HEWITT ET AL.: DEVELOPMENT AND USE OF SONAR MAPPING 



system which are multiplexed and sampled at 

 appropriate times by the analog-to-digital con- 

 verter. These are: 



The start pulse — the trigger pulse for the sonar 

 transmitter. 



The sonar signal — the 1,000 cycle band width 

 detected video from the sonar receiver. 



The ship's speed — a DC voltage from the ship's 

 log proportional to speed. 



The hour mark — a pulse from the ship's preci- 

 sion simplex clock system occurring at the 

 end of each hour. 



The start pulse initiates the program, which 

 then counts 200 sample times before recording 

 data. Two hundred fifty samples are then taken 

 between 200 and 450 m, to be operated on by the 

 program as previously described. A running 

 count of the number of start pulses occurring 

 after the beginning of each new hour is kept and 

 used as a time base for all events recorded during 

 that hour. During data reduction, this count is 

 divided into 60 min and used to provide absolute 

 time data. 



The ship's speed is recorded with each target, 

 and may be used to calculate the area surveyed. It 

 is used in the data collection program to deter- 

 mine when a hydrographic and/or biological sta- 

 tion has been reached and to suspend data record- 

 ing while on station; start pulses continue to be 

 counted, however, thus the time at the beginning 

 and end of the station is recorded. 



In shipboard operation, the system requires no 

 attendance. Prior to leaving the dock, the com- 

 puter is started, and the hour counter is preset to 

 the current time. The sonar system is then 

 started and may be left in operation 24 h a day or 

 turned off at night. In either case, the data collec- 

 tion program will begin sampling automatically 

 at 0800 each morning and continue until 1600 

 each afternoon, except while on station. There are 

 six memory storage fields in the PDP8/I of 4,096 

 words each. One field is used for programming 

 and temporary data storage. The other five fields 

 provide final storage for 3,300 targets, at six data 

 words per target. At the end of the day ( 1600 h) 

 the data collection program in field zero is re- 

 placed by a general computational program used 

 in the PDP8/I called FOCAL. This program 

 change is accomplished automatically from a pre- 

 recorded magnetic tape cartridge. With FOCAL 

 programming, the stored target data is now re- 



duced, summarized, and dumped onto periph- 

 eral mass storage capable of holding the entire 

 cruise. 



When the output is finished, the collection 

 program is reread into field zero, and the com- 

 puter waits for 0800 h the following morning to 

 again begin data recording. 



Field testing of this system was conducted in 

 July 1974, by comparing computer listings of 

 events with the corresponding wet paper records. 

 The system proved to have a greater resolution 

 than was felt necessary and the criteria for a 

 target block changed to two coincident and con- 

 secutive samples above threshold. Ten samples 

 below threshold rather than five were judged 

 adequate to terminate a target on any given line. 

 A variable threshold based on an integrated 

 value of volume reverberation is being developed. 



The system was field tested under a wide vari- 

 ety of conditions and judged satisfactory for our 

 requirements. Figure 7 describes a cumulative 



20 40 60 80 100 120 140 

 MEAN CLASS DIAMETER (meters) 



Figure 7. — Cumulative frequency diagram of school count 

 and horizontal school area from a sample taken during the 

 field test of an automated sonar system in July 1974. 



frequency diagram for school count and horizon- 

 tal school area. A median school size of 30 m 

 agrees with data from previous cruises. 



293 



