Sandipa Singh

The WHOI micro-modem: an acoustic communications and navigation system for multiple platforms

The micro-modem is a compact, low-power, underwater acoustic communications and navigation subsystem. It has the capability to perform low-rate frequency-hopping frequency-shift keying (FH-FSK), variable rate phase-coherent keying (PSK), and two different types of long base line navigation, narrow-band and broadband. The system can be configured to transmit in four different bands from 3 to 30 kHz, with a larger board required for the lowest frequency. The user interface is based on the NMEA standard, which is a serial port specification. The modem also includes a simple built-in networking capability which supports up to 16 units in a polled or random-access mode and has an acknowledgement capability which supports guaranteed delivery transactions. The paper contains a detailed system description and results from several tests are also presented

Analysis of channel effects on direct-sequence and frequency-hopped spread-spectrum acoustic communication

Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles, moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance.

The WHOI micromodem-2: A scalable system for acoustic communications and networking

A successor to the WHOI Micromodem-1 underwater acoustic modem has recently been developed. The Micromodem-2 has the same compact form-factor as the Micromodem-1 and will support all of the existing applications for the Micromodem-1, as well as interoperate with the Micromodem-1. Existing acoustic communications protocols using phase-shift keying (PSK) as well as frequency-hopping frequency-shift keying (FH-FSK) are supported, as are navigation features including narrow-band and broadband long-baseline (LBL) navigation. The Micromodem-2 is significantly more capable than the Micromodem-1 in computational ability and memory, bandwidth, non-volatile data storage, user expansion interfaces, and real-time clock precision. The expanded capabilities will allow new communications algorithms, modulation, errorcorrection methods, navigation features, and networking capabilities to be implemented. The improvements in processing capability and acoustic interfaces on the Micromodem-2 allow it to operate at acoustic frequencies from approximately 1kHz to 100kHz. The significant increases in available non-volatile storage enable the Micromodem-2 to capture data in-situ for diagnostic and research purposes. The Micro mo dem-2s firmware architecture is similar to the Micromodem-1s firmware architecture, using a real-time operating system based on modular signal processing blocks. It has been improved to increase modularity and facilitate future portability, and it offers significant improvements in timing for use with navigation and networking applications.

Integrated acoustic communication and navigation for multiple UUVs

Integrated acoustic communication and navigation for underwater vehicles provides dual functionality in a single system which reduces cost, size, power and potential acoustic interference. A combination of time and code division multiplexing is used to allow long baseline navigation and bidirectional communication for a number of vehicles operating simultaneously in the same area. The communication system uses multi-rate phase coherent modulation to maximize throughput for the uplink from a vehicle to a central node, and code-division multiple-access using a simple frequency-hopping scheme. The data rate and modulation methods are completely programmable and may be selected based on the application requirements and the propagation conditions present between the different vehicles in the network. While one or several vehicles may use time-multiplexed long-baseline (LBL) navigation, when the number of vehicles grows large, the time between fixes becomes unacceptably long. Thus in addition to LBL, a second mode of navigation is also available for use with swarms of vehicles such as the surf-zone crawlers proposed for mine counter-measure applications. This second mode employs navigation broadcasts initiated by the central controller and incorporates three additional fixed nodes that transmit in response. The time differences of arrival measured at the vehicle are used to determine its location.

Underwater Acoustic Navigation with the WHOI Micro-Modem

The WHOI Micro-Modem is a compact, low-power acoustic transceiver that can provide both acoustic telemetry and navigation. Its size and versatility make it ideal for integration in autonomous underwater vehicles (AUVs). The modem supports the use of both broadband and narrowband transponders for long baseline navigation systems, has a modem-to-modem ranging capability, and can be configured to provide synchronous oneway ranging, when integrated with a precision clock. This paper gives an overview of the different navigation systems supported by the Micromodem and presents the results from field tests conducted on the SeaBED AUV in deployments in Greece, the Bluefin AUV, and whale localizations in the Stellwagen Bank Marine Sanctuary

Multi-band acoustic modem for the communications and navigation aid AUV

An acoustic communications system with the capability to operate at multiple data rates in two frequency bands has been designed and developed for use in 21-inch AUVs. The system is specifically designed around the 21-inch diameter Bluefin Robotics AUV, though it could be adapted to smaller vehicles (12-inch), or similar free-flooded vehicles. The system includes both high (25 kHz) and mid-frequency (3 kHz) modems and supports data rates from 80 bps to more than 5000 bps. Both of the modems utilize four-channel arrays to increase reliability. The high-frequency modem is also used to support multi-vehicle navigation via one-way travel time measurements using synchronized clocks on all of the vehicles in a work group

An acoustically-linked deep-ocean observatory

A buoy-based observatory that uses acoustic communication to retrieve data from water column and seafloor instruments has been developed and deployed in 2362 m of water offshore Vancouver Island. The system uses high-rate (5000 bps) acoustic modems that are power-efficient (on order 1000 bits per joule) to telemeter data from an ocean bottom seismometer and a sensor monitoring a cold seep site near the Nootka fault. The buoy includes a Linux-based embedded controller, the modem base station and meteorological sensors. Data is off-loaded from the buoy using ftp, and remote login capability allows the acoustic communication schedule to be modified when instruments are added or removed from the network. The system has been operational for one year, typically transferring more than 500 Kbytes of data per day from two seafloor instruments.

Acoustic communications and navigation under Arctic ice

Initial results of experiments performed under Arctic ice have shown that acoustic communications and navigation can be performed on scales of 10-100 km using relatively inexpensive and compact hardware. Measurements of the impulse response at ranges of 10 and 75 km reveal extensive scatter and both resolvable and unresolvable rays. Phase coherent communication using adaptive equalization was successful up to ranges of 70-90 km at data rates of 5-10 b/s. As the SNR drops to levels too low for phase coherent communication, short FM sweeps (5-10 s), are shown to provide sufficient gain to provide lower rate communications and also support navigation.

Acoustic communication in very shallow water: results from the 1999 AUV Fest

The 1999 AUVFest was held near Gulfport, Mississippi in a shallow-water area with depths between 3 and 8 meters. The acoustic channel was characterized by multipath spanning up to 20 msec and high noise levels from snapping shrimp. Acoustic communication testing was performed with the REMUS AUV transmitting both known test data and realtime status information. In addition, a series of ship-to-ship tests were performed in order to characterize stationary performance at ranges of one to five kilometers. Data rates of 60-5000 bits per second were transmitted and, despite the very shallow water conditions, the communication link was reliable at ranges up to 5 km at all data rates.

Nondestructive imaging of fragile sea-floor vent deposit samples

X-ray computed tomography was used to detail the internal structure of sea-floor hydrothermal vent samples. A third-generation industrial computed tomography (CT) scanner with a microfocus tube was used to scan a black smoker chimney and cores taken from a white smoker chimney and a block of Fe-rich sulfide. Images of the black smoker chimney clearly show sulfide- versus anhydrite-dominated areas. Display of pore space in three dimensions shows the complex geometry of the main flow conduit, and also much smaller (2–3 mm diameter) conduits within the chimney wall that parallel the main flow conduit. Images of the white smoker sample document the continuity of an anastomosing ∼1-mm-diameter flow conduit, and the pronounced anisotropy of porosity. Tube structures presumed to be casts of worm tubes are clearly evident in images of the Fe-rich sulfide sample. X-ray CT is an excellent technique for rapidly identifying the internal structure of porosity and mineralogy of fragile hydrothermal precipitates on scales of tens of microns to hundreds of millimeters, and data can be used to deduce styles of fluid flow and other processes involved in vent deposit formation.