ACOUSTIC POSITIONING OF OCEANOGRAPHIC INSTRUMENTS
`
`D.L. McKeown
`
`Atlantic Oceanographic Laboratory
`Bedford Institute of Oceanography
`Dartmouth, Nova Scotia, Canada
`
`SUMMARY
`
`An acoustic technique for positioning oceano—
`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-
`tioned. Particular consideration is given to the pro—
`blem of operating such a system in areas of very rugged
`topography where direct acoustic paths from instru-
`ment
`to transponder may be obscured. Accuracy and
`repeatability of the technique utilizing both direct
`and surface reflected acoustic paths between instru-
`nent 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-
`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 latter two 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
`
`a need has arisen
`Within the past few years,
`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
`to determine their positions.
`It has been shownli2
`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, making the
`
`The
`system readily transferable between ships.
`acoustic bottom markers may be either beacon pingers
`which emit acoustic energy at predetermined times
`controlled by an internal clock or acoustic trans—
`ponders which respond to external acoustic inter—
`rogations 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 from long—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 I.
`time T = 0,
`the ship interrogates both trans-
`At
`ponders at a frequency fc. Only two transponders,
`A and B, are shown although three are required for
`an unambiguous fix.
`Each transponder replies at its
`own unique frequency,
`fA and f8,
`to permit
`identifi—
`cation thus measuring slant ranges SAS and 585'
`Ship
`Dosition 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 T seconds at time
`T 2 1/2 + NT, where N is an integer. This acoustic
`pulse, at a frequency fc’ interrogates the transponders
`Aboard ship,
`which respord at frequencies fA and fB‘
`the instant of interrogation is known, hence, slant
`ranges SPS’ SPAS and SPBS are measured. After making
`appropriate allowance to 5A5 for ship movement during
`the interval
`;/2,
`the slant range from pinger to
`transponder A is
`
`PAS ' SAS
`
`(1)
`
`and similarly slant ranges to all other transponders
`can be determined.
`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 instrument are on,
`their positions can be
`found more quickly by solving a set of spherical
`equations centered on the ship and transponders.
`
`
`SOUND VELOCITY AND REFRACTION
`
`The slant range measurements described above
`are actually travel
`time measurements.
`To convert
`these measured travel
`times to true slant ranges,
`sound velocity variations in the working area
`must be measured and each travel
`time converted to
`slant range through applications 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
`
`1
`
`ION 1051
`ION 1051
`
`

`

`lowered to 1000 metres and raised in TOO—metre
`was
`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 l. This error
`in depth determination is caused by transponder survey
`and range measurement errors and has been dealt with
`elsewhere.3s“
`
`REPEATABILITY, DIRECT AND SURFACE REFLECTED PATHS
`
`An important parameter of an acoustic position—
`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.
`
`to examine
`A second experiment was carried out
`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—
`tive to 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.3=“ 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-
`ful
`interrogations between the pinger and transponder
`ATE—3 were a result of surface reflected signals.
`Occasional surface reflected interrogations of trans—
`ponders ATB—l and 2 were also noted. Three-dimensional
`pinger coordinates and siant ranges from pinger to
`transponders were computed for all successful direct
`and surface reflected interrogations as summarized in
`Table 1.
`
`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 TUPOGRAPHY
`
`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-
`ciently far off the bottom to 'see' over such
`obstructions but its position would then become uncer-
`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-
`strated that the measured pinger—surface-transponder
`
`slant range, SPRT’ 15
`
`S
`
`PRT : [(
`
`X -X )2 + (YP-YT)2 + (ZP+ZT+2D)
`P
`T
`
`]2
`
`(2)
`
`3/
`
`2
`
`and the equivalent direct slant range SPT is
`
`5PT = [<SPRT)
`
`- 4(ZT+D)(Z +D)]%
`P
`
`(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—
`gated at a common frequency of ll kHz and replying
`at unique frequencies from 9 to l3 kHz.
`They are
`fitted with 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-
`vision for synchronization with an external clock,
`a repetition rate selectable from 10 to 100 seconds,
`and an output frequency of ll kHz.
`The shipboard
`system is illustrated in Figure 3.
`The interroga-
`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
`l2 kHz echo sounder transducer, and a transducer
`mounted in a l.22 metre Braincon V-fin.
`
`No constraints were placed on ship movement
`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 was chosen. Table 1
`indicates that
`pinger coordinates determined by direct and surface
`reflected slant range measurements agree within
`l0 metres and the total
`root—mean-square variation in
`position does not exceed l7.6 metres in either case.
`The measured pinger—transponder ATB—l and ATE—2 slant
`ranges show a significantiy lower standard deviation
`than pinger to ATB—3 surface reflected slant ranges.
`It has been shown 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 reflection and
`Vol.2 -151
`
`ABSOLUTE ACCURACY
`
`To test absolute positioning accuracy of the
`the pinger was attached to a Guildline
`system,
`Model 8l00 conductivity-temperature-depth (CTD)
`instrument capable of measuring instrument depth
`with an accuracy of 2 metres.
`The instrument package
`
`2
`
`

