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International

Technical Note JSN-81-64

August 1981

FISH RAGU (FISH, RADIO-RECEIVING AND GENERALLY USEFUL)

R. L. Garwin

SRI International 16 11 North Kent Street Arlington, Virginia 22209

" Fish Ragu (Fis h, Radio - Rece i ving a nd Genera lly Useful ) " JAS ON Technica l Note JSN - 81 - 64. (OB008 1FTS H)

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18. SUPPLEMENTARY NOTES

19. K£ v WOAOS ; Co," .. "". on , . ... , .. .. 0. il 'I.e .... , ... e"<1 io . ",,'y by OIOC _ n .. mootl

ST~TEGIC SU&~INE COMXUNl~~TIONS

VL? CO~ICATIONS SU!1'1ARINE VLF RECEIVERS

:10. ASS"l"I'IACT ICo"""". OR ' ..... " . 110. ,I n..: .... 'v ... 0 io . ", ily 0.,. CIO.~ ""mo. , '

Tho concep t of using a SOkg self-propelled hody a, a receiver for V1F signals 1s presented . This " fish " ..... ould operate a few IDe:te r 5 below che 5ur face and communic3.te Co a submarine via high frequency acoustics . Zinc .;)xide ba tteries provide sufficient power for all of the " fish's" oper.3tions including a 24 hour endurance a c a 4 knot speed . The "fish" could also be used as a sonar receiver.

• sensor of submar ine radiated noise and as a VHF or UHF rece ive r .

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TABLE OF CONTENTS

I NTRODUCTION .. ... .. ....... ....... .. ... .. ... ...... ... RADIO-LINK FUNCTION ...... .. .... .. ... ... ..... ... .... . STATI ONKEEP I NG ........ ....... ........ , .......... ... . 3. 1 3 . 2

NOR.11AL STATIONKEEPING FALL- BACK STATIONKEEPING

... ....... .. ..... ... ....

SKETCH .. ... ...... .. .. .......... ... .... ... .. .... ... .. 4 . 1 4 . 2 4 . 3

PROPULSION TRAIN RADIO SUBSYSTEM ACOUSTIC SUBSYSTEM

... .. .... ... ... ....... .. ...... . ... .. .. .. ....... ..... ........ .

SUBK~RINE MODIFICATIONS .. .. .. ... .. ...... .. ... .... .. . DETECTABILlTY ...... .. ....... .... .... ....... ...... ... 6.1 6 . 2 6. 3

ACOUSTIC-LIW~ DETECTABILITY ...... ...... ....... . KELV I N WAKE PROPULS ION NOISE

........... .... .. .............. .. ....... .......... ........... .. '

OTHER USES .. ... ......... ... ......... .. .... .. ....... . SUl-IMARY ..... .......... ... ... ........ .. .. .. ........ .. DISTRIBUTION LIST ...... .. .. ..... ... ... ......... .....

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I INTRODUCTION

Blue - green lasers and ELF are massive technological approaches

to improving a single link in the chain for one- way communication to

s ubmarines . In both cas es, the submarine (or at least the

communications receiver) would still be restricted in operating depth,

bu t less severely than by the few-rneter limit~tion imposed by skin depth

of VLF. There should be interest in another approach to improving the

communications link to the submarines, which ;eems far cheaper , better,

and more generally useful than either BGL or ELF .

The concep t is simply to employ a re lay contained in a small

"fish" which patrols directly above the subma:-ine , submerged a few

meters in the water and containing ferrite - roc! antennas for receiving

the normal VLF signal. The VLF band is simply translated in frequency

and modulated as FM or A"I or.to a megahertz aCDustic signal

( A = 1.5 mm ) r adia t e d t o the submarine from a transducer on the belly

of the fish . The a ttenuation of megahenz sOI:nd in the ocean is such

that the transmission cannot be heard at long range, and the acoustic

power output required fol' eXcl:!Jlent signal to noise ratio over a

k ilohertz band to a rec e i \'e r 10 cm diam at 40C ill. is less than a

ni crowatt.

1

At l east t wo s uch fish (and probabl y many more) would be housed

in a module attached to the submarine , wh i ch would provide recharging

facili t ies fo r tlleir bat teries. A fish would weigh about 50 kg and

cculd trave l fo r about an hour at 10-knot or all day at 4- knot speed .

