Acoustic doppler system and method

A survey system including a multibeam echo sounder having a projector array and a hydrophone array in a Mills Cross arrangement uses a multicomponent message to ensonify one or more fans to estimate a Doppler velocity.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to underwater acoustical systems, methods for using underwater acoustical systems, and methods for processing and using the data they produce. In particular, the invention relates to survey systems including sonar systems capable of making Doppler measurements such as Doppler velocities associated with multifan ensonification of fixed or moving targets.

Discussion of the Related Art

A month after the Titanic struck an iceberg in 1912, English meteorologist Lewis Richardson filed a patent at the British Patent Office for an underwater ranging device. Modern day successors to Richardson's invention are often referred to as SONAR (sound navigation and ranging) devices.

Modern day SONAR devices include ones capable of making Doppler measurements to determine velocities. Where there is relative motion between a target and a SONAR that ensonifies the target, echoes from the target may be used to determine relative target velocities. For example, where a SONAR moves relative to a fixed target, echoes from the target may be used to determine SONAR velocities.

Such Doppler velocity measurements are subject to multiple sources of error. Such errors limit the utility of an otherwise useful survey and navigation aid technology.

SUMMARY OF THE INVENTION

The present invention provides a multifan survey system and method. Multifan survey operations may be useful in multiple survey tasks including bathymetry, water column monitoring, forward look survey, Doppler velocimetry, Doppler current profiling, and motion stabilization.

Doppler velocimetry may benefit from multifan operation with advantages including use of one or more of forward/backward steered fans that allow for a Janus-like configuration of beams from a multi-beam echo sounder. Doppler estimates like Doppler velocity log (“DVL”) estimates may be made. Doppler estimates like Acoustic Doppler Current Profiling (“ADCP”) estimates may be made.

In an embodiment, an acoustic Doppler system for estimating a relative velocity between an acoustic source and reflectors and/or scatterers in multiple underwater fans comprises: one or more transducers in one or more projector arrays included in the acoustic source; a transmitter for transmitting a message via the one or more projector arrays, the message for ensonifying i>=2 fans; components of the message including at least one pulse pair for each fan and each fan including j>=8 beams; one or more transducers of the one or more projector arrays and plural transducers of one or more hydrophone arrays in a Mills Cross arrangement; the hydrophones for sensing reflected and/or scattered returns and a receiver for processing reflected and/or scattered return signals; and, the return signals processed via autocorrelation of pulse pairs to calculate for each of (i*j) beams respective Doppler radial velocity estimates DRVi,j; wherein simultaneous consideration of a plurality of the DRVi,jestimates provides an estimated source velocity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosure provided in the following pages describes examples of some embodiments of the invention. The designs, figures, and description are non-limiting examples of the embodiments they disclose. For example, other embodiments of the disclosed device and/or method may or may not include the features described herein. Moreover, described features, advantages or benefits may apply to only certain embodiments of the invention and should not be used to limit the disclosed invention.

As used herein, the term “coupled” includes direct and indirect connections. Moreover, where first and second devices are coupled, intervening devices including active devices may be located therebetween.

Multibeam Echo Sounder

FIGS. 1A-Eshow a survey system including a multibeam echo sounder system and describe exemplary multibeam echo sounder embodiments.FIG. 1Gshows a legend100G of selected symbols appearing onFIGS. 1C-F.

FIG. 1Ashows a survey system in accordance with an embodiment of the present invention100A. The survey system includes an echo sounder system such as a multibeam echo sounder system102which may be mounted on a surface vehicle or vessel, on a waterbody bottom, on a remotely operated vehicle, on an autonomous underwater vehicle, or the like. As is further described below, echo sounder and/or survey system outputs114may be contemporaneous with echo sounder processing of hydrophone data as in some embodiments for bathymetry or non-contemporaneous with processing of hydrophone data as in some embodiments for waterbody bottom classification.

