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ABSTRACT

Sonobuoy is a device used to detect and identify objects moving in the

water. Typically, a sonobuoy is used to detect submarines by either listening

for the sounds produced by propellers and machinery (passive detection) or by

bouncing a sonar "ping" off the surface of the submarine (active detection).

Multi-static techniques are also used for submarine detection and localization.

Multi-static operations utilize separate active source and passive receiver

sonobuoys.

A sonobuoy is a device which is dropped into the ocean and used to

gather acoustic data. There are a number of different types of sonobuoys,

designed for a variety of applications from anti-submarine warfare to whale

research. All sonobuoys are characterized by being very rugged, built to

withstand severe weather and extreme temperature and pressure, and many are

also designed to be essentially disposable, as loss of a sonobuoy is quite

common. A sonobuoy (aportmanteau ofsonarandbuoy) is a relatively small

(typically 4 inches, or 124 mm, in diameter and 36 inches, or 910 mm, long)

expendable sonar system that is dropped/ejected from aircraft or ships

conducting anti-submarine warfare orunderwater acoustic research.

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CONTENTS

1. INTRODUCTION

2. HISTORY

3. CONCEPT OF OPERATION

4. GPS SONOBUOY

5. GPS EQUIPPED SONOBUOY SYSTEM

6. DETAILED DESCRIPTION OF THE SHIPBOARD SYSTEM

7. CONCLUSIONS

8. REFERENCE

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INTRODUCTION

During the Cold War, passive detection in deep water was the strategy

of choice to covertly track nuclear submarines around the world. Since former

Soviet Union and NATO relations have changed, detection needs have

fluctuated. An increase in the number of diesel electric submarines under the

flag of third world nations has led to an increase in the interest in active

sonobuoys and shallow water detection techniques. Sonobuoy Tech Systems

offers a full line of sonobuoys and technical support to address modern Anti-

Submarine Warfare (ASW).

Sonobuoys with different characteristics other than those described can

be designed and built to customer requirements, following a careful analysis of

needs. Sonobuoy TechSystems has designed and manufactured many

sonobuoy variations over the years, and continues to make high-performance,

high-reliability sonobuoys.

The buoys are ejected from aircraft in canisters and deploy upon water

impact. An inflatable surface float with a radio transmitter remains on the

surface for communication with the aircraft, while one or more hydrophone

sensors and stabilizing equipment descend below the surface to a selected

depth that is variable, depending on environmental conditions and the search

pattern. The buoy relays acoustic information from its hydrophone(s) via

UHF/VHF radio to operators onboard the aircraft.

The sonobuoy owes its development to the Allied need to monitor

submarine traffic in the First World War. With the development and

deployment of the German U-Boat, the Allies realized that they would be

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powerless against the Germans unless they had a way to identify and track the

German U-Boats. The result was the development of early sonar systems,

which used sound waves in a variety of ways to identify objects moving

through the ocean. Planes started dropping sonobuoys into the Atlantic to track

the course of U-Boats, and ever since then, these devices have been refined

and retooled for an assortment of purposes.

There are two main parts to a sonobuoy: the buoy itself, and a radio

transmitter. When a sonobuoy is dropped into the water, the buoy detaches

from the transmitter, allowing the transmitter to float on the surface of the

water while the sonobuoy sinks below. As the sonobuoy gathers data, it passes

the information on to the transmitter, which in turn transmits the data to an

aircraft or ship. When it is possible to do so, the sonobuoy will be retrieved

after use.

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Sonobuoy being loaded onto an USN P-3 Orion aircraft

A basic sonobuoy is simply passive, recording the sounds of the water

it is immersed in. These sounds can sometimes be quite interesting, as in

addition to revealing passing ship traffic, a sonobuoy will also record the

sounds of ocean life. Militaries use sonobuoys to watch out for submarines and

other hazards, while scientific researchers utilize the data to find out more

about the life in the ocean. A scientific sonobuoy also often collects data about

currents, temperatures, and pressure.

