`
`
`Ex. PGS 1051
`
`
`
`EX. PGS 1051
`
`
`
`
`
`

`

`OF OCEANOGRAPEIC INSTRUMENTS
`ACOUSTIC POSITIONING
`0. L . McKeown
`At1 a n t i c Oceanographic Laboratory
`Bedford Institute of Oceanography
`Dartmouth, Nova Scotia, Canada
`
`SUMMARY
`
`An acoustic technique for positioning
`oceano-
`graphic instruments in three dimensions
`a t any point
`column o r on the ocean floor is described.
`in the water
`The system u t i l i z e s an array of acoustic transponders
`on the sea floor
`and an
`acoustic source controlled
`by
`on the device to
`an internal clock installed
`be posi-
`t o the pro-
`tioned. Particular consideration is given
`of very rugged
`blem of operating
`such a system in areas
`from instru-
`topography where direct acoustic paths
`ment to transponder may be
`obscured. Accuracy and
`repeatability of the technique utilizing
`both d i r e c t
`and surface reflected acoustic paths
`between instru-
`ment and transponder are examined experimentally.
`an experiment to position a bottom
`Results of
`sampling device utilizing such
`a multipath are
`presented.
`
`INTRODUCTION
`
`Modern oceanographic studies such as surveys
`with submersible or bottom sampling devices
`have
`created a demand for precise undetvdater navigation
`and positioning systems.
`The most practical means
`of achieving precise positional
`measurement under
`the ocean's surface is through the
`use of
`acoustic
`techniques classified as short-baseline
`and long-
`In i t s
`baseline acoustic positioning systems.
`minimum configuration, the former utilizes
`one
`ocean floor acoustic marker, one acoustic marker
`on the device to
`be positioned, and a hydrophone
`array on the support vessel
`and t h e l a t t e r
`two o r
`more acoustic bottom markers,
`an acoustic source
`on the instrument to
`be positioned, and a single
`shipboard transducer.
`Methods of utilizing short
`baseline and long baseline systems
`have been
`described in the literature.'
`
`few years, a need has arisen
`Within the past
`a t t h e Bedford I n s t i t u t e of Oceanography
`f o r a
`system capable of positioning instruments
`and
`equipment in the ocean and on the ocean floor with
`a r e l a t i v e accuracy or repeatability of b e t t e r
`than 20 metres. The system must operate without
`t o the surface, function
`any hard-wire connection
`with instruments generating significant acoustic
`noise, and operate i n reaions of very rugged
`bottom topography such as the Mid-Atlantic Ridge.
`
`POSITIONING SYSTEM
`The positioning technique chosen i s based on
`the long baseline range-range concept, that is,
`s l a n t ranges from a ship and instrument to
`two o r
`more acoustic markers on the ocean floor are measured
`I t has been shown1y2
`to determine their positions.
`t h a t such a system is the best choice for precision
`surveying and navigation. One additional advantage
`over short baseline bearing-bearing or range-bearing
`systems i s t h a t extensive nonportable transducer
`i n s t a l l a t i o n s aboard ship are avoided, Faking the
`
`between ships. The
`system readily transferable
`acoustic bottom markers may be
`e i t h e r beacon pingers
`which e r i t a c o u s t i c energy a t predetermined times
`controlled by an internal clock or acoustic trans-
`ponders which respond to external acoustic inter-
`rogations b u t t h e l a t t e r a r e more suitable as bottom
`markers for this type
`of application because of their
`on internal power sources and
`longer operating life
`freedom fror long-term clock drift.
`The acoustic
`unit on the instrurrent to
`be positioned may a l s o be
`e i t h e r a transponder or a beacor! pinger. A beacon
`pinger was chosen since i t i s unaffected by high
`the instrument may generate
`acoustic noise fields
`and can be readily resynchronized with
`a shipboard
`clock periodically to avoid clock drift problems.
`
`shown in Figure 1.
