NEMO - NIXIE Enhanced Modular Option Surface Ship Torpedo Defense (SSTD) Program Update Test Results

Transcription

1 NEMO - NIXIE Enhanced Modular Option Surface Ship Torpedo Defense (SSTD) Program Update Test Results John P. Fumo, CTO ArgonST, Inc Fair Lakes Circle, Fairfax, VA Abstract NIXIE Enhanced Modular Option (NEMO) has completed a series of domestic and international at-sea tests to demonstrate its ability to determine the AN/SLQ-25A/C surface ship torpedo defense system readiness. This paper presents an overview of the NEMO instrument, a description of the NEMO field change kit, and the NEMO results from the at-sea tests. Background The AN/SLQ-25A surface ship torpedo defense SSTD is the surface ship s primary defense against torpedo attack. This system consists of a towed acoustic countermeasure, high power amplifiers, an arbitrary waveform generator, and the associated computer console that provides the user interface for command and control. This system is commonly referred to as the NIXIE system. The NEMO instrument is used in conjunction with the NIXIE system to assess the overall system performance and NIXIE readiness. Overview NEMO provides real-time built-in test (BIT) and environmental monitoring of the AN/SLQ-25A towed counter measure subsystem. NEMO monitors the acoustic sound pressure level emitted by the NIXIE towed countermeasure. NEMO also provides the environmental measurements of conductivity, temperature, and pressure. NEMO is attached to the aft connector of the TB-14A towed countermeasure which is towed by the Littoral Fiber Optic Tow Cable (LFOTC) tow cable. Figure 1 NEMO Instrument NEMO samples the acoustic sound pressure level (SPL), sea water temperature, pressure, and sound velocity during operations of the towed countermeasure. Data is collected and sent to the AN/SLQ-25A console via the fiber-optic tow cable (FOTC). The information is processed to determine the relative SPL being transmitted by the NIXIE, the tow body depth, and the local sound speed. Figure 1 is a photograph of the actual NEMO instrument. NEMO is enclosed in a waterproof housing approximately 6 in length and 2 in diameter. Figure 2 NEMO Circuit Card Assembly, CCA At the heart of the NEMO instrument is an embedded Texas Instruments TMS320VC33 digital signal processor (DSP), see Figure 2. The DSP is responsible for collecting the time domain samples from the embedded acoustic hydrophone. The acoustic energy is transformed into an electrical signal using a Polyvinylidene Fluoride (PVDF) Piezo Film transducer. This type of sensor was selected due to its small size and relatively flat acoustic spectrum. The PVDF transducer output is amplified, conditioned, and then sampled using an Analog Devices AD bit sigma-delta converter. The DSP utilizes its internal direct-memory-access (DMA) controller to collect 1024 samples each cycle utilizing a double-buffered memory architecture. The DMA process runs in parallel with the DSP foreground task. In the foreground, the DSP engine calculates a 2048 point Fast-Fourier Transform (FFT) with 50% overlap, Hanning windowed, to estimate the power spectral density of the local sound pressure level. The results of each PSDE are sent to a PIC micro-controller (PIC18F6520) which in turn, bundles the PSDE with the other sensor data into a fiber optic communication packet for transmission over the fiber-optic tow cable (FOTC). 1

2 Sensor Resolution (bits) Temperature 16 Pressure 16 Sound Velocity 16 Humidity 8 Voltage 8 Current 8 Table 1 NEMO Sensor Resolution The PIC micro-controller is responsible for sampling the platinum resistive thermal device (PRTD) temperature sensor, the pressure sensor, the sound velocity sensor, and a relative-humidity sensor; used as a leak detector. The PIC also collects the primary DC bus voltage and total DC current the instrument is consuming. The Humidity, DC voltage, and DC current is used to create the built-in test (BIT) status of the instrument. Table 1 shows each sensor and its sampled resolution. NEMO Enhancements The original NEMO conductivity sensor was redesigned to use a non-contact (i.e., magnetic) sensing technique to eliminate the NEMO electrical signals from interfacing directly with the seawater. This engineering change required a new transducer (i.e., transformer bobbin) which allows the seawater to flow through the center. This new sensors is extremely accurate and robust. module seals the instrument from the seawater environment. Temporary Installation Figure 4 HMAS Sydney, Temp-Alt Figure 4, shows the C-winch temporary installation on the aft flight deck. The C-Winch (referred to as the Charlie Winch) is the primary handling system used during the deployment, tow, and recovery of the NIXIE countermeasure. On the US Navy ships the C-winch has a permanent installation below deck; typically in the aft section of the ship. The NIXIE is streamed through a sea water access port located on the transom of the ship. Deployment NEMO connects to the AFT connector on the TB-14A (i.e., NIXIE tow body). The 100ft drogue line that originally was connected to the end cap on the aft blank connector is now directly connected to the eyelet at end of NEMO instrument. The drogue line stabilizes the towing of the TB-14A by providing an axial force applied at the end of the unit. Figure 3 Conductivity Improvement Figure 3, illustrates the bobbin and mechanical configuration. The entire transducer and conditioning electronics is contained on a removable module to allow services. An o-ring around the perimeter of the 2 Figure 5 NEMO readied for deployment