Data acquired by multibeam echo sounder systems104include data from echo sounder listening devices such as hydrophones (e.g., transducers) that receive echoes which are related to the acoustic/pressure waves emanating from the echo sounder projectors but have returned by virtue of an interaction with inhomogeneities of many kinds. The interactions may take the form of reflection or scattering. The inhomogeneities, also known as reflectors and scattering centers, represent discontinuities in the physical properties of the medium. Exemplary scattering centers may be found in one or more of i) an ensonified volume of the waterbody such as a water column, ii) upon the ensonified surface of the bottom, or iii) within the ensonified volume of the sub-bottom.

Scattering centers of a biological nature may be present in the water column, as they are a part of the marine life. Scattering centers of a nonbiological nature may be present in the water column in the form of bubbles, dust and sand particles, thermal microstructure, and turbulence of natural or human origin, such as ships' wakes. Scattering centers on the surface of the bottom may be due to the mechanical roughness of the bottom, such as ripples, or be due to the inherent size, shape and physical arrangement of the bottom constituents, such as mud, sand, shell fragments, cobbles and boulders, or due to both factors. Scattering centers in the sub-bottom may be due to bioturbation of the sediments, layering of different sediment materials within the bottom or buried manmade structures such as pipelines.

Data processing within the echo sounder system may include contemporaneous processing of hydrophone data106, for example to obtain Doppler velocity data, bathymetric data, and/or backscatter data. Data processing may also include non-contemporaneous processing of multibeam echo sounder system data108, for example to characterize bottom conditions or the water column.

Data processing may include utilization of complementary or other data. For example, contemporaneous processing of hydrophone data106may utilize contemporaneous110and/or non-contemporaneous112data such as contemporaneously collected geographic positioning system (“GPS”) data, sound speed measurements, attitude, and navigational information. For example, non-contemporaneous processing of echo sounder system data may utilize contemporaneous110and/or non-contemporaneous112data such as non-contemporaneously collected waterbody bottom composition data and tidal records.

FIG. 1Bshows portions of an exemplary multibeam echo sounder system (“MBES”)100B. The echo sounder system includes a transducer section120and an acoustic transceiver122. The echo sounder system may include a transceiver interface such as an interface module124and/or a workstation computer126for one or more of data processing, data storage, and interfacing man and machine. Exemplary transducers, shown here in a Mills Cross arrangement120, include a transmitter or projector array130and a receiver or hydrophone array140. Projectors in the projector array may be spaced along a line that is parallel with a keel line or track of a vehicle or vessel to which they are mounted which may be referred to as an along track arrangement. In some embodiments, a receiver of the transceiver122has an operating frequency range corresponding to that of the projectors and/or the hydrophones.

During echo sounder operation, sound or pressure waves emanating from the projector array travel within a body of water and possibly within the bottom beneath the body of water and in doing so may undergo interactions, such as reflections or scattering, which disturb the propagation trajectory of the pressure waves. Some of the reflections or echoes are “heard” by the hydrophone array. See for example the disclosure of Etal, U.S. Pat. No. 3,144,631, which is included herein by reference, in its entirety and for all purposes.

The acoustic transceiver122includes a transmitter section150and a receiver section170. The acoustic transceiver may be configured to transmit to one or more projector arrays130and to receive from one or more hydrophone arrays140. Unless otherwise noted, the term transceiver does not require common packaging and/or encapsulation of the transmitter and receiver.

In various embodiments, a projector array may be a single projector array regardless of the geometry, arrangement, or quantity of devices employed. For example, where a plurality of projectors forms a plurality of spatially distinct projector groups, the plural projectors are a single projector array if they are operated to ensonify the entirety of a swath on a single ping, for example a swath of waterbody bottom or a swath of water column. In various embodiments: i) a single projector array may ensonify the entirety of a swath on a single ping; ii) a plurality of projector arrays may ensonify the entirety of a swath on a single ping; and, iii) a plurality of projector arrays ensonify multiple swaths on a single ping.

