PAKT IV: DATA COLLECTION AND ANALYSIS 



53. In this section, the FRF data acquisition system, data collection 

 techniques, and data analysis procedures are discussed. 



Digital Wave Data 



Recorders/signal conditioning 



54. The data acquisition system consisted of a primary and backup re- 

 corder and associated electronics for signal conditioning prior to recording. 

 Two different primary recorders were used to collect the wave data. Prior to 

 October 1980, the primary system transmitted analog data signals via telephone 

 lines from the FRF to Ft. Belvoir, Virginia, where the data were recorded in 

 digital form on a Modcomp 11/25 minicomputer. After October, a Data General 

 NOVA-4 minicomputer located in the FRF laboratory building was used to collect 

 the data. In addition, a backup system consisting of a Lockheed Store 7 (FM) 

 recorder located at the FRF was used to record data when the primary system 

 was known to be inoperative. Frequently during storm conditions the backup 

 system was run simultaneously with the primary system to ensure that wave 

 data were obtained. A second FM recorder located at CERC (Ft. Belvoir) was 

 used to play these tapes into the Modcomp so that the data recorded could be 

 digitized. 



55. Regardless of the system used, the voltage signal from the sensors 

 required certain conditioning. For the phoneline/Modcomp system, the signal 

 was first amplified and biased to ensure a 0- to 5-V range, then converted to 

 a frequency-modulated (FM) signal by exciting a voltage-controlled oscillator 

 (VCO) . That signal was then transmitted to Ft. Belvoir via telephone line 

 where a discriminator was used to convert it back to a voltage signal. This 

 signal was fed into a demultiplexer and converted to a serial data stream 

 which was then sampled by the Modcomp. For the NOVA-4 and FM recording sys- 

 tems, the 0- to 5-V signal was fed directly into the recorders. However, 

 since the FM recorder operated on a maximum output of 3 V, it linearly scaled 

 the 0- to 5-V signal by a factor of 3/5. 



Collection 



56. The signal from the wave sensors was routinely sampled four times 

 per second for 20 minutes every 6 hours beginning as near as possible to 0100, 



25 



