INSTRUMENTATION AND METHODS 



STD-DDL System 



A Plessey Environmental Systems, Model 

 9040, S/T/D Environmental Profiling System 

 (STD) was the primary sampling instrument 

 at 199 hydrographic stations during the 1971 Ice 

 Patrol. During the May multiship sur\ey, 

 ROCKAWAY and EVERGREEN each used "a 

 Plessey Environmental Systems, ]\Iodel 8114A, 

 Digital Data Logger (DDL) with the 9040 STD 

 system. Four channels of information were 

 scanned at sample rates from ten scans per sec- 

 ond to one scan every two seconds. STD fre- 

 quencies representing depth, temperature, and 

 salinity wei-e converted to binary and recorded 

 on a 7 channel IBM compatible magnetic tape 

 at a bit density of 200 bpi. The fourth informa- 

 tion channel was a\ailable for recording sound 

 velocity on the DDL, but it was never used. The 

 tape format for each STD cast consisted of a 

 station identifier (up to 8 characters) followed 

 by any number of data records, depending on the 

 maximum depth and lowering rate of the cast. 

 Each record consisted of the temperature and 

 salinity information at 32 depth levels. An av- 

 erage one thousand meter cast was composed of 

 about 2400 data levels recorded on approximately 

 80 records. Three computer progi-ams were de- 

 veloped by CGOU to reduce the number of data 

 levels to a more manageable figure of 50 to 100 

 which would still accurately represent the origi- 

 nal water column. 



The computer programs were developed for a 

 Control Data Corporation (CDC) 3300 com- 

 puter. The first program, RFIL, inputs the STD 

 and DDL constants and reads the records to be 

 processed from the magnetic tape. The digitized 

 frequencies were translated from binary to engi- 

 neering units of depth (meters to tenths), tem- 

 perature (C° to hundredths), and salinity (ppt 

 to thousandths). The values were printed out 

 so that an initial quality control check of the 

 data could be made. In addition, a tape output 

 provided the necessary input to the second pro- 

 gram. When a rapid sample rate such as 0.1 



second was used, a specific depth level might 

 show up several times. AVhile these temperature 

 and salinity values were always close, they gen- 

 erally did not agree exactly, probably as a result 

 of sensor lag. This indicates that the descent 

 rate of the underwater sensor probably should 

 be less than the 25 to 60 meters per minute pres- 

 ently being used. The output from the first 

 program was normally around 2400 levels of 

 data. 



Program AVERS eliminated all levels where 

 the depth had not increased from the preceding 

 level. This decreased the number of levels by 

 about one third. Temperature and salinity values 

 were then averaged over successive 2.5 meter 

 intervals. If there were less than five samples 

 in an interval, the interval was expanded to in- 

 clude five consecutive levels in the average. This 

 step brought the number of levels down to be- 

 tween 150 and 200. 



The third program, STSP, determined the 

 standard and significant levels, whose averaged 

 values of temperature and salinity would accu- 

 rately rejjresent the original water column. 

 Standard levels were taken at the averaged 

 depths falling closest to certain depths we had 

 specified. In 1971, we used the minimum re- 

 corded depth, 10, 20, 30, 50, 75, and 100 meters, 

 every 25 meters to 300 meters, and then every 

 50 meters to 1000 meters. The first test for 

 significant levels consisted of fitting a cubic curve 

 successfully through five consecutive tempera- 

 ture data points. If the curvature at the mid- 

 point exceeded an absolute value of 0.005, the 

 second, third, and fourth points were compared 

 with the data points immediately above and be- 

 low. A level was significant if it departed from 

 a straight line between the adjacent points by 

 more than 0.04° C. for temperature (more than 

 0.06°/oo for salinity). The second test compared 

 the differences between the curvature of two suc- 

 cessive midpoints. If the absolute value of the 

 difference exceeded 0.005, we again checked the 

 departure of the point from the adjacent points, 



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