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

`

`ACOUSTIC POSITIONING OF OCEANOGRAPHIC INSTRUMENTS
`
`D. L. McKeown
`
`Atlantic Oceanographic Laboratory
`Bedford Institute of Oceanography
`Dartmouth, Nova Sc0tia, Canada
`
`SUMt-IARY
`
`An acoustic technique for positioning oceano(cid:173)
`graphic instruments in three dimensions at any point
`in the water column or on the ocean floor is described.
`The system utilizes an array of acoustic transponders
`on the sea floor and an acoustic source controlled by
`an internal clock installed on the device to be posi(cid:173)
`tioned. Particular consideration is given to the pro(cid:173)
`blem of operating such a system in areas of very rugged
`topography where direct acoustic paths from instru(cid:173)
`ment to transponder may be obscured. Accuracy and
`repeatability of the technique utilizing both direct
`and surface reflected acoustic paths beb1een instru(cid:173)
`ment and transponder are examined experimentally.
`Results of an experiment to position a bottom
`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 underwater 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(cid:173)
`baseline acoustic positioning systems.
`In its
`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 the 1 atter b1o or
`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. 1
`
`Within the past few years, a need has arisen
`at the Bedford Institute of Oceanography for a
`system capable of positioning instruments and
`equipment in the ocean and on the ocean floor with
`a relative accuracy or repeatability of better
`than 20 metres. The system must operate without
`any hard-wire connection to the surface, function
`with instruments generating significant acoustic
`noise, and operate in regions of very rugged
`bottom topography such as the Mid-Atlantic Ridge.
`
`POSITIONING SYSTEM
`
`The positioning technique chosen is based on
`the long baseline range-range concept, that is,
`slant ranges from a ship and instrument to two or
`more acoustic markers on the ocean floor are measured
`It has been shown 1 • 2
`to determine their positions.
`that such a system is the best choice for precision
`surveying and navigation. One additional advantage
`over short baseline bearing-bearing or range-bearing
`systems is that extensive nonportable transducer
`installations aboard ship are avoided, ~aking the
`
`system readily transferable between ships. The
`acoustic bottom markers may be either beacon pingers
`which emit acoustic energy at predetermined times
`controlled by an internal clock or acoustic trans(cid:173)
`ponders which respond to external acoustic inter(cid:173)
`rooations but the latter are more suitable as bottom
`markers for this type of application because of their
`longer operating life on internal power sources and
`freedom fran· lono-term clock drift. The acoustic
`unit on the instrument to be positioned may also be
`either a transponder or a beacon pinger. A beacon
`pinger was chosen since it is unaffected by high
`acoustic noise fields the instrument may generate
`and can be readily resynchronized with a shipboard
`clock periodically to avoid clock drift problems.
`
`The interrogation cycle is shown in Figure 1.
`At time T = 0, the ship interrogates both trans(cid:173)
`ponders at a frequency f c. Only tl~o transponders,
`A and B, are sh01·m although three are required for
`an unambiguous fix. Each transponder replies at its
`o~m unique frequency, fA and f8, to permit identifi-
`cation thus measuring slant ranges SAS and s85 . Ship
`position can then be found by an interative least
`squares solution. Prior to deployment, a clock in the
`pinger attached to the instrument to be positioned is
`synchronized with a shipboard clock.
`It emits acoustic
`pulses at a repetition rate of , seconds at time
`T 1/2 + N:, where N is an integer. This acoustic
`pulse, at a frequency fc' interro9ates the transponders
`which respord at frequencies fA and f8. Aboard ship,
`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
`the interval ~12. the slant range frow pinger to
`transponder A is
`
`(1)
`
`and similarly slant ranges to all other transyonders
`can be detenrined.
`Instrument position can then be
`found by an iterative least squares solution.
`If only
`two transponders are successfully interrogated and it
`is known which side of the transponder A, B baseline
`ship and instru~ent are on, their positions can be
`found more quicrly by solving a set of spherical
`equations centered on the ship and transponders.
`
`~-t!2l_D_ VELOCITY AriD REFRACTION
`
`The slant range measurements described above
`are actually travel time measurements. To convert
`these measured travel ti~es to true slant ranges,
`sound velocity variations in the working area
`must be measured and each travel tiree converted to
`slant range throu9h applicatiors of Snell's Law to
`correct for refraction effects. A simpler procedure
`is to multiply each travel time by an appropriate value
`
`Vol.2 150
`
`Ex. PGS 1051
`
`

