 km link at frequencies of 36 and 39 Mhz, for which SRDE was the Design Authority, was established between St.Lawrence (Isle of Wight) and Gozo (Malta). The link was the longest range ionospheric scatter link in service and the transmitted power was 40 KW with 20 db gain aerials. The channel capacity was 200 bauds, 4 teleprinter channels; an error detection and correction system was provided.

Equipment was made at SRDE to record the propagation data. The propagational data was converted to binary coded decimal digits and a sequence of these digits was punched out at suitable time intervals.



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