A fish .... ould normally swim at constant speed and heading unless

C OQ~nded by the host submarine (via the megahertz acoustic link) to

change r.eading, translate sideways by a specified amount, translate

longit udi nally by a spe.:.1f i ed amount, change speed, o r r e turn t o its

housing (change mode). The fish would be tracked by a lO-cm di am

trans duce r on the subma~lne. which .... ould a l so serve for 2- way

con::::nJ n ica t for. •

Assuming zinc- l.ickel oxide (Zn-NiO) batteries at SO w-h/ kg (as

produc~d in l arge quan tHies by General Motors on its Delco- Remy pilot

lir.e f or electric ... ·ehicles) we assume 20 kg of the t otal fish weight of

50 kg is batteries. Thus the ener gy available is 1 kwh .

Such a fi~h wo~ ld have an endurance of only abou t 7 min at

20-- knot (and 2 min at ~O-knot) speed. Fo r ve r y- high-spe ed, long­

<!ndur-l:1ce traverse, it !Li ght be be tter to equip one of the fish with an

optiona l t o '..l line, bu t t he l ine could be a s imple mechanica l attachme"nt,

since ::he ir.telligence I.oul d be transmit ted via the acoust i c link, and

the electroni cs suppli e d forever by the batteries.

2

Additional functions for such a fish could be a quiet sonar

receiver , a fish to sense submarine radiated noise as a function of

directi on , and the like. It could also be equipped with a body-mounted

whip antenna (o r l ow-gain a rray) fo r deployment in case the VLF signal

disappears and VHf or UHF corn:r.unciation (via satellite) is desired with

satellit es .

3

II R.ADI~LI.'lK FUNCTION

Since the VLF band i s so noisy , only modest frequency

selectivit y and ga in need be pr ovided 1n the fish t o r eceive t he signal .

It 1s amplified sligh tly and modulated onto the megahertz acoustic

transducer , ~ith an antenna patte rn limited t o the downwa r d quarter

sphere . The acoustic signal could be ~ide-band FH, with the deviation

maintained by the received VLF s i gna l o r by noise if no VLF s1gna l is

received .

4

II I STAT!ONKEE P ING

The fish should r emain direc tly above t he submar i ne {o r at some

convenient {small] angle offse t to the vertical. This prevents the

no rmal varia ti on in sound velocity with depth from causing significan t

signal re f r action or shado~ine .

3 . 1 Normal Stationkeeping

Ordi narily the stationkeeping is oaintained by erro r signals

derived from the high-gain megahertz acoustic an t enna on the submarine .

Thi s IO- cm diam antenna has a beam~idth of about I deg ree. and is to be

used to derive fish velocity and position . The co~parison of the fish

velocity and pOS ition with t hose desi red should r esult in t he

transmission of c ommands f r om t he s ubma rine to the fi sh to corr ect its

position, velocity, or heading . In order to min i mize the consequences

of a loss of transmmission, the fish should have sufficient intel ligence

(via a microprocessor) so that position modification could be

accomplis hed by a position-change command, rather ::han by a command to

change velocity (which would have t o be paired wit~ a later compensation

of that velocity change in o r der t o re s ult in a ne t change of position

only) .

5

3.2 Fa ll- back St a tionkeeping

When the submarine is at the s ur face , it obviously canno t

communicate from a transduce r atop t he submarine t o the belly of the

fish . Under the se ci rcums t ances, the fish s hould patrol, submerged , at

some distance abeam t he submarine . A few-meter offset can be maintained

by refere nce to a 4- element megahertz transduce r mounted abeam the

submarine.

Launching and recovery of t he fi s h should be done l argely open

loop . The fish ~ould be commanded by the megahertz link and tracked to

within a few feet of its housing. At that point , a "h ome " command would

be issued. which would re su lt i n t he fish dropping down and forging

ahead into its housing, where it would be r etained by a cl~mp. valve , or

the like. It would t hen be r echarged th rough electrical connec tions to

the battery. Launching the fish is s imple r, since it nee d only drift

out of the housing, climb and advance to a point at whi ch it can be

picked up by the acoustic link .

6

IV SKETCH

The po~er source would be a Zn-NtO battery of about 1 kwh

capac ity . Scaling required-power as velocity cubed and displacement to

the 2/3 po~~ r gives a requirement of about 0 . 4 kw for operation at 10-

knot and about a 0.03 k· ... f or operation at 4- knot. The battery voltage

would be conve rted by an efficient solid-state converter to ope rate a

varia bl e-s peed e o tor connected to the propeller.

4 . 1 Propuls i on Train

At lO-knot speed, the Brenouilli pressure is about

1. 2 x 105 dyne /cm2 , about equal to the hydrostatic pressur e at 1 m

depth . A requ i red propulsive power of 500 w (at 10-knot speed)

corresponds t o a propulsi on for ce of about 107 dyne or 104 gm force.

Thus , a propeller area exceeding 100 cm 2 would reduce the propulsion

pressur e to below the Bre~ouill i pressure and should allow propeller

opera t ion ~l thcut cavitation at depth exceeding 1 m. At 10-knot speed,

a propell e r r o t a tion rate nn the orde r of 600 rpm seems suitable .