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HISTORY

With the technological improvement of the submarine in modern

warfare, the need for an effective tracking system was born. Sound Navigation

And Ranging (SONAR) was originally developed by the Britishwho calledit ASDICin the waning days ofWorld War I. At the time the only way to

detect submarines was by listening for them (passive sonar), or visually by

chance when they were on the surface recharging theirbattery banks or by

massive air patrols with lumbering airships and biplanes. Sonar saw extremely

limited use and was mostly tested in the Atlantic Ocean with few naval

officers seeing any merit in the system. With the end of WWI came the end to

serious development of sonar in the US, a fact that was to be fatal in the early

days of World War II. However, considerable development of ASDIC took

place in the UK, including integration with a plotting table and weapon.

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P-3 Orion paradropping a sonobuoy

The ravaging wolf-packs ofU-boats in WWII made the need for sonar a

priority. With millions of tons of shipping being sunk in the Atlantic, there

was a need to locate submarines so that they could be sunk or prevented from

attacking. Sonar was installed on a number of ships along with Radio

Detection and Ranging (RADAR) to detect surfaced submarines. While sonar

was a primitive system, it was constantly improved.

Battle of the Atlantic (19391945)

Modern anti-submarine warfare grew from the WWII convoy and battle

group movement through hostile waters. It was imperative that submarines be

detected and neutralized long before the task group came within range of an

attack. Aircraft-based submarine detection was the obvious solution. The

maturity of radio communication and sonar technology made it became

possible to combine a sonar transducer, batteries, a radio transmitter and whip

antenna, within a self-contained air-deployed floating (sono)buoy. The

advancement in sonobuoy technology, it could be argued, eventually led to the

development of entire classes of aircraft (such as the P-2 Neptune, S-2

Tracker,S-3B Viking and P-3 Orion) to anti-submarine warfare (ASW).

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AN/SSQ-47B active pinger ranging sonar sonobuoy and carry case (note

eight sided for stacking.)

Early sonobuoys had limited range, limited battery life and were

overwhelmed by the noise of the ocean. They first appeared towards the end of

WWII [add NDRC Div 6 ref & photo here] but it is doubtful that they saw

operational use until the Cold War. They were also limited by the use of

human ears to discriminate man-made noises from the oceanic background.

However, they demonstrated that the technology was viable. With the

development of better hydrophones, the transistorand miniaturization, and the

realisation that very low frequency sound was important, more effective

acoustic sensors followed. The sonobuoy went from being an imposing six feet

tall, two feet diameter sensor to the compact suite of electronics it is today.

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CONCEPT OF OPERATION

Sonobuoys are classified into three categories: active, passive and special

purpose.

Active sonobuoys emit sound energy (e.g. "pings") into the water andlisten for the returning echo before transmittingusually range and

bearinginformation via UHF/VHF radio to a receiving ship or

aircraft.

Passive sonobuoys emit nothing into the water but rather listen,

waiting for mechanically generated sound waves (for instance, power-

plant, propeller or door-closing and other noises) from ships or

submarines, or other acoustic signals of interest, to reach the

hydrophone that are then transmitted via UHF/VHF radio back to a

receiving ship or aircraft.

Special purpose sonobuoys relay various types of oceanographic data

to a ship, aircraft, or satellite. There are three types of special-purpose

sonobuoys in use today. These sonobuoys are not designed for use in

submarine detection or localization.

o BTThe bathythermobuoy (BT) relay bathythermographic or

salinity readings, or both, at various depths.

o SARThe search and rescue (SAR) buoy is designed to operate

as a floating RF beacon. As such, it is used to assist in marking

the location of an aircraft crash site, a sunken ship, or survivors at

sea.

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o ATAC/DLCAir transportable communication (ATAC) and

down-link communication (DLC) buoys, such as the UQC, or

"gertrude", are intended for use as a means of communication

between an aircraft and a submarine, or between a ship and a

submarine.

This information is analysed by computers, acoustic operators and

TACCOs to interpret the sonobuoy information. Any noise that a submarine

makes is a potential death knell, so few submariners are communicative.