`

`In all experiments
`of harmonic mean sound velocity. 3
`to date, this latter approach has led to slant range
`errors 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.
`
`OPERATION IN RUGGED TOPOGRAPHY
`
`In areas such as the Mid-Atlantic Ridge, bottom
`topography is extremely rugged, thus, there is every
`likelihood that pinger-transponder paths will be
`obscured. The transponder could be suspended suffi(cid:173)
`ciently far off the bottom to 'see' over such
`obstructions but its position would then become uncer(cid:173)
`tain. Alternately, a surface reflected or multipath
`signal between pinger and transponder could be used.
`For a pinger at (Xp, YP, Zp) and a transponder at
`(XT' YT' ZT)' as shown in Figure 2, it can be demon(cid:173)
`strated that the measured pinger-surface-transponder
`slant range, SPRT' is
`
`and the equivalent direct slant range SPT is
`
`(3)
`
`where the reference plane for depth is a horizontal
`plane through the shipboard transducer at depth D.
`
`SYSTEM HARDWARE
`
`The acoustic transponders used are American
`Machine and Foundry (AMF) Model 322 units interro(cid:173)
`gated at a common frequency of 11 kHz and replying
`at unique frequencies from 9 to 13 kHz. They are
`fitted 1~ith buoyancy in the form of six 0. 4-metre
`Corning Glass spheres, a radio beacon and a flashing
`light to 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(cid:173)
`vision for synchronization with an external clock,
`a repetition rate selectable from 10 to 100 seconds,
`and an output frequency of 11 kHz. The shipboard
`system is illustrated in Figure 3. The interroga(cid:173)
`tion cycle is controlled by the clock and control
`unit. Acoustic signals are received and converted
`to binary coded decimal
`(BCD) slant ranges by an AMF
`Model 205 four-channel range-range receiver. These
`slant ranges are recorded in a number of formats as
`shown, as well as routed to a digital computer for
`real-time positioning. Several different transducer
`installations have been used successfully including
`a special purpose hull-mounted transducer, a standard
`12 kHz echo sounder transducer, and a transducer
`mounted in a 1.22 metre Braincon V-fin.
`
`ABSOLUTE ACCURACY
`
`To test absolute positioning accuracy of the
`system, the pinger was attached to a Guildline
`Model 8100 conductivity-temperature-depth (CTD)
`instrument capable of measuring instrument depth
`1~ith an accuracy of 2 metres. The instrument package
`
`was 1 m~ered to 1000 metres and raised in 1 DO-metre
`increments. At 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
`change in depth determined by the CTD for 20 samples
`was 8.4 metres, every pinger-transponder path being a
`direct one of the type shown in Figure 1. This error
`in depth deter~ination is caused by transponder survey
`and range measurement errors and has been dealt with
`elsewhere. 3 • 4
`
`REPEATABILITY, DIRECT AND SURFACE REFLECTED PATHS
`
`An important parameter of an acoustic position(cid:173)
`ing system is its repeatability or ability to define
`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 at 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
`repeatability, was 3,0 m for 482 samples.
`
`A second experiment was carried out to examine
`the repeatability of three-dimensional instrument
`positioning and slant range measurement. A pinger,
`P, was moored on the bottom within a transponder triad
`as shown in Figure 4. The ship then steamed back and
`forth through this triad determining its position rela(cid:173)
`tive to both the acoustic transponders and radar trans(cid:173)
`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(cid:173)
`ponders and pinger were determined by a technique des(cid:173)
`cribed previously. 3 • 4 Knowing the absolute position of
`the ship, transponders, and pinger, it was possible to
`compute the expected direct and surface reflected
`pinger-transponder slant ranges. Virtually all success(cid:173)
`ful interrogations beb:een the pinger and transponder
`ATB-3 were a result of surface reflected signals.
`Occasional surface reflected interrogations of trans(cid:173)
`ponders ATB-1 and 2 were also noted. Three-dimensional
`pinger coordinates and slant ranges from pinger to
`transporders were computed for all successful direct
`and surface reflected interrogations as summarized in
`Table 1.
`
`No constraints were placed on ship move~ent
`during this experiment, thus, a significant portion
`of the fixes occurred when the ship was on or near
`a baseline as it passed back and forth through the
`triad. Such fixes introduce considerable positional
`error leading to a poorer repeatability than in the
`Guildline CTD comparison experiment where a more
`optimum geometry ~:as chosen. Table 1 indicates that
`pinger coordinates determined by direct and surface
`reflected slant range measurements agree within
`10 metres and the total root-mean-square variation in
`position does not exceed 17.E metres in either case.
`The measured pinger-transponder ATB-1 and ATB-2 slant
`ranges show a significantly lower standard deviation
`than pinger to ATB-3 surface reflected slant ranges.
`It has been sho~m that range measurement accuracy is
`directly proportional to signal-to-noise ratio at the
`receiver. 3 The surface reflected path has a poorer
`signal-to-noise ratio than direct path because of
`scattering of energy at the point of reflectinn and
`
`Vul.2- 151
`
`Ex. PGS 1051
`
`