4 .2 Radio Subsystem

The radio subsys tem sh(uld consist of two ferrite- core antenna

coils faT Vi,.F recC!ption i ndependen t of VLF pola rization and propagation

direction , ane appropriate dep l oyable satellite antennas for use when

VLF is down or for t r ansmi ssion from the submarine to satellite via a

7

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burst low-probability- of - intercept (LPt) system. The e l ec t ronics should

be fed from the main battery , constituting a negligible drain.

4.3 Acoustic Subsystem

For the radio-link function, t he acoustic subsystem is a low­

gain an t enna on the belly of the fish, transmitting signals to the

su~rine and receiving signals froe the submarine . The two signals may

he ~ltiplexed onto the same tran sduce r, since only a modest bandwidth

is required for each at the megahertz frequency.

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V SUBMARINE MODIFICATIONS

The submarine must be provided with a housing for seve ral RAGU

fish , probably two acoustic tracking antennas in order that constant

coverage by a fish at the surface may be obtained (and to allo~ for

failure s ) , power supply to th e hou sing , and co mmunicat i ons through the

hull f o r the signa l and recharging power of a few kw (pea k ). Only small

hull penetrations are required.

The modifications are best accomplished via a single ,

consolidated housing and electronics subsection , faired to the submarine

surface. The size i s that r equired to accommoda te three or f our 50- kg

fish--about a meter long, 30- cm high , and perhaps a mete r broad.

9

VI DETECTABILITY

Naturally one does not .... ant t o add t o the detectability of the

bare submarine, a~d one .... ants t o reduce the potent ia l detection

possibilit ies by the trailing .... ire or the to .... ed buoy no .... used for

r eceiving VLF communications .

6.1 Acoust ic - Link Detectability

The tracking antenna on the submarine has more tha n 40 dB ga in

and operates at a distance not exceeding some 400 m. If desired, the

signal can be broad-band so as t o reduce the probability of inte r cept by

a subma r ine hunter.

6 . 2 Kelvin Wake

A body about 30-cm diao travelling at 10- kno t produces a flo ....

velocLcy .... hich falls of! as the inverse cube of the radiu s from t he body

in the plane perpendicular t o the axis . At 1 m distance , this flow

amo unts to only abou t O,l-knot and is limi t ed t o a region about 1 meter

1n extent . It would corres pond to an eleva tion of the .... ater surface by

0 . 01 c~--difficult to detect among the norma l disturbances .

6 . 3 Propul siOl! Noi~.':..

At a rna:<ir.:um turn rate of 10 rps (and with a three- bladed

propeller combined with hila tera l symme try of the fish and propeller

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shroud), the alternating forces should be e:<:ceedingly small compared

with those on a submarine. Further~ore , since the structure is so small

compared I.'ith the \.,'ave le ngth of sound wave s in this frequency r ange,

the radiation efficiency 15 ouch further reduced. By holding the water

surface speed over the propeller below cavitation speed at the operating

depth , one can avoid high-frequency noise. These are design constraints

on the propulsion system, and seem readily satisfied . Any problems can

be solved by increasing the diameter or the propeller .

11

VII OTHER USES

Such a fi s h operat ed from submarines could be used to provide a

displaced listening ?ost, far fr om the internal noise of t he

subcarine. h ,",auld be even core valuabl,~ 1£ deployed from surface

ships, because of t he much higher no ise level of the ship and t he ocean

surface . Na t urally, the fish could be used for a subma r ine at great

depth to detect blue- g r een laser radiation o r ELF! It could also be

used t o map accura tely t he r a d i ated acoustic f ield from a submarine , to

intercept active s ona r s i gnals before they r each the subma r ine , and the

like.

12

VIII SUNl'lARY

By the use of consume r t echnology in the propulsion system ,

struc ture, and elec tronics of such a fish, it wo uld seem possible t o

replicate it for $1,000 . Naturally the housing, tracking sys tem,

command generator. and t he like on the suboarine ~ould be mo r e costly.

The fish ~~GU would be a major aid to the flexib i lity and covertness of

submarine opera t ions, wh ile ~intaining excel l en t receive capabi lity on

VLF signa l s . If the fix ed - base VLF transmitters I.'ere destroyed. TACAMO

aircraft would take ove r. If all U. S. VLF signals disu?peared. a VHF or

UHF antenna could be deployed from the fish RAGU at appointed t imes to

listen to dark satel l i t es .

In view of the modest en try cost and substant ia l poten tial

benefits , the characte r iza tion and deve l opment of ;>uch n fish should be

under taken now .

13

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