Active and/or passive sonobuoys may be laid in large fields or barriers for

initial detection. Active buoys may then be used for precise location. Passive

buoys may also be deployed on the surface in patterns to allow relatively

precise location by triangulation. Multiple aircraft or ships monitor the pattern

either passively listening or actively transmitting in order to drive the

submarine into the sonar net. Sometimes the pattern takes the shape of a grid

or otherarray formation and complexbeamformingsignal processing is used

to transcend the capabilities of single, or limited numbers of, hydrophones

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Sonobuoy deployment procedures after impacting water.

GPS SONOBUOY

Very often, the need arises to calibrate underwater acoustic sources in

open sea environment. The use of a sonobuoy as a free floating, acoustic

receiver linked to a ship through a very high frequency (VHF) transmitter on

the buoy and a VHF receiver on board the vessel is a convenient solution.

With a remote acoustic receiver, a ship is free to tow an acoustic source in

close proximity to the sonobuoy receiver and information on the source

characteristics can be gained by analyzing the sonobuoy's acoustic data. One

of the problems with a free-floating receiver is in knowing its exact location. A

sonobuoy does not present a large radar target, and tracking one on radar is

difficult. If the buoy is equipped to send information on its position (latitude

and longitude) along with its acoustic data, then the problem is reduced. Fig.

below shows the concept of a GPS equipped sonobuoy system. Defence

Research Establishment Atlantic (DREA) conceived and constructed an early

version sonobuoy incorporating GPS positional information in the data

telemetry in 1997.

In December 1998, DREA issued a contract to Hermes Electronics Inc.

to design and produce a specialpurpose sonobuoy that features an acoustic

hydrophone and a low-gain preamplifier. This allowed the sonobuoy to

operate in environments of high sound pressure levels typical of close rangelow frequency active (LFA) sound sources. The response of the calibrated

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acoustic sensor is omni-directional, which is suitable for characterizing LFA

towed acoustic sources. The sonobuoy is equipped with a Global Positioning

System (GPS) receiver and the data from the GPS receiver is embedded in the

radio frequency (RF) transmission from the buoy. his paper describes the

sonobuoy system, including the buoy configuration, the shipboard receiver

system and the software to retrieve the GPS positional data.

GPS Sonobuoy Concept

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GPS EQUIPPED SONOBUOY SYSTEM

The GPS equipped sonobuoy system block diagram. The diagram

illustrates the functional blocks of the system (both hardware and software)

and their connection to other system components. The GPS equipped

sonobuoy system is divided into two functional units: the sonobuoy itself (Fig.

a) and the shipboard system (Fig. b). The buoy is considered first. An acoustic

hydrophone and electronic preamp are housed in a pressure vessel. This

assembly is referred to as the lower electronics unit (LEU). The output

acoustic signal is connected to the upper electronics unit (UEU) through a wire

and compliant suspension. The UEU also houses the GPS engine and the GPS

antenna. A microprocessor in the UEU programs the GPS receiver to send

specific binary data words at 9600 baud to the transmitter. This binary data

modulates a 57.6 kHz sub-carrier. The sub-carrier and the acoustic data fromthe hydrophone assembly combine to frequency modulate the RF carrier of the

sonobuoy transmitter in the UEU. The surface float is a small air-filled vinyl

bag. The VHF transmitter antenna runs from the top of the 300 mm inflatable

float to a metal plate at the top of the UEU. An end-of-life scuttling circuit is

disabled allowing for post-mission recovery of the sonobuoy.

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On board the ship, a VHF receiver tuned to the correct channel receives

the sonobuoy RF signal. The receiver output is connected to two filters. A low

pass filter set at 3000 Hz filters the acoustic data. A separate band pass filter

(pass band 50 kHz to 63 kHz) retrieves the sonobuoy GPS data. Thus the 57.6

kHz sub-carrier with the GPS data is isolated. A separate custom decoder

circuit restores the RS-232 binary data words from the sub-carrier. This data is

fed to the communications port (COM port) of a personal computer (PC).

