44 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



cussed in the paper on cable design. ^ Suffice it to say here that after meas- 

 uring the loss of a length of cable in the factory and computing the sea- 

 bottom loss, the computed result was a little greater than the actual 

 sea-bottom loss. The difference, i.e., the laying effect, was approxi- 

 mately proportional to frequency, and was greater for deep-sea than for 

 shallow-water conditions. Its existence was confirmed by precise mea- 

 surements on trial lengths of cable, made early in 1955 in connection 

 with cable-laying tests near Gibraltar. 



Laying effect had been suspected from statistical analysis of less pre- 

 cise tests of repeater sections of cable generally similar to transatlantic 

 cable, as measured in the factory and as laid in the vicinity of the Baha- 

 mas. However, the repeater design had of necessity been estabhshed 

 before the Gibraltar test results were known. The consequence was a 

 small systematic excess of repeater gain over computed cable loss, ap- 

 proximately proportional to frequency, and amounting at the top 

 frequency to about 0.04 db per nautical mile on the average, for deep- 

 sea conditions, and about 0.025 db per mile for shallow-water condi- 

 tions. While this difference appears small, it would accumulate in a 

 transatlantic crossing of some 1,600 miles of deep sea and 350 miles of 

 relatively shallow sea, to about 75 db at the top frequency. This would 

 be enough to render almost half of the channels useless if no remedial 

 action were taken. 



After the system is laid, changes in cable loss or repeater gain may 

 occur. These may be caused by temperature effects and by aging of cable 

 or repeaters. 



Comprehensive studies* of the expected amounts of temperature 

 change were made both before and after the 1955 laying. These gave an 

 estimate of to ±1 degree F annual variation in sea-bottom temperature 

 in the deep-sea part of the route (about 1,600 nautical miles) and per- 

 haps ±5° F on the Continental shelves (about 330 nautical miles). Use 

 of these figures leads to a db5-db variation in system net loss at 164 kc 

 from annual temperature changes. If the deep-sea bottom temperature 

 did not change at all, the estimated net loss variation due to temperature 

 would be about half of this. 



Control of Misalignment 



The first line of defense against variations leading to misalignment was 

 the design and production of complementary repeaters and cable. This 

 is discussed in companion papers. 



* Factual data on deep sea bottom temperatures are elusive. Many of the exist- 

 ing data were acquired by unknown methods vmder unspecified circumstances, 

 using apparatus of unstated accuracies. Statistical analj'sis of selected portions 

 of the data leads to the quoted estimates. 