The echo sounder may further include a means such as an interface module124for interconnection with the transceiver122. This interface module may provide, among other things, a power supply for the transceiver, communications with the transceiver, communications with the workstation computer126, and communications with other sources of data such as a source of contemporaneous GPS data.

The workstation computer126may provide for one or more of data processing such as data processing for visualization of survey results, for data storage such as storage of current profiling data, bathymetry data, sound speed data, and backscatter data, for user inputs, and for display of any of inputs, system status, and survey results.

FIG. 1Cshows portions of an exemplary multibeam echo sounder system (“MBES”)100C. The echo sounder system includes a transducer section120, a transmitter section150, and a receiver section170. Some embodiments include a sensor interface section190and/or a management section192. And in some embodiments it is the management block that signals and/or provides instructions to the signal generators159.

The transducer section includes transducers for transmitting acoustic messages and transducers for receiving acoustic messages. For example, a transducer section may include an array of projectors130and an array of hydrophones140.

Projectors in the projector array130may include piezoelectric elements such as ceramic elements. Element geometries may include circular and non-circular geometries such as rectangular geometries. Some projectors have an operating frequency range of about 10 kHz to 100 kHz, of about 50 kHz to 550 kHz, or about 100 to 1000 kHz.

Hydrophones in the hydrophone array140may include piezoelectric elements such as ceramic elements. Element geometries may include circular and non-circular geometries such as rectangular geometries. Some hydrophones have an operating frequency range of about 10 kHz to 100 kHz, of about 50 kHz to 550 kHz, or about 100 to 1000 kHz.

During operation of the projector array130and hydrophone array140, the transmitter section excites the projector array, an outgoing message137emanates from the projector array, travels in a liquid medium to a reflector or scattering center138, is reflected or scattered, after which a return or incoming message139travels to the hydrophone array140for processing by the receiver170. Notably, the acoustic/pressure wave input136received at the hydrophone array140may include a Doppler shifted or otherwise modified version of the transmitted message137along with spurious signal and/or noise content.

The transmit section150may include a signal generator block158, a transmit beamformer block156, a summation block154, and a power amplifier block152. The transmit section provides for generation of or for otherwise obtaining one or more signals or message components158that will be used to compose a message137. Notably, a message may be composed of multiple signals or not. Where a message is composed of multiple signals, the message may contain i) signals in parallel (superposed), ii) signals that are serialized (concatenated), or iii) may be a combination of parallel and serial signals.

The transmit beamformer block156receives the signal(s) from the signal generator block158where beamforming for each signal takes place. The beam(s) are combined in the summation block154to construct a parallel, serial, or combination message M. In the power amplifier block152, the time series voltages of the message are amplified in order to excite or drive the transducers in the projector array130. In an embodiment, each transducer is driven by a respective amplifier.

The receive section170includes multiple hydrophone signal processing pipelines. In an embodiment the receive section includes a hardware pipelines block/analog signal processing block172, a software pipelines block/digital signal processing block174, a receive beamformer block176, and a processor block178. The receive section provides for isolating and processing the message137from the input136received at the hydrophone array140. For example, some embodiments process echoes to determine Doppler velocities and/or depths as a function of, among other things, round trip travel times.

In the hardware pipeline block172, plural hydrophone array transducers of the hydrophone array140provide inputs to plural hardware pipelines that perform signal conditioning and analog-to-digital conversion. In some embodiments, the analog-to-digital conversion is configured for oversampling where the converter Fin (highest input frequency) is less than Fs/2 (one half of the converter sampling frequency). In an embodiment, a transceiver122operates with a maximum frequency of about 800 kHz. In an embodiment the transceiver utilizes analog-to-digital converters with sampling rates in a range of about 5 to 32 MHz. In an embodiment the transceiver utilizes analog-to-digital converters with sampling rates of about 5 MHz or about 32 MHz.