`The interrogation cycle is
`b o t h trans-
`A t tirre T = 0 , the ship interrogates
`ponders a t a frequency f c . Only two
`transponders,
`A and B, are shown although three are required for
`a t i t s
`an unambiguous f i x . Each transponder replies
`own unique frequency,
`f A and f B , t o permit identifi-
`cation thus measurins slant ranges
`SAs and SBs. Ship
`position can then be found by an i n t e r a t i v e l e a s t
`squares solution. Prior to deployment,
`a clock in the
`be positioned is
`pinger attached to the instrument to
`a shipboard clock.
`synchronized with
`I t emits acoustic
`pulses a t a repetition rate of T seconds a t time
`T = ~ / 2 + NT, where N i s an integer. This acoustic
`a frequency fc, interro9ates the transponders
`pulse, at
`f A and f g . Aboard ship,
`\.rhich respord at frequencies
`the instant of interrogation is known, hence, slant
`ranges Sps, SPAS and SpBs are measured. After
`making
`appropriate allowance to
`SAs for ship movement during
`range frorr pinger t o
`the interval
`: / 2 , the slant
`transponder A
`i s
`
`'PA = ' P A S - 'AS
`
`(1)
`
`and similarly slant ranges to all other trans;onders
`can be deterrrined. Instrument position
`can then be
`found by an iterative least squares solution. If only
`and i t
`two transponders are successfully interrogated
`A , B baseline
`i s known which side of the transponder
`ship and instruvent are o n , their positions can be
`found more quickly by solving a s e t of spherical
`equations centered on the ship and transponders.
`
`SOl!?4D VELOCITY AllC REFRACTION
`
`above
`The s l a n t range measurements described
`are actually travel time
`measurercents. To convert
`t o true slant ranges,
`these measured travel times
`sound velocity variations in the
`working area
`must be measured and each travel
`t i r e converted to
`of S n e l l ' s Law t o
`s l a n t range throuah applicatiors
`correct for refraction effects.
`A simpler procedure
`by an appropriate value
`is to multiply
`each travel time
`
`\:'ol.?
`
`- 150
`
`Ex. PGS 1051
`
`

`

`was lowered t o 1000 metres and raised in 100-metre
`increments. A t each depth,
`a number of acoustic
`interrogations were completed and pinger depth computed.
`Absolute depth measurement accuracy of the system
`defined as the standard deviation of the differences
`between mean change in depth measured acoustically and
`by the CTD f o r 20 samples
`change in depth determined
`was 8.4 metres, every pinger-transponder path being
`a
`1 . This e r r o r
`d i r e c t one of the type
`shown in Figure
`in depth deterrination is
`caused by transponder survey
`and range measurement e r r o r s and has been dealt with
`el sewhere. $ 4
`
`REPEATABILITY, DIRECT AND SURFACE REFLECTED PATHS
`An important parameter of an acoustic position-
`t o define
`ing system i s i t s r e p e a t a b i l i t y o r a b i l i t y
`the relative positions of
`sample stations particularly
`in the case of surface reflected signals.
`To assess
`repeatability, the mean acoustic depth of the Guildline
`CTD used in the above experiment was found a t each
`depth increment
`and the difference between this depth
`and each individual measurement of depth
`computed. The
`standard deviation of these differences,
`a measure of
`r e p e a t a b i l i t y , was 3.0 m f o r 482 samples.
`A second experiment was carried o u t to examine
`the repeatability
`of three-dimensional instrument
`A pinger,
`positioning and s l a n t range measurement.
`P , was moored on the bottom within
`a transponder triad
`4. The ship then
`steamed back
`and
`as shown in Figure
`f o r t h through this triad determining its position rela-
`t i v e t o both the acoustic transponders
`and radar trans-
`ponders located at geodetic stations
`on shore as well
`as recording the pinger-acoustic transponder-ship slant
`ranges. The absolute positions of the acoustic trans-
`ponders and pinger were determined by a technique des-
`cribed previously.3y4
`Knowing the absolute position
`of
`and pinger, i t was possible t o
`the ship, transponders,
`cospute the expected direct
`and surface reflected
`pinger-transponder slant ranges. Virtually all success-
`ful interrogations
`between the pinger
`and transponder
`ATE-3 were a r e s u l t of surface reflected signals.