`

`TABLE 1.
`FINGER COORDINATES FOR DIRECT AND SURFACE REFLECTED INTERROGATIONS
`Surface Reflected
`Direct
`Mean #1 Std. Dev.
`No. of
`Mean #2 Std. Dev.
`(m)
`(m)
`Fixes
`(m)
`(m)
`
`Coordinate
`
`No. of
`Fixes
`
`#1 - #2
`(m)
`
`XP
`
`Yp
`
`ZP
`S1ant 1-P
`
`S1ant 2-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
`
`899
`
`899
`
`899
`38
`
`9
`
`2187.1
`
`3181.3
`
`2218.2
`2002.9*
`
`1621.8*
`
`12.1
`
`9.7
`
`8.4
`19.4
`
`12.3
`
`-5.5
`
`—9.9
`
`+2.4
`45.0
`
`+4.5
`
`5 2866.0 12.6 863 2874.7* 14.9SIant 3—P -8.7
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`A POSITIONING EXPERIMENT
`
`* These are equiva1ent direct s1ant ranges as determined from equation (3) and pinger depth = 2218.2 metres.
`increased attenuation due to 1onger path 1ength.
`by means of a surface ref1ected interrogation between
`pinger and transponder.
`The a1ternate soiution of sus-
`pending the transponder high enough off the bottom to
`avoid b1ind areas is not viabIe since its position
`becomes very uncertain.
`The onset of mu1tipath can be
`determined by p1otting a cross-section of the bottom
`topography from pinger to transponder or monitoring
`pinger transponder s1ant ranges as the instrument
`is
`iowered from the surface.
`In a11 39 Towerings of
`bottom samp1ing devices to date in rugged topography,
`such a mu1tipath has been detected and used success—
`fu11y for positioning.
`If no constraints are p1aced
`on ship movement reiative to the transponder array,
`repeatabi1ity of pinger position is 17.6 metres by
`both direct and surface ref1ected signais from pinger
`to transponder.
`If fix geometry is optimized, that
`is, neither ship nor pinger near a base1ine, abso1ute
`accuracy of depth measurement by direct signa1 path
`is 8.4 metres and repeatabi1ity 3.0 metres. There is
`a region of
`'cross—over' between direct and surface
`refiected paths in which positioning is poor.
`No
`exp1anation for this 'cross—over'
`region has been found
`yet. Positioning in this region shouid be avoided by
`choosing transponder positions re1ative to samp1e
`stations such that the latter do not require positioning
`at depths corresponding to this 'cross-over' region.
`
`To test the princip1es out1ined above, an
`experiment was conducted in which a samp1ing station
`on the bottom at a depth of 2714 metres was chosen
`to generate a surface ref1ected path between pinger
`and transponder ATB-T at a depth of 2397 metres. Onset
`of mu1tipath condition was predicted to occur at a depth
`of 2490 metres from an examination of the cross-section
`of bottom topography between the dri11ing station and
`ATE-1 as shown in Figure 5.
`The second transponder
`was moored 5592 metres from ATS—1 at a depth of
`2290 metres. There were no osbtructions between the
`samp1e station and this transponder, hence, a11
`interrogations were successfu11y completed by a direct
`path between pinger and transponder.
`The variation
`in ship and dri11 (pinger) northing and easting and
`dri11 depth as it was dep1oyed and recovered are
`shown in Figure 6.
`The onset and cessation of mu1ti-
`path interrogations occurred at 2501 metres and
`2515 metres pinger depth respective1y as predicted
`from Figure 5. There was a “cross—over‘ during which
`a1ternate direct and mu1tipath signa15 were obtained
`from the pinger—ATB—1 path.
`Figure 6 shows that,
`in
`this region, horizonta1 positioning by the surface
`ref1ected path was poor initia11y but
`improved at
`greater pinger depths. Litt1e cross—over error was
`noted in pinger depth.
`It was found that appiication
`of exact refraction corrections instead of using
`harmonic mean sound ve10city to convert travei
`time
`to s1ant range did not significant1y improve the
`'cross—over' error. Computer simuiations for the
`geometry of this station indicated that the 'cross—
`over'
`region was not caused by the aTgorithms used,
`errors in s1ant range measurement, or errors in
`determining transponder base1ine 1ength.
`Simu1ated
`errors in transponder or shipboard transducer depths
`caused the northing and easting of the dri11 position
`to be disp1aced upon onset of a mu1tipath signa1 but
`did not produce the form of
`‘cross—over‘ distortion
`i11ustrated in Figure 6.
`
`1.
`
`2.
`
`3.
`
`4.
`
`REFERENCES
`
`Van C1acar, H., ”Acoustic position reference
`methods for offshore dri11ing,“ Offshore
`Techno1ogy Conference Proc., Houston, Texas,
`May 18-21, 1969, V01. II, pp. 467-482.
`
`Fain, 6., “Error ana1ysis of severai bottom
`referenced navigation systems for sma11 sub—
`mersib1es,“ Proc. 5th Annua1 Marine Techno1ogy
`Society Conference, Nashington, 0.0.,
`June 15-18, 1969.
`
`McKeown, D.L., ”Survey techniques for acoustic
`positioning arrays,” submitted to Navigation,
`Journa1 of the Institute of Navigation, 1974.
`
`McKeown, D.L., and R.M. Eaton, “An experiment
`to determine the repeatabi1ity of an acoustic
`range-range positioning system,“ to be
`presented at the Internationa1 Symposium on
`App1ications of Marine Geodesy, Co1umbus,
`Ohio, June 3-5, 1974.
`
`SUMMARY AND CONCLUSIONS
`
`It has been shown that an oceanographic instru-
`ment or bottom samp1ing device can be positioned in
`three dimensions by fitting it with a suitab1e acoustic
`pinger and uti1izing ocean fioor acoustic transponders.
`In areas of very rugged topography such as the Mid-
`At1antic Ridge, positioning is readi1y accomp1ished
`
`Vol.2 ~ 152
`
`3
`
`