A second GPS receiver is located on the upper deck of the ship and

serves as a base station. The corresponding RS- 232 data stream from the base

station is fed to a second COM port on the PC. This allows the software

installed and running on the PC to process real time relative kinematic (RTK)

positioning information of the relative location of the remote sonobuoy with

respect to the shipboard base station.

Fig.a. GPS Equipped Sonobuoy

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Fig.a GPS ship Board System

Figure 2. GPS Equipped Sonobuoy System Block Diagram

Several information windows are available in the software graphical

user interface (GUI). One such window is the plotting routine that traces the

navigational track of the base station and the remote sonobuoy with a pixel

"crumb" trail. Since GPS updates (epochs) occur every second, a clear picture

of the past movements of all remotes and the base can be gained at a glance.

The shipboard system hardware is apable of receiving and tracking up to four

buoys and one base station simultaneously. The software running on the PC

can be configured for up to 20 buoys simultaneously.

Hydrophone and Lower Electronics Unit

Hermes Electronics Inc. modified a regular production sonobuoy

(Model AN/SSQ53D(2) DIFAR) to use as a test bed for the GPS equipped

sonobuoy. The hydrophone and preamp module comprise the lower

electronics unit (LEU). The omni hydrophone is unchanged from the

production AN/SSQ53D(2) DIFAR and the directional hydrophones, although

present, are not used. The sensitivity of the omni hydrophone is -210 dB re 1

Volt/Pa at 1kHz. The standard LEU circuit board is replaced with a board

containing only a preamplifier and line driver for the omni hydrophone.

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The standard pre-whitening filter circuitry and the electronics to process

the directional (DIFAR) channels are also removed and the original

preamplifier is modified to provide lower gain. This ensured that the high

signal levels of LFA acoustic sources do not overload the sonobuoy system.

When a 1000 Hz acoustic signal with a sound pressure level of 194.9 dB re 1

Pa is present at face of the sensor, the shipboard sonobuoy receiver produces

an output signal of 1.0 volts RMS (0.0 dBV). Two filter circuits in the LEU

shape the frequency response of the preamplifier. A high pass filter at 104 Hz

and a low pass filter at 3000 Hz form a pass band esponse that is nominally

flat in the region from 100 Hz to 3000 Hz. Acoustic signals below100 Hz and

above 3000 Hz are out of band for the LFA application of the GPS equipped

sonobuoy.

The response of the hydrophone and preamp is dependent on depth. Fig.

below shows a typical normalized response versus frequency for three

different depths. A calibration curve for each GPS equipped sonobuoy ensures

that the response of each buoy is known.

Figure : GPS Sonobuoy Normalized Frequency Response Versus Depth

Cable Suspension and Deployment

A standard A size canister houses the GPS sonobuoy. Fig. 2a shows

the layout of the various components: the float, the UEU, the suspension pack,

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the LEU, and the acoustic sensors. Prior to deployment, the user must program

the buoy for the intended mission. The program options such as the RF

channel number, the deployment time and the depth of the acoustic receive

system are selected through a menu-driven push button choice on the side of

the canister. The deployment times are half hour, one hour, two, four and eight

hours and the available depths are shallow (30 m), medium (120 m) or deep

(300 m). Referring again to Fig. a, the suspension cable consists of a

spring/mass system to reduce the effects of the surface wave motion on the

receive sensors. A large spring/mass system is effective in reducing the wave

motion on the receive sensor. However, the surface area of the mass increases

the horizontal displacement between the sub-surface system and the surface

float, especially in large shear currents or high surface winds.

Thus a small 0.3 m circular damper plate is used to increase the

effective mass and at the same time, minimize the horizontal displacement due

to drag. Hermes Electronics modeled the performance of the 0.3 m circular

damper plate. At a depth of 300 m, the model predicted a horizontal

displacement of 10 m in a shear current of 2 knots.