`trans-
`Occasional surface reflected interrogations of
`ponders ATB-1 and 2 were also noted. Three-dimensional
`fron! pinger t o
`and slant ranges
`pinger coordinates
`transpopders were computed for all successful direct
`and surface reflected interrogations as
`summarized in
`Table 1 .
`
`of harmonic mean sound ~ e l o c i t y . ~ I n all experiments
`t o d a t e , t h i s l a t t e r
`approach has led
`t o s l a n t range
`e r r o r s of less than 7 metres, the
`peak occurring at
`about 6000 metres range, for ship-transponder paths
`and 2.5 metres error for pinger-transponder paths
`once the pinger is
`below the thermocline.
`
`bottom
`
`OPERATION IN
`
`RUGGED TOPOGRAPHY
`as the Mid-Atlantic Ridge,
`I n areas such
`topography is extremely rugged, thus, there is every
`1 i kel i hood that pinger-transponder paths wi 11 be
`obscured. The transponder could be suspended s u f f i -
`bottom t o ‘see’ over such
`ciently far off the
`obstructions b u t i t s p o s i t i o n would then become uncer-
`tain. Alternately,
`a surface reflected or multipath
`signal between pinger and transponder could be used.
`Y p , Z p ) and a transponder at
`For a pinger at
`( X p ,
`(XT, YT, Z T ) , as shown in Figure
`2 , i t can be demon-
`s t r a t e d t h a t t h e measured pinger-surface-transponder
`slant range,
`SpRT, i s
`
`’PRT = [(X P
`
`
`T
`-X )2 + (Yp-YT)2
`
`+ (Zp+ZT+2D)2]’
`
`( 2 )
`
`and the equivalent direct slant
`
`range SpT i s
`
`SPT = [ ( S ~ R T ) - ~(ZT’D) (Zp+D)IL’
`
`(3)
`
`a horizontal
`where the reference plane for depth is
`D.
`plane through the shipboard transducer at depth
`
`SYSTEM HARDWARE
`
`The acoustic transponders used a r e American
`Machine and Foundry (AMF) Model
`322 units interro-
`gated a t a common frequency of 11 kHz and replying
`a t unique frequencies
`are
`from 9 t o 13 kHz. They
`fitted with buoyancy in the
`form of
`s i x 0.4-metre
`Corning Glass spheres,
`a radio beacon and a flashing
`l i g h t t o permit recovery.
`The beacon pinger is
`an
`AMF Model 360
`unit with
`an internal clock having
`a
`drift rate equivalent to
`0.75 metres per day, pro-
`an external clock,
`vision for synchronization with
`10 t o 100 seconds,
`a repetition rate selectable fror
`11 kHz. The shipboard
`and an
`output frequency of
`system is illustrated in Figure
`3. The interroga-
`tion cycle is controlled
`by the clock and control
`unit. Acoustic signals are received
`and converted
`(BCD) s l a n t ranges by an AMF
`to binary coded decimal
`Model 205 four-channel range-range receiver.
`These
`a number of formats as
`slant ranges are recorded in
`t o a d i g i t a l computer f o r
`shown, as well as routed
`real-time positioning. Several different transducer
`i n s t a l l a t i o n s have been used successfully including
`a special purpose hull-mounted transducer,
`a standard
`and a transducer
`12 kHz echo sounder transducer,
`mounted in a 1.22 metre Braincon V-fin.
`
`No constraints were placed on ship movement
`during this experiment, thus,
`a significant portion
`or near
`of the fixes occurred
`when the ship was on
`i t passed back and forth through the
`a baseline as
`t r i a d . Such fixes introduce considerable positional
`error leading to
`a poorer repeatability than in the
`Guildline CTD corparison experiment where a more
`1 indicates that
`optimum geometry was chosen. Table
`pinger coordinates determined
`by d i r e c t and surface
`reflected slant
`range measurements agree within
`10 metres and the total root-mean-square variation in
`not exceed 17.6 metres in either case.
`position does
`The measured pinger-transponder ATB-1 and ATE-2 s l a n t
`ranges show a s i g n i f i c a n t l y lower standard deviation
`t o ATB-3 surface reflected slant ranges.