In the software pipeline block174, the hardware pipelines172provide inputs to the software pipelines. One or more pipelines serve each of the hydrophones in the hydrophone array. Each software pipeline may provide, among other things, downconversion and/or filtering. In various embodiments, the software pipeline may provide for recovery of a message from a hydrophone input136. In an embodiment, hydrophone may be served by pipelines for one or more of interpreting, distinguishing, deconstructing and/or decoding a message such as a multicomponent message.

In the receive beamforming or steering block176, the software pipelines174provide beamformer inputs. Beamformer functionality includes phase shifting and/or time delay and summation for multiple input signals. In an embodiment, a beamformer is provided for each of multiple coded signals. For example, where software pipelines operate using two coded signals, inputs to a first beamformer are software pipelines decoding a first code and inputs to a second beamformer are software pipelines decoding a second code.

In the processor block178, the beamformers of the beamformer block176provide processor inputs. Processor functionality may include any one or more of bottom detection, backscatter processing, data reduction, Doppler processing, acoustic imaging, and generation of a short time series of backscatter sometimes referred to as “snippets.”

In an embodiment, a management section192and a sensor interface section190are provided. The management section includes an interface module194and/or a workstation computer196. The sensor interface section provides for interfacing signals from one or more sensors ES1, ES2, ES3such as sensors for time (e.g. GPS), motion, attitude, and sound speed.

In various embodiments, control and/or control related signals are exchanged between the management section192and one or more of the power amplifier block152, software pipelines block174, transmit beamformer block156, receive beamformer block176, signal generator block158, processor block178. And, in various embodiments sensor interface section data190are exchanged with the management section192and the processor block178.

FIG. 1Dshows portions of an exemplary multibeam echo sounder system (“MBES”)100D. The echo sounder system includes a transducer section120, a transmitter section150, and a receiver section170. Some embodiments include an interface section190and/or a management section192.

In the embodiment shown, a message153incorporating quantity N signals, for example N coded signals, is used to excite plural projectors in a projector array and a receiver having quantity T hardware or software pipelines and (T*N) hardware or software pipelines may be used to process T hydrophone signals for recovery of echo information specific to each of the N coded signals.

In various embodiments, first and second serialized signals within the same message may be identical as with coded pulse pairs associated with Doppler velocity measurements described below.

The transmitter section150is for exciting the projector array130. The section includes a signal generator block158, a transmit beamformer block156, a summation block154, and a power amplifier block152.

The signal generator block158may generate quantity N signals or message components, for example N coded signals (e.g., Scd1. . . ScdN). In some embodiments, each of plural signals within a message shares a common center frequency and/or a common frequency band. And, in some embodiments, each of plural signals within a message has a unique, non-overlapping frequency band.

A transmit beamformer block156receives N signal generator block outputs. For each of the N signals generated, the beamformer block produces a group of output beam signals such that there N groups of output beam signals.

The summation block154receives and sums the signals in the N groups of output beams to provide a summed output153.

The power amplifier block152includes quantity S amplifiers for driving respective projectors in the projector array130. Each power amplifier receives the summed output or a signal that is a function of the summed output153, amplifies the signal, and drives a respective projector with the amplified signal.

An array of quantity T hydrophones140is for receiving echoes of acoustic/pressure waves originating from the projector array130. The resulting hydrophone signals are processed in the receiver section170which includes a hardware pipeline block172, a software pipeline block174, a receive beamformer block176, and a processor block178.

In the hardware pipeline block172, T pipelines provide independent signal conditioning and analog-to-digital conversion for each of the T hydrophone signals.

In the software pipeline block174, (T*N) software pipelines may provide downconversion and/or filtering for each of the T hardware pipeline outputs. Means known in the art, for example filtering such as band pass filtering, may be used to distinguish different signals such as signals in different frequency bands. As shown, each of T hardware pipeline outputs181,182,183provides N software pipeline inputs a,b and c,d and e,f (i.e., 3*2=6 where T=3 and N=2).