GPS Receiver in the Sonobuoy

The GPS sonobuoy uses a Rockwell (Conexant) Jupiter LP (Model

TU30-D160-011) GPS receiver. It is a single frequency receiver in the L1

band (1575.42 MHz). The 12 parallel channel capability allows simultaneous

tracking of 12 GPS satellites. The low power (LP) version features lower

power consumption than the conventional model, requiring only 145 mW for

continuous operation at 3.3 VDC. Low power consumption is an important

consideration as a saltwater battery powers the buoy. One can configure the

Jupiter LP receiver to accept either a passive or an active antenna. Hermes

Electronics produced 20 buoys with passive antennas (Micropulse model

1621AW/C) and 10 buoys with active antennas (SiGem model

SGM3902PMX). The active antenna provides a gain of 13 dB providing bettersatellite reception. This active antenna comes at the price of increased power

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consumption. The SiGem antenna draws 7 mA when operating at 3.3 volts.

The active antenna version of the sonobuoy was first used during the DREA

sea trial in March 2001.

Although only three buoys featuring the active antenna were

successfully deployed, they demonstrated improved satellite reception over the

passive models. Where the passive-antenna buoy might receive four to five

GPS satellites per epoch, the active-antenna buoy would track five to six GPS

satellites. It is important to note that at least four satellites are required to

satisfy the relative positioning algorithm. The stability of the position solution

increases with the number of satellites received. Sea state plays a large part in

the ability of the GPS engine to remain locked on the satellite signals.

The random movement of the float as it is buffeted by the surface

waves deteriorates the satellite signal lock considerably. This effect is difficult

to quantify and will become the focus of future trials.

GPS Interface Board

The Jupiter LP GPS receiver resides as a "daughter board" on the GPS

Interface Board, a custom printed circuit Hermes Electronics designed and

produced. The GPS interface board sits in the upper electronics unit of the

AN/SSQ53D(2) DIFAR. The UEU has a spare card slot available and only

minor alterations in the other boards are necessary to accommodate the

presence of the GPS Interface board. The circuitry includes a crystal oscillator

operating at 3.6864 MHz that clocks a micro-controller, the Amtel

AT90S2313-4. This device is programmed at the time of manufacture and it

communicates with the Jupiter receiver when power is applied to the buoy. It

tells the Rockwell Jupiter receiver to turn off some default messages, what

data to send, and at what baud rate to send the data. Table 1 shows the

execution steps of the micro-controller program.

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DREA consulted Waypoint Consulting of Calgary, Canada, authors of

the navigational software package, RTKNav, to determine which GPS data

words are required from the sonobuoy. These critical words for the Rockwell

Jupiter GPS engine are Message 1000 (Geodetic Position Status) and Message

1102 (Measurement Time Mark). Some default messages (1002, 1003 and

1108) are still sent even after the Disconnect-All-Messages command is

delivered to the Jupiter receiver. Table 2 shows all the GPS binary messages

transmitted from the GPS receiver after the programming routine is executed

as well as the number of words/bits in each of the messages (two bytes per

word, 10 bits per byte). Thus the GPS buoy transmits a total of 8600 bits of

binary data per second at 9600 baud. Data is present for 0.896 seconds and

absent for 0.104 seconds before the next data transmission begins. A 16-stage

counter divides the 3.6864 MHz crystal oscillator used to clock the micro-

controller.

This produces a 57.6 kHz square wave. The binary data from the GPS

receiver toggles this square wave on and off in accordance with the polarity of

the binary data and the result is an amplitude shift-keyed (ASK) modulation ofthe 57.6 kHz signal. This encoded sub-carrier modulates the sonobuoy RF

carrier. A variable resistor on the GPS interface board adjusts the sub-carrier

signal amplitude to ensure correct deviation of the RF carrier in the sonobuoy

transmitter circuit.

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Table 1. GPS Interface Micro-Controller Program

Table 2. GPS Binary Data Transmitted

Sonobuoy Transmitter

The frequency modulation (FM) transmitter circuitry of the GPS

equipped sonobuoy is based on the phase locked loop (PLL), 99-channel VHF

transmitter developed at Hermes Electronics for the USN AN/SSQ53E

sonobuoy. This transmitters full power output is typically 1 watt. The GPS

sonobuoy incorporates design changes to the transmitters modulation

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crossover frequency circuitry. The full-scale deviation of the RF carrier is 75

kHz.