`

`TABLE 1. PINGER COORDINATES FOR DIRECT AND SURFACE REFLECTED INTERROGATIONS
`
`Coordinate No. of
`Fixes
`
`Direct
`Mean #1 Std. Dev.
`(m)
`(m)
`
`No. of
`Fixes
`
`Surface Reflected
`Mean #2 Std. Dev.
`(m)
`(m)
`
`#l - #2
`(m)
`
`Xp
`
`Yp
`
`Zp
`
`Slant 1-P
`
`Slant 2-P
`
`Slant 3-P
`
`760
`
`760
`
`760
`
`1496
`
`1413
`
`2181.6
`
`12.6
`
`3171.4
`
`2220.6
`
`1987.9
`
`1626.3
`
`9.4
`
`8.0
`
`6.6
`
`5.1
`
`5
`
`2866.0
`
`12.6
`
`899
`
`899
`
`899
`
`38
`
`9
`
`863
`
`2187 .l
`
`12. l
`
`3181.3
`
`2218.2
`
`9.7
`
`8.4
`
`-5.5
`
`-9.9
`
`+2.4
`
`2002.9*
`
`19.4
`
`-15.0
`
`1621 .8*
`
`12.3
`
`2874.7*
`
`14.9
`
`+4.5
`
`-8.7
`
`A POSITIONING EXPERIMENT
`
`* These are equivalent direct slant ranges as determined from equation (3) and pinger depth = 2218.2 metres.
`by means of a surface reflected interrogation between
`increased attenuation due to longer path length.
`pinger and transponder. The alternate solution of sus(cid:173)
`pending the transponder high enough off the bottom to
`avoid blind areas is not viable since its position
`becmr.es very uncertain. The onset of multipath can be
`determined by plotting a cross-section of the bottom
`topography from pinger to transponder or monitoring
`pinger transponder slant ranges as the instrument is
`lowered from the surface.
`In all 39 lowerings of
`bottom sa"-pling devices to date in rugged topography,
`such a multipath has been detected and used success(cid:173)
`fully for positioning.
`If no constraints are placed
`on ship movement relative to the transponder array,
`repeatability of pinger position is 17.6 metres by
`both direct and surface reflected signals from pinger
`to transponder.
`If fix geometry is optimized, that
`is, neither ship nor pinger near a baseline, absolute
`accuracy of depth measurement by direct signal path
`is 8.4 metres and repeatability 3.0 metres. There is
`a region of 'cross-over' bet>-:een direct and surface
`reflected paths in which positioning is poor. No
`explanation for this 'cross-over' region has been found
`yet. Positioning in this region should be avoided by
`choosing transponder positions relative to sample
`stations such that the latter do not require positioning
`at depths corresponding to this 'cross-over' region.
`
`To test the principles outlined above, an
`experiment was conducted in 1-1hich a sampling station
`on the bottom at a depth of 2714 metres was chosen
`to generate a surface reflected path between pinger
`and transponder ATB-1 at a depth of 2397 metres. Onset
`of multipath condition was predicted to occur at a depth
`of 2490 metres from an examination of the cross-section
`of bottom topography between the drilling station and
`ATB-1 as shm·m in Figure 5. The second transponder
`was moored 5592 metres from ATG-1 at a depth of