In the receive beamformer block176, (T*N) software pipeline block174outputs are used to form N groups of beams. A beamformer is provided for each of N codes. For example, where there are T=3 hydrophones and software pipelines process N=2 codes, inputs to a first beamformer are software pipelines processing the first code a1, c1, e1and inputs to a second beamformer are software pipelines processing the second code b1, d1, f1.

In the processor block178, N processors receive respective groups of beams formed by the beamformer block176. Processor block178data are exchanged with a management section192and sensor interface190data ES1, ES2, ES3are provided to the management section and/or the processor block.

In various embodiments control signals from the management block192are used to make power amplifier block152settings (e.g., for “S” power amplifiers for shading), to control transmit156and receive176beamformers, to select software pipeline block174operating frequencies, and to set signal generator block158operating frequencies.

As the above illustrates, the disclosed echo sounder transmitter may construct a message incorporating N components such as N coded signals. And, the echo sounder may utilize a receiver having T hardware pipelines and (T*N) software pipelines to process T hydrophone signals for recovery of echo information specific to each of the N message components.

FIGS. 1E-Fshow portions of an exemplary multibeam echo sounder system (“MBES”)100E-F. The echo sounder system includes a transducer section120, a transmitter section150, and a receiver section170. Some embodiments include an interface section190and/or a management section192.

In the embodiment shown, a message153incorporating first, second, and third message components such as coded signals Scd1, Scd2, Scd3where N=3 is used to excite three projectors in a projector array, and a receiver having three hardware pipelines and nine software pipelines is used to process three hydrophone signals T=3 to recover echo information specific to each of the N message components.

The transmitter section150is for exciting the projector array130. The section includes a signal generator block158, a transmit beamformer block156, a summation block154, and a power amplifier block152.

In the signal generator block158, signals are constructed, generated, recalled and/or otherwise provided. Here, an exemplary process is depicted with e.g., N=3 signal generators. In respective beamformers of the beamformer block156, multiple beams are generated from each signal. In a summation block154, the beams are combined to produce a summation block output signal or transmit message153.

The transducer block120includes a projector array130and a hydrophone array140arranged, for example, as a Mills Cross. As shown, there are three projectors131in the projector array and three hydrophones141in the hydrophone array. In the power amplifier block152, the summed signal or transmit message153is an input to power amplifiers driving respective projectors.

Applicant notes that for convenience of illustration, the projector and hydrophone counts are limited to three. As skilled artisans will appreciate, transducer arrays do not require equal numbers of projectors and hydrophones nor do the quantities of either of these types of transducers need to be limited to three. For example, a modern multibeam echo sounder might utilize 1 to 96 or more projectors and 64 to 256 or more hydrophones.

The array of T=3 hydrophones141is for receiving echoes resulting from the acoustic/pressure waves originating from the projector array130. The resulting hydrophone signals are processed in the receiver section170which includes a hardware pipeline block172, a software pipeline block174, a receive beamformer block176, and a processor block178.

In the hardware pipelines block172, each of T=3 hardware pipelines processes a respective hydrophone141signal through analog components including an analog-to-digital converter. In the embodiment shown, a hardware pipeline provides sequential signal processing through a first amplifier, an anti-aliasing filter such as a low pass anti-aliasing filter, a second amplifier, and an analog-to-digital converter.

In the software pipelines block174, each of the T=3 hardware pipeline outputs is processed through N=3 software pipelines with downconversion and band pass filtering. In the embodiment shown, a software pipeline provides sequential signal processing including processing through a mixer (an oscillator such as local oscillator may be coupled to the mixer), a bandpass filter, and a decimator. Communications may occur via communications links between any of the processor block178, the signal generator block158, the hardware pipelines block172, the software pipelines block174, the and the beamformer block176. See for exampleFIGS. 1C-D.