The GPS data causes the RF carrier to deviate 7.5 kHz while the

acoustic signal from the hydrophone deviates the RF carrier by a maximum of

67.5 kHz. With the GPS signal and the acoustic signal at the maximum, the

full-scale output of the standard sonobuoy receiver, Model AN/ARR-75 is 2.0

volts RMS. The possibility of interference between the fundamental VHF

frequecy of the transmitter and the GPS receiver was investigated. Tests on the

first 31 channels of the sonobuoys RF band (162 MHz to 174 MHz) revealed

no interference problems in the reception of the GPS signals. The ASK

modulation of the GPS data does, however, result in side band noise evident in

the base band spectrum of the sonobuoy receiver output. Discrete component

filters in the transmitter prevent this noise from contaminating the acoustic

frequency band of interest. Spectrum measurements indicate that the side band

signal levels at frequencies below 3000 Hz are down by 65 dB below the 57.6

kHz sub-carrier fundamental. Thus, no adverse effects of the sub-carrier areevident in the acoustic signals.

DETAILED DESCRIPTION OF THE SHIPBOARD

SYSTEM

Shipboard Sonobuoy Receiver

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As stated, the sonobuoy receiver is a Model AN/ARR-75. Capable of

receiving four buoys at one time, this FM receiver can be tuned to the first 31

RF channels of the 99-channel sonobuoy band. Channel numbers are

interleaved: Channel 1 is at 162.250 MHz, Channel 17 is at 162.625 MHz,

Channel 2 is at 163.000 MHz up to Channel 31 at 173.125 MHz and channel

16 at 173.500 MHz. A band of 375 kHz separates each channel. During the

first DREA trial (February 2000) the VHF antenna system was mounted on the

uppermost deck of CFAV Quest and did not use an antenna preamplifier.

Maximum range of reception was about three to four kilometers. In the

second DREA trial (March 2001) an RF amplifier with 16 dB of gain was

inserted in the antenna line about 20 meters from the antenna. The antenna

itself was raised about 10 meters higher than the previous trial to the ships

main mast. These changes extended the reception range of the sonobuoy to

over eight kilometers. Sea state also plays a large part in limiting the range of

the GPS reception from the buoy. High waves that wash over the sonobuoy

surface float, or shield the transmission from the buoy to the ship cause seriousdropouts in the RF signal and as a result in both the GPS and acoustic data. In

the two trials conducted, reasonable contact with the sonobuoy was maintained

up to sea state 3. Noticeable degradation in the quality of GPS data

transmissions occurred in sea state 4 and above. Seas higher than sea state 5

were not experienced in either trial.

Acoustic and GPS Data Filters and GPS Data Restoration

The shipboard receiver output feeds two filter configurations. A low

pass filter (eight pole Butterworth) set at 3000 Hz recovers the acoustic signal

that is then recorded and analyzed to determine the power spectrum of the

hydrophone signal. A band pass filter (eight pole Butterworth) set to 50 kHzand 63 kHz recovers the GPS data. This extracts the ASK modulated sub-

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carrier (57.6 kHz) that contains the sonobuoy GPS data. A custom electronics

decoder chassis demodulates the sub-carrier using a Mitel integrated circuit

MT8840 (data-over-voice modem) configured as a demodulator. The GPS

binary data appears at the chassis output in an RS-232 format (9600 baud,

eight data bits, no parity, one stop bit). The processing software running on the

personal computer requires this data at a COM port. Once received the

software assigns a remote designator to the data (R1 for example). Each

sonobuoy deployed must have a dedicated sonobuoy receiver channel, a

separate acoustic and GPS data filter, and a GPS data demodulator circuit.

DREAs current shipboard GPS system will accommodate up to four

sonobuoys simultaneously transmitting data.

Base GPS Receiver on the Ship

The base GPS station is mounted on board the ship. The base station

GPS antenna is the point from which the measurements of range and bearing.