`

`
`
` SURFACE
`Z =0
`
`
`interrogations .
`
`u)
`
`SHIP‘ TRANSPONDER ' S-IIP INTERROGATION AT T =0
`
`
`
`Figure 2. Surface reflected pinger (P) - surface (R)
`— acoustic transponder (T) signal path.
`
`Figure 1. Acoustic signal paths for ship and pinger
`
`h) PlNGER-TRANSPONDER-SHIP INTERROGATION AT T=O+Tl2
`
`
`
`TIME+ 4 BCD
`
`SLANT RANGES
`
`
`PAPER TAPE
`PUNCH
`
`DIGITAL
`PRINTER
`
`DIGITAL
`COMPUTER
`
`
`
`
`
`
`
`
`TRANSDUCER
`
`AMF POWER
`AMPLIFIER
`
`CH. I D-A
`ANALOGUE
`
`
`CONVERTER
`RECORDER
`
`
`AMF
`
`CODER
`
`CH.4D-A
`CONVERTER
`
`
`ANALOGUE
`RECORDER
`
`
`
`Figure 3. Block diagram of shipboard acoustic positioning interrogation and data acquisition system.
`Vol.2 - 153
`
`4
`
`

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`
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`
`Figure 4.
`
`Acoustic transponder array and pinger (P).
`location to determine repeatability for direct
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`
`Vol.2 - 154
`
`5
`5
`
`