`2290 metres. There were no osbtructi ons betv1een the
`sample station and this transponder, hence, all
`interrogations ~1ere successfully completed by a direct
`path between pinger and transponder. The variation
`in ship and drill (pinger) northing and easting and
`drill depth as it v1as deployed and recovered are
`sho1-1n in Figure 6. The onset and cessation of multi(cid:173)
`path interrogations occurred at 2501 metres and
`2515 metres pinger depth respectively as predicted
`from Figure 5. There was a 'cross-over' during which
`alternate direct and multipath signals were obtained
`from the pinger-ATB-1 path. Figure 6 shm-1s that, in
`this region, horizontal positioning by the surface
`reflected path was poor initially but improved at
`greater pinger depths. Little cross-over error was
`noted in pinger depth.
`It was found that application
`of exact refraction corrections instead of using
`harmonic mean sound velocity to convert travel time
`to slant range did not significantly improve the
`'cross-over' error. Computer simulations for the
`geometry of this station indicated that the 'cross(cid:173)
`over' region \>laS not caused by the algorithms used,
`errors in slant range measurement, or errors in
`determining transponder baseline length. Sinulated
`errors in transponder or shipboard transducer depths
`caused the northing and easting of the drill position
`to be displaced upon onset of a multipath signal but
`did not produce the form of 'cross-over' distortion
`illustrated in Figure 6.
`
`SUt~MARY Arm CONCLUSIOt~S
`
`It has been shovm that an oceanographic instru(cid:173)
`ment or bottom sampling device can be positioned in
`three dimensions by fitting it with a suitable acoustic
`pinger and utilizing ocean floor acoustic transponders.
`In areas of very rugged topography such as the t~id­
`Atlantic Ridge, positioning is readily acco~1plished
`
`1.
`
`2.
`
`3.
`
`4.
`
`REFERENCES
`
`Van Clacar, H., "Acoustic position reference
`methods for offshore drilling," Offshore
`Technology Conference Proc., Houston, Texas,
`May 18-21, 1969, Vol. II, pp. 467-482.
`
`Fain, G., "Error analysis of several bottom
`referenced navigation systems for small sub(cid:173)
`mersibles," Proc. 5th Annual t~arine Technology
`Society Conf~rence, Washington, D.C.,
`June 15-18, 1969.
`
`t·1cKeov:n, D.L., "Survey techniques for acoustic
`positioning arrays," submitted to Navigation,
`Journal of the Institute of Navigation, 1974.
`
`McKeown, D.L., and R.~1. Eaton, "An experiment
`to determine the repeatability of an acoustic
`range-range positioning system," to be
`presented at the International 5ymposium on
`Applications of Marine Geodesy, Columbus,
`Ohio, June 3-5, 1974.
`
`Vol.2 · 152
`
`Ex. PGS 1051
`
`