`than pinger
`I t has been shown t h a t range measurerent accuracy
`i s
`a t the
`directly proportional to signal-to-noise ratio
`receiver.3 The surface reflected p a t h has a poorer
`signal-to-noise ratio than direct path because of
`scattering of energy at the point
`of reflection and
`Vo1.2 - 151
`
`ABSOLUTE ACCURACY
`
`of the
`To test absolute positioning accuracy
`system, the pinger
`was attached to a Guild1 ine
`Model 8100 conductivity-terperature-depth (CTD)
`instrument capable of measuring instrument depth
`with an accuracy of 2 metres. The instrument package
`
`Ex. PGS 1051
`
`

`

`TABLE 1. PINGER COORDINATES FOR DIRECT
`
`AND SURFACE REFLECTED INTERROGATIONS
`
`Re fle ct ed
`
`Surface
`
`
`Coordinate No. o f
`F i x e s
`-
`
`Direct
`Mean 81 Std. Dev.
`(m)
`(m)
`
`No. o f
`F i x e s
`(m)
`
`
`
`Mean 12 Std. Dev.
`(m)
`
`#1 - 1 2
`(m)
`
`2181.6
`
`12.6
`
`XP
`
`YP
`
`ZP
`
`S l a n t 1-P 1496
`
`760
`
`760
`760
`
`3171.4
`
`2220.6
`
`1987.9
`
`S l a n t 2-P
`
`1413
`
`1626.3
`
`S l a n t 3-P
`
`5
`
`2866.0
`
`12.6
`
`9.4
`
`8.3
`
`6.6
`
`5.1
`
`899
`
`89 9
`
`89 9
`
`38
`
`9
`
`863
`
`2187.1
`
`12.1
`
`3181.3
`
`2218.2
`
`2OO2.9*
`
`1621.8*
`
`2874.7*
`
`9.7
`
`8.4
`
`19.4
`
`12.3
`
`14.9
`
`-5.5
`
`-9.9
`
`+2.4
`
`-1 5 .O
`
`+4.5
`
`-8.7
`
`as determined from equation (3)
`
`and pinger depth = 2218.2 metres.
`
`* These a r e e q u i v a l e n t d i r e c t s l a n t r a n g e s
`in cre ased at ten ua t io n due t o l o n g e r p a t h l e n g t h .
`
`A POSITIONING EXPERIMENT
`
`above, an
`To t e s t t h e p r i n c i p l e s o u t l i n e d
`experiment was conducted i n which a sampling station
`on the bottom
`a t a depth of
`2714 metres was chosen
`to generate a s u r f a c e r e f l e c t e d p a t h
`between pinger
`and transponder ATB-1 a t a d e p t h o f 2397 metres. Onset
`o f m u l t i p a t h c o n d i t i o n was p r e d i c t e d t o o c c u r a t
`a depth
`o f 2490 metres from
`an examination of the cross-section
`of bot t om topog ra p h y b e t wee n th e dri lli ng stati on
`and
`ATB-1 as shown i n F i g u r e 5. The second
`transponder
`was moored 5592 metres from ATE-1 a t a d e p t h o f
`2290 metres. There were
`no o s b t r u c t i o n s between t h e
`sample s t a t i o n and this transponder,
`hence, a l l
`i n t e r r o g a t i o n s were successfully completed by
`a d i r e c t
`path between pinger
`and transponder. The v a r i a t i o n
`i n s h i p and d r i l l ( p i n g e r ) n o r t h i n g
`and e a s t i n g and
`d r i l l d e p t h as i t was deployed and recovered are
`shown i n F i g u r e 6. The onset and cessation of mul ti-
`p a t h i n t e r r o g a t i o n s o c c u r r e d a t
`2501 metres and
`2515 metres pinger depth respectively
`as p r e d i c t e d
`from Figure 5. There was a 'cross-over' during which
`a l t e r n a t e d i r e c t and m u l t i p a t h s i g n a l s were obtained
`from the pinger-ATB-1 path. Figure
`6 shows t h a t , i n
`t h i s r e g i o n , h o r i z o n t a l p o s i t i o n i n g b y t h e s u r f a c e
`r e f l e c t e d p a t h was p o o r i n i t i a l l y b u t i m p r o v e d a t
`was
`g r e a t e r p i n g e r d e p t h s . L i t t l e c r o s s - o v e r e r r o r
`noted i n p i n g e r d e p t h .