Each software pipeline may have a single mixer and/or each hardware pipeline may have no mixer. A processor178may control gain of a first and/or a second hardware pipeline amplifier. A processor may provide for tuning, for example via a processor controlled oscillator coupled with a mixer.

In the receive beamformer block176, each of N=3 beamformers processes signals. As such, i) a first set of three software pipeline outputs corresponding to a first coded signal are processed by a first beamformer, ii) a second set of three software pipeline outputs corresponding to a second coded signal are processed by a second beamformer, and (iii) a third set of three software pipeline outputs corresponding to a third coded signal are processed by a third beamformer. Notably, beamformers may be implemented in hardware or software. For example, one or more beamformers may be implemented in one or more field programmable gate arrays (“FPGA”).

In the processor block178, each of N=3 processors are for processing respective beamformer outputs. Here, a first plurality of beams generated by the first beamformer is processed in a first processor, a second plurality of beams generated by the second beamformer is processed in a second beamformer, and a third plurality of beams generated by the third beamformer is processed in a third beamformer.

Processor outputs interconnect with a management section192. Notably, one or more processors may be implemented in a single device such as a single processor or digital signal processor (“DSP”) or in multiple devices such as multiple signal processors or digital signal processors.

Complementary data may be provided via, inter alia, a sensor interface section190that is interfaced with a plurality of sensors ES1, ES2, ES3. The sensor interface module may provide sensor data to the management section192and/or to processors in the processor block178.

The management section192includes a sonar interface194and/or a workstation computer196. In various embodiments control signals from the management block192are used for one or more of making power amplifier block152settings (e.g., for array shading), controlling transmit beamformers156and receive beamformers176, selecting software pipeline block174operating frequencies, setting set signal generator block158operating frequencies, and providing processor block178operating instructions.

Applicant notes that the echo sounder systems ofFIGS. 1C-Fmay be used to process hydrophone returns from targets i) present within an ensonified volume of the water body, ii) upon an ensonified surface of the bottom, or iii) lying within an ensonified volume of the bottom.

In various embodiments, the MBES ofFIGS. 1E-Fdistinguishes among signals based on frequency or frequency band. In various embodiments, the MBES ofFIGS. 1E-Fdoes not distinguish among signals using matched filtering.

MBES Message Cycle

FIG. 2Ashows a first message cycle200A. The cycle includes a sequence of operations with transmission of a message during a time t1and reception of a message during a time t3. Transmission of a message refers to a process that excites the projector array130and reception of a message refers to a complementary process including message echo receipt by the hydrophone array140. A wait time t2that varies primarily with range, angle, and sound speed may be interposed between the end of the message transmission and the beginning of the message reception. This wait time may be determined by the sonar range scale setting or round trip travel time for the longest sounding range, for example a return from the most distant observed location or cell in a swath ensonified by the projector array. In some embodiments, the message transmit length is in a range of 10 to 60 microseconds. In some embodiments, the transmit message length is about 5-15 milliseconds or 10 milliseconds.

FIG. 2Bshows a second message cycle200B. Here, a transmitted message includes multiple coded message components. During transmission of the message, each of the message components is steered as by beamformers156to ensonify a respective strip or fan of a waterbody bottom as is further explained below. Each of the transmitted message components results in a similarly coded message component return. Decoding in the receiver separates these returns such that data specific to each fan is available for analyses.

Multimode Doppler Operations

As mentioned above, the MBES disclosed inFIGS. 1A-Fmay be operated in multiple modes. These modes include various Doppler Velocity Log (“DVL”) modes and various Acoustic Doppler Current Profiling (“ADCP”) modes.

When operating the MBES in a typical DVL navigation mode, the MBES may be placed on a moving platform such as a surface vessel for targeting a stationary reflector such as a waterbody bottom.

When operating the MBES in a typical ADCP current profiling mode, the MBES may be placed on a stationary platform such as a waterbody bottom for targeting reflectors and/or scatterers entrained in a moving water column.