To the sonobuoy are referenced. DREA uses a Rockwell Jupiter GPS engine

housed in a small chassis. This commercially available development kit

includes an active GPS antenna, an RS-232 interface electronics, and a power

supply. Switch settings on the chassis configure the output data format. Data is

set for Rockwell binary words (vice NMEA-0183 ASCII text strings) at 9600

baud, eight stop bits, no parity, one stop bit. The RS-232 output data is sent to

an unused COM port on the PC. The processing software assigns the Base

designator to the data.

RTKNav Software Program

Waypoint Consulting (Calgary, Canada) produces computer software

that performs real time GPS processing. Their product, RTKNav includes real

time processing for up to 20 remote receivers, using features like kinematic

ambiguity resolution, moving baseline, and heading determination.

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RTKNav runs on Windows 9x/NT and is able to create windows that

display the updated satellite data, solution data (range and bearing of the

remotes), processing and receiver status at every epoch. A plot of the base

station and its remotes can also be viewed and modified to focus on a specific

receiver. DREA has tracked up to three remotes on sea trials. Discussion on

the nature of the processing is outside the scope of this document but

Waypoint Consultings web site provides more information. The below

figures are show samples of actual window displays of RTKNav during the

DREA trials. Fig. 4 is taken from the trial on 14 February 2000. Several data

windows are displayed.

Figure: Sample RTKNav Window Display

The Plot View Master window (pixel map). the Plot View

Master window display from RTKNav during the DREA trial on 24 March

2001. The pixel map displays the vessel (CFAV Quest designated Base in

blue) proceeding on a closest-point-of-approach to the drifting sonobuoy

(designated R1 in red). The past track of the sonobuoys drift is very clear

and the zoom features of RTKNav allow close inspection on the precision of

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the relative position data. The ring markers are spaced at 30m. Very few

pixels lie outside a 10 m track. Notice that the red pixels are not evenly spaced

like the blue pixels. This depicts the track of one remote sonobuoy receiver

(R2 in green) and the track of the moving base (research vessel CFAV Quest

designated B in blue).

Sample RTKNav Window Display

It traces the ships bow-tie maneuver over the course of almost three

hours and shows the sonobuoys drift during that same time. The Solution

Data View Remote 2 window indicates positional information of the

sonobuoy at the time of the last positional fix. The window displays the

latitude (Lat) and longitude (Lon) as well as the relative position (Local Level)

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of the remote from the base. This information is given as an easterly offset (E)

in meters and a northerly offset (N) in meters. Negative values represent

westerly and southerly offsets respectively. The Vector View Master

window shows calculated range and bearing (true) of the remotes R1 and R2.

Indicates that the fixes from the sonobuoy are not received each second. Sea

state, antenna shielding and loss of satellite lock preclude data reception at

every epoch. However, there are plenty of data points to determine the course

of the sonobuoy.

CONCLUSIONS

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DREA deployed 17 GPS equipped sonobuoys with passive GPS

antennae during the trail in Feb 2000. Four failed to operate properly.

Problems included faulty RF transmission, and incorrect GPS data

composition. Accurate location fixes were maintained for ranges greater than

four kilometers. Five buoys with active GPS antennae were deployed during

the trial in March 2001. Two buoys failed to operate properly. Buoys with

active antennae generally tracked more satellites and reported more location

fixes than buoys with passive antennae. Tracking the position of the surface

float to an accuracy of 10 meters is routine. In conditions of low sea state and

robust GPS satellite reception, accuracy to within 3 meters is possible.

Improvements in the shipboard VHF receiver antenna system extended the

range out to eight kilometers. The inherent difference in horizontal

displacement between the sonobuoy surface float and the underwater sensor

ultimately limits the accuracy of determining the exact location of the acoustic

sensor. Hermes Electronics Inc. has provided DREA with a useful tool for

assessing the performance of high power underwater sound sources. Plans to

use the present system to bring the GPS capability to a full sensitivity,directional sonobuoy are underway. This will require new modulation

techniques for the GPS binary data. Smaller, and lower power GPS receivers

are also under consideration to help offset the increase in power required when

the functionality of the DIFAR circuitry is restored and the gain is increased to

that of a standard sonobuoy.

REFERENCE

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www.ieee.org

www.sonobuoy.com