ver, The attenuation variation with temperature is also important. In a system costs are high compared with alternative technologies (Flat- buried cable, the temperature change will not exceed an annual man, 1989). variation of +10° C, and the prevailing annual mean temperature will lie typically between +5° C and +15° C. The temperature coef- 15.3.6 Coaxial cable for submarine ficient is taken as 2x10-3 per degree centigrade at frequencies of telecommunications 500kHz or more, increasing at lower frequencies to 2.8x10-3 per degree centigrade at 60kHz. The special requirements of submarine telecommunication systems, which provide many international telephone links across oceans and shallower seas, result in a coaxial cable design rather different from 15.3.3.4 Crosstalk the land-line product. Both cable and submerged repeater equip- ment have to be designed and manufactured to rigorous standards of performance and consistency coupled with high reliability and As previously discussed, the structure of coaxial pairs is such that long life expectancy. high values of crosstalk suppression are obtained between pairs. The preferred system configuration uses a single coaxial cable Poor crosstalk on an installed cable is evidence of a fault condition, with the two directions of transmission transmitted in different such as a broken outer conductor. frequency bands separated by directional filters at each repeater The most demanding systems crosstalk requirement arises from (Worthington, 1986). consideration of sound programme transmission in the same carrier frequency band on adjacent coaxial pairs. In practice the limiting factor tends to be crosstalk within the carrier and repeater equip- 15.3.6.1 Deep-sea cable ment and not within the coaxial cable system. This cable is used where there is no risk of damage from fishing 15.3.3.5 activity or ships' anchors. It needs no mechanical protection but Group delay must have tensile strength and be able to withstand deep water pressure. The design used by STC comprises a centra