`It was f o u n d t h a t a p p l i c a t i o n
`o f e x a c t r e f r a c t i o n c o r r e c t i o n s i n s t e a d o f u s i n g
`harmonic mean sound v e l o c i t y t o c o n v e r t t r a v e l t i m e
`t o s l a n t r a n g e d i d n o t s i g n i f i c a n t l y i m p r o v e t h e
`'cross-over' error. Computer simulations for the
`geometry o f t h i s s t a t i o n i n d i c a t e d t h a t t h e ' c r o s s -
`ov er ' reg ion was n o t caused by the algorithms used,
`e r r o r s i n s l a n t r a n g e measurement, o r e r r o r s i n
`determining transponder baseline length. Sirlulated
`errors in transponder or shipboard transducer depths
`caused the northing
`and e a s t i n g o f t h e d r i l l p o s i t i o n
`t o be displaced upon o n s e t o f a m u l t i p a t h s i g n a l b u t
`form o f ' c r o s s - o v e r ' d i s t o r t i o n
`did not produce the
`i l l u s t r a t e d i n F i g u r e 6.
`
`SUMMARY AtjC CORCLUSIONS
`
`shown t h a t an oceanographic instru-
`It has been
`ment or bottom sampling device can
`be p o s i t i o n e d i n
`by f i t t i n g i t w i t h a s u i t a b l e a c o u s t i c
`three dimensions
`pinger and u t i l i z i n g ocean floor acoustic tran spo n ders.
`I n areas of very rugged topography such
`as the Mid-
`A tl ant i c Ridge, po s it ioning is rea d ily a cco mpl ishe d
`
`be
`
`by means o f a surface reflect ed interrogation between
`sus-
`pinger and transponder.
`The a1 t e r n a t e s o l u t i o n o f
`pending the transponder high
`enough o f f t h e b o t t o m t o
`a v o i d b l i n d a r e a s i s n o t v i a b l e s i n c e i t s p o s i t i o n
`becorres very uncertain.
`The o n s e t o f m u l t i p a t h c a n
`determined by plotting
`a cross-section of the bott om
`t o transponder or monitoring
`topography from pinger
`pinger transponder slant ranges
`as t h e i n s t r u m e n t i s
`l o w e r e d f r o m t h e s u r f a c e . I n a l l
`39 l o w e r i n g s o f
`bottom sarpling devic es t o date in rugged topography,
`such a multipath has been detected
`and used success-
`I f no constraints are placed
`f u l l y f o r p o s i t i o n i n g .
`on ship movement r e l a t i v e t o t h e t r a n s p o n d e r a r r a y ,
`r e p e a t a b i l i t y o f p i n g e r p o s i t i o n i s 1 7 . 6 m e t r e s b y
`b o t h d i r e c t and surface reflected signals from pinger
`If f i x g e o w t r y i s o p t i m i z e d , t h a t
`to transponder.
`i s , n e i t h e r s h i p n o r p i n g e r n e a r
`a baseline, absolute
`accuracy of depth measurement by
`d i r e c t s i g n a l p a t h
`i s
`i s 8.4 metres
`and r e p e a t a b i l i t y 3.0 metres. There
`a region of 'cross-over' bet ween direc t
`and surface
`No
`r e f l e c t e d p a t h s i n w h i c h p o s i t i o n i n g i s p o o r .
`e x p l a n a t i o p f o r t h i s ' c r o s s - o v e r ' r e g i o n
`has been found
`y e t . P o s i t i o n i n g i n t h i s r e g i o n s h o u l d
`be avoided by
`choosing transponder positions relative to sample
`s t a t i o n s s u c h t h a t t h e l a t t e r
`do n o t r e q u i r e p o s i t i o n i n g
`at depths corresponding to this 'cross-over' region.
`
`1.
`
`2.
`
`3.
`
`4.