As skilled artisans will appreciate, acoustic Doppler measurements may be used to determine velocity and the velocity determined may be, whether in a DVL or an ADCP mode, a relative velocity between the MBES and the reflector or source of the backscattered acoustic energy.

FIGS. 3A-Dshow an exemplary vessel equipped with a multibeam echo sounder300A-D. See for example the echo sounders ofFIGS. 1A-E. As seen inFIG. 3A, an MBES array package304is affixed to a vessel302, for example to a bottom of the vessel.

Within the array package304is an along track array of projectors308and a cross track array of hydrophones310. The projector array is for excitation by a transmit message such as the message ofFIG. 2AorFIG. 2B. The hydrophone array is for receiving echoes of the transmitted message. As explained below, a crossed array arrangement such as a Mills Cross arrangement of the projector and hydrophone arrays enables the echo sounder to operate with crossed transmit and receive beams wherein the cross intersection identifies a particular waterbody location, area, or cell. The crossed arrays may be in a perpendicular or a substantially perpendicular arrangement. Substantially perpendicular refers to generally small deviations from perpendicular caused by any of array assembly tolerances, mounting tolerances, adjustment tolerances, and the like.

FIG. 3Bshows bottom ensonification300B. In particular, an across track strip or fan of a waterbody bottom312is ensonified by the projector array308. Note the along track projector array308ensonifies an across track fan. As shown, the projected beam311has a wide across track aperture angle θt1as compared with a relatively narrow along track aperture angle θt2. Echoes from this ensonified fan may be received by the hydrophone array310.

DVL bottom-tracking mode: In light of the multimode Doppler operations discussed above, it will be appreciated thatFIGS. 3A-Billustrate operation of an MBES in a DVL bottom-tracking mode where the MBES is moving and the reflector(s), for example reflectors on a sea floor, are assumed to be stationary. DVL water-tracking mode: Another configuration called DVL water-tracking mode exploits backscatter from a moving water layer within the water column to estimate a moving MBES' relative velocity through the water. ADCP mode: Additionally, if the MBES ofFIGS. 3A-Bis stationary and oriented such that a vertical strip of the water column is ensonified from above or below, the MBES mode of operation becomes an ADCP mode. Each ofFIGS. 3C-Fmay be viewed in a similar manner to visualize multifan ensonification of the water column during an ADCP or DVL water-tracking mode of operation.

FIG. 3Cshows bottom ensonification and echoes that result from the bottom300C. In particular, echoes from the ensonified across track fan312are received by the hydrophone array310. As shown, the received beam321has a wide along track aperture angle θr1as compared with a relatively narrow across track angle θr2. And, as shown, the hydrophone array beam may be steered to observe or read a set of along track strips331,332,333. . . that intersect the ensonified fan312at multiple adjacent or overlapping locations. Data such as bathymetric data may be obtained from and associated with each of these intersecting locations or areas340such that each time an across track fan is ensonified, multiple receiving beams observe multiple receiving strips and provide bathymetric data at multiple locations along the ensonified fan.

Just as a single ensonified fan312may be observed or read by multiple receiving beams321, so too may multiple ensonified fans be observed or read by multiple receiving beams.

FIG. 3Dshows multifan bottom ensonification300D. Here, the projector array is steered to produce multiple adjacent or overlapping ensonified strips or fans that are oriented across track. While any number of fans, such as 2, 3, 4, 5, 10 or more fans, may be projected, the example ofFIG. 3Dshows five projected fans comprising a center fan flanked by Forward A and Aft A fans which are flanked by Forward B and Aft B fans respectively. As before, multiple receiving beams351provide a set of along track receiving strips361,362,363. These receiving strips intersect the multiple fans372.

When a receiving strip362intersects multiple fans, a plurality372of cells340may be observed. And, when multiple receiving strips361,362,363. . . intersect multiple fans, a grid-like or two dimensional zone370results and bathymetric data may be obtained from each of the cells identified by intersections within the zone.

