, I ! - , , 08/00/8 1 International Technical Note JSN-81-64 August 1981 FISH RAGU (F ISH, RADIO-RECEIVING AND GENER ALLY USEFUL) R. L. Garwin SRI International 16 11 North Kent Street Arlington, Virginia 22209 " Fi sh Ragu (Fis h, Radio - Rece i ving and Ge ner a ll y Usef ul )" JAS ON T ec h nica l Note JSN - 81 - 64. (OB008 1FTSH) oscoz I f LSft
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FISH RAGU (Fish, Radio-Receiving and Generally Useful)
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,
I !
-, ,
t11~ 08/00/8 1
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)
oscoz I f LSft
UNCLASS IFIE D SECU~ I TY CI.AS$Ii<,eAT!ON OF "H 'S PAGE 1 .... ~.n 0 ... En • • ,.."
REPORT DOCUMENTATION PAGE READ INsrRUCTJOfVS BEFORE COMPLETING FORM
FlE~OFl r N UM8EFI 2 , GOVT ACCESSION '0 . , Ae C ,p'E/IIT"S CATALOG NuMIEPo
JS~ -8 1-64
" . TITL~ I,nd S .. """., •• TYPl; OF REPOAT & PE RIOD COVEFlEO
FISH :tAGU (FISH. ~~IO-~ECEIVING AND GENERALLY Technical Repor ! USEF1.JL)
•• PERFOAMING oo::c; . REPD?! " NUMIEFI
7 . AUT"ORhl , CONTRACT OR G"''''NT NI,/M!lERI.,
R. L. Garwin MDA-903- 78- c- OOe6 , PERFORMING ORGANIZATION NAME A N D AOQRESS ' O. ""OOl"' ''''''' ELEMENT . PR:l'ECT . TAS",
SRI International AREA, & .... 0'1 0; 1.1"',1' NI • .'M8EFlS
1611 North Ken t Stree t Arlington, VA 22209
12. i'lEI'ORT 0 ':'1( 113. 1110. OF PAGES 11 . CON r I>O l.,-,NO OFFICE NA M ;; "'NO AOQRESS
Au .. st 1981 Defense Acvanced Projects Research Agency 1 ;
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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
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 .
I DO , ~~:M" 1473 UNCLASS IFIED EDITI ON OF T NOV 65 IS OBSOl.ETE
"SS' ~'C" T' ON O~ T"' S PAGE ' W~.n 00 " ~ .. ,. , . ",
00, ~~~~~3'473( 3ACK J EDITIO N OF , NOV 6S IS OBSOLE TE SECUI'I 'TY CL.IISS,F'C.IIT'ON O·F TH'S PAGE 'W ..... 0 . , . En ...... '
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VIII
TABLE OF CONTENTS
I NTRODUCTION .. ... .. ....... ....... .. ... .. ... ...... ... RADIO-LINK FUNCTION ...... .. .... .. ... ... ..... ... .... . STATI ONKEEP I NG ........ ....... ........ , .......... ... . 3. 1 3 . 2
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
....
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.
8
. .
. .'
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
10
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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