`
`REFEREWCES
`-~
`
`" A c o u s t i c p o s i t i o n r e f e r e n c e
`Van Clacar, H.,
`rrethods f o r o f f s h o r e d r i l l i n g , " O f f s h o r e
`Technology Conference Proc., Houston, Texas,
`May 18-21, 1969,
`Vol. 11, pp. 467-482.
`
`" E r r o r a n a l y s i s o f s e v e r a l b o t t o m
`Fain, G.,
`referenced navigation systems for small sub-
`mersibles," Proc. 5th Annual Marine Technology
`D . C . ,
`Society Conference, Washington,
`June 15-13, 1969.
`
`f o r a c o u s t i c
`IkKeown, D.L., "Survey techniques
`posit ioning arrays, " submit ted to Nav igation,
`J o u r n a l o f t h e I n s t i t u t e o f N a v i g a t i o n , 1 9 7 4 .
`
`R.M. Eaton, "An experiment
`McKeown, D.L., and
`t o d e t e r m i n e t h e r e p e a t a b i l i t y o f
`an a c o u s t i c
`range-range positioning system," to be
`p r e s e n t e d a t t h e I n t e r r a t i o n a l Z y n p o s i u n
`A p p l i c a t i o n s o f K a r i n e Geodesy, Columbus,
`3-5, 1374.
`Ohio, June
`
`on
`
`Vo1.2 - 152
`
`Ex. PGS 1051
`
`

`

`SHIP- TRANSPONDER - SHIP INTERROGATION AT
`
`01
`
`T = 0
`
`(P) - s u r f a c e (R)
`F i g u r e 2. S u r f a c e r e f l e c t e d p i n g e r
`- a c o u s t i c t r a n s p o n d e r (T) s i g n a l p a t h .
`
`F i g u r e 1. A c o u s t i c s i g n a l p a t h s f o r s h i p a n d p i n g e r
`i n t e r r o g a t i o n s .
`
`bl PINGER-TRANSPONDER-SHIP
`
`INTERROGATION AT T=O+T/Z
`
`ICLoCKI
`
`TIME+ 4 BCD
`SLANT RANGES
`
`4
`PRINTER I
`UNIT .
`
`CONTROL
`I Ir
`I
`I
`
`I
`
`I
`
`t
`
`TRANSDUCER
`
`AMPLIFIER
`
`AMF CODER
`
`TRIGGER
`
`-
`
`ANALOGUE
`RECORbER
`
`CH. 4 D-A
`-CONVERTER
`
`Ex. PGS 1051
`
`

`

`Nt
`
`t
`
`I
`
`IOoo
`
`DEPTH + 2000 m
`
`* DEPTH +IO00 m
`7
`*
`
`ATB-3
`2 =2293 m
`
`DEPTH
`METRES
`
`-
`
`200-
`
`a
`
`
`
`-
`
`-
`
`0
`
`78001
`
`I
`1000
`
`I
`2000
`
`I
`
`
`3000 4000
`METRES
`
`I
`
`5000
`
`.m
`E
`
`
`
`"6800
`
`"
`
`O
`
`n
`
`+ DIRECT
`
`Figure 4.
`
`and pinger ( P )
`Acoustic transponder array
`location to determine repeatability for direct
`and surface reflected acoustic paths
`
`EASTING
`METRES
`
`23001
`2500 I
`
`2400
`
`DEPTH
`METRES
`2600
`
`VERTICAL EXAGGERATION
`
`104
`
`t
`I
`
`NORTHING
`METRES
`720
`
`88001
`
`680 " " " " " " " "
`
`CR. 71-002
`STN 110
`
`-
`
`---.*
`
`-1. *-*.
`
`2800 I
`
`0
`ATB-I
`
`4851 m
`DRILL
`
`EASTING
`METRES,,,,
`
`8600L
`
`between
`Figure 5. Cross-section of bottom topography
`bottom sampling
`s t a t i o n ( d r i l l ) and
`acoustic transponder ATB-1.
`
`and easting and
`Figure 6 . Ship and drill northing
`drill depth as functions of time for
`bottom
`sampling s t a t i o n .
`
`Vo1.2 - 154
`
`Ex. PGS 1051
`
`