`

`--------------------------~~--------L--- SURFACE
`
`Z=O
`
`A
`
`a)
`
`SHIP- TRANSPONDER - SHIP INTERROGATION AT T ~ 0
`
`T
`
`Figure 2. Surface reflected pinger (P) - surface (R)
`- acoustic transponder (T) signal path.
`
`Figure 1. Acoustic signal paths for ship and pinger
`interrogations.
`
`bl PINGER-TRANSPONDER-SHIP INTERROGATION AT T=O+T/2
`
`BCD
`TIME
`
`CONTROL
`UNIT
`
`TIME+ 4 BCD
`SLANT RANGES
`
`PAPER TAPE
`PUNCH
`
`DIGITAL
`PRINTER
`
`DIGITAL
`COMPUTER
`
`0
`
`TRANSDUCER
`
`AMF POWER
`AMPLIFIER
`
`TRIGGER
`
`AMF CODER
`
`TRIGGER
`
`CH.I D-A
`CONVERTER
`
`ANALOGUE
`RECORDER
`
`CH.4 0-A
`CONVERTER
`
`ANALOGUE
`RECORDER
`
`Figure 3. Block diagram of shipboard acoustic positioning interrogation and data acquisition system.
`
`Vo1.2- 153
`
`Ex. PGS 1051
`
`

`

`N
`
`5000
`
`4000
`
`ATB-2
`z;2461 m
`
`3000
`
`METRES
`
`2000
`
`1000
`
`ATEi-1
`Z ;f664 m
`
`" DEPTH + 1000 m
`•
`/
`
`DEPTH + 2000 m
`
`DEPTH +1000 m
`
`..
`
`100
`
`ATB-3
`Z ;2293 m
`
`DRILL
`DEPTH
`METRES
`
`800
`
`600
`
`400
`
`200
`
`7800
`
`7600
`
`7400
`DRILL
`NORTHING
`METRES
`o"a-·-·
`72oo
`o
`.. - .. .,.. .. ...,. ... _........_.,...,._ :nc:PJ c'=tdJO~::;a:::-""'::lc~D:I
`"o
`°D:l
`
`o
`
`0
`
`0
`
`7000
`
`1000
`
`2000
`
`3000
`METRES
`
`4000
`
`5000
`
`E
`
`Figure 4.
`
`Acoustic transponder array and pinger (P)
`location to determine repeatability for direct
`and surface reflected acoustic paths
`
`DRILL 840
`EASTING
`METRES 8200~
`
`8800
`
`+ DIRECT
`0 MULTIPATH
`co
`8600
`"a,
`83 DIRECT+MULTIPATH
`.............. -+ ... ·~n:.-.. ~~~::co
`~co-.............. ...
`-!ball
`
`::::Lt~~~--~~~~~~~~~~~~~~~~
`
`7800
`
`7600
`
`SHIP
`7400
`NORTHING
`METRES
`720
`
`I
`t
`I
`I
`-12490m
`I
`
`-
`
`2300
`
`2500
`
`DEPTH
`METRES
`
`2600
`
`2700
`
`VERTICAL EXAGGERATION 10•1
`
`2800L---~OL-------------------------~4~85~1-m-----
`ATB-1
`DRILL
`
`Figure 5. Cross-section of bottom topography between
`bottom sampling station (drill) and
`acoustic transponder ATB-1.
`
`6sod-~-L~~~~~~~~~~~~~L-~~~~
`
`88oof
`
`8600~
`
`CR. 71-002
`STN. 110
`
`SHIP
`
`8400;··-·--······ •••• _
`
`EASTING
`METRES
`8200
`
`~
`
`8oooL
`
`-·-••
`•• .•.
`
`~ ... ~ ... """ -..
`· .... ______ ......
`
`78ooL---~,o~~2ko~~3o~~4~0~-s~o~-e6~o~~7*o~~oon-~~
`TIME MINUTES
`
`Figure 6. Ship and drill northing and easting and
`drill depth as functions of time for bottom
`sampling station.
`
`Vol.2 · 154
`
`Ex. PGS 1051
`
`