3 Figure 5, shows the NEMO instrument attached to the TB-14A NIXIE tow body before deployment. Test Results The data stream from the NEMO devices is sent using an ASCII format. This was done to aid during the analysis and data collection phase of the NEMO development (i.e., the use of standard laptop software to collect and assess the data packets). Figure 6, shows a typical NEMO data set. Figure 7 NEMO Results, Condo and Humidity The Humidity sensor is used as a leak detector. The graph shows the small deviation due to the ADC sampling (i.e., +/- 1bit uncertainty ). In general, the Humidity detector should not change. Figure 6 NEMO Data Packets, Typical During the at-sea test operations data from the NEMO tests was collected on a laptop computer configured with a fiber optic media converter. The fiber optic converter transforms the fiber optic data stream into electrical equivalent data stream which in turn connects to a standard USB interface. To aid in analysis a Matlab script was created to convert the data packets into a readable format for plotting and charting. Figure 7, shows the conductivity ad humidity sensor outputs. The conductivity measurements are well behaved showing a mean conductivity of ~36mS, with a small variation due to depth; which are consistent with the expected results for seawater conductivity (ocean mean = 35 ms). The small variation associated with depth is normal as surface conductivity values are typically lower at the surface. The ocean had been churned up due to storm activity in the days preceding the test which was evident from the small overall variation in readings over the measurement duration. The air temperature was ~7degC. Figure 8 NEMO Results, Depth and Temperature Figure 8, shows the depth (i.e., pressure sensor) and sea water temperature during a recovery operation. The samples numbered from approximately 1500 to 2000 and again from 2500 to 3000 are the points where the crew where detaching the range pingers from the tow cable; as indicated by the level tow depth. The temperature chart shows the small temperature variation from deployed to the point the instrument was placed on deck. The water temperature was actually warmer than the deck of the ship which is typical for the mid- November timeframe on the range (i.e., it was cold!). 3

4 Figure 9 NEMO Results, Waterfall Display Figure 9, depicts a waterfall display of the power spectral density estimation (PSDE) of the sound pressure level (SPL) for an operational NIXIE system. The high band on the NIXIE system is purposely switched off so the NIXIE system would not interfere with the acoustic range tracking pingers. Figure 11NEMO Results, Waterfall Display, AIR Figure 11, indicates the NIXIE instrument was disable during the recovery of the NIXIE tow body. Small variations in SPL at bin #10 and packets #200, 1000, 2300, etc. shows NEMO s ability to function as an acoustic intercept receiver (AIR). Figure 12 HMAS Sydney Figure 10 NEMO Results, Water Fall Display, SPL Test Figure 10, depicts the waterfall display for the NIXIE. The blue box areas indicate disabling (i.e., indicating the absence of acoustic energy) and then re-enabling each of the power amplifier bands starting with XLF, LF, and MF. Summary The NEMO instrument performed extremely well throughout all of the at-sea tests. All indications are that the original issues with the fiber optic communications link have been resolved. This is principally due to the increased sensitivity in the fiber optic transceiver dynamic range improvement; 28dB vs. 6db, new receiver sensitivity vs. old receiver sensitivity, respectively. In total, NEMO sent 8304 packets of data over the fiber optic communication link at a baud rate of Each packet contains 318 chars, which is equivalent to bits or 264,067,200 total bits transferred, with error free transmission. NEMO also 4

5 showed its ability to effectively assess the NIXIE system performance and real-time monitoring and reporting of the environmental data (i.e., pressure, temperature, and conductivity) surrounding the tow body to allow the determination of tow body depth and estimate the local sound speed. Acknowledgements The author would like to thanks the captain and crew of the HMAS Sydney at-sea tests for their help configuring the ship for this evolution and supporting all of the at-sea testing of the NIXIE and NEMO subsystem. About the Author Mr. Fumo is chief technology officer (CTO) at ArgonST, Inc. in Fairfax, Virginia. John previously held the position of CEO of Sonatech, Inc and Electro- Optical, Inc. in Santa Barbara, CA and has been involved in the oceanographic research and development area for most of his engineering career. Mr. Fumo holds a BSEE from Cal State Fullerton, and an MSEE from University of California, Santa Barbara. 5