Applicant notes that as shown inFIG. 3Deach of the fans has opposed cross-track boundaries that are essentially straight lines. This presentation is idealized. In practice, these opposed fan boundaries may be curved. For example, fan outlines on a waterbody bottom may be parabolic in shape with a cross-track major dimension. Transmit beamforming and/or other than planar waterbody bottoms may contribute to fans having other than straight cross-track boundaries but that does not preclude locating the centers of the cells340.

Advantages of multifan operation may include increased survey speed resulting from, for example, an extended along track zone of ensonification, redundancy via overlapping of zones (e.g., where a fifty percent overlap between pings may provide two looks at every waterbody bottom location observed), and imaging a given target from multiple aspects. For example, imaging from multiple aspects including at nadir and from two opposing off-nadir sides. For example, imaging from multiple aspects including front, overhead, and behind.

In various embodiments, realizing the benefits of a multifan survey system requires an MBES capable of distinguishing between echoes returned from each of the fans. And, in various embodiments, any of temporal, spectral, or code separation techniques may be used to relate an echo to the fan from which it originated. In some embodiments, frequency separation is used to associate returns with particular fans as is further explained below. And in an embodiment, temporal separation is used to distinguish message components in a multicomponent message. And, in an embodiment, temporal separation is used to distinguish message components in a multicomponent message transmit over one or more message cycles.

FIG. 3Eshows a transmitted message ensonifying five fans300E. Here, an MBES projector array308transmits380five formed beams381-385to a center fan, to Aft A and Forward A fans flanking the center fan, and to peripheral Aft B and Forward B fans. Each of the five formed beams381-385ensonifies a respective fan with one of five differing frequency band signals.

In the example shown, Aft B fan is ensonified with signal1in frequency band A by the first beam381, the Aft A fan is ensonified with signal2in frequency band B by the second beam382, the Center fan is ensonified with signal3in frequency band C by the third beam383, the Forward A fan is ensonified with signal4in frequency band D by the fourth beam384, and the Forward B fan is ensonified with signal5in frequency band E by the fifth beam385. Notably, five messages may be sent in five different frequency bands to ensonify the five fans. The messages may be sent concurrently and separated in the receiver by frequency band.

FIG. 3Fshows returns300F from the ensonified fans ofFIG. 3E. Here, an MBES hydrophone array310receives returns390from the center fan393, from Aft A and Forward A fans flanking the center fan392,394, and from peripheral Aft B and Forward B fans391,395.

It is noted that in some embodiments, one or multiple projector and/or hydrophone arrays may be used in connection with multifan operations. For example, multiple projectors or projector arrays might be used to simultaneously ensonify fans in multiple fixed look directions. For example, multiple hydrophones or hydrophone arrays might be used to simultaneously acquire returns from multiple fixed look directions.

Acoustic Doppler Measurements

As shown above, an MBES may be designed, built, and operated to ensonify multiple fans. For example, a 256 beam system that ensonifies three fans can acquire data from 3*256 beams. In the case of bathymetry and data from 3*256 waterbody bottom locations, this multiplicity of measurements may be used, for example, to improve survey speed and/or the density of survey measurements. In the case of navigation, this multiplicity of measurements may be used, for example, to improve the accuracy of velocities determined using acoustic Doppler techniques.

Where a source emits acoustic signals and a target moving relative to the source reflects the signals, acoustic Doppler techniques may be used to determine a velocity of the target relative to the source. For example, changes in acoustic wavelength of reflected signals (e.g., returns, echoes) may be used to determine a radial component of velocity. In some cases, the source is fixed and the target is moving. And, in some cases, the target is fixed and the source is moving.

FIG. 4Ashows a flowchart of steps that may be used in determining velocities from multifan data400A. In a first step402, a message is constructed. The message is for transmission from an MBES source moving relative to a waterbody bottom at velocity Vsource. In various embodiments, the message includes a pulse pair such as a pair of identical pulses.

In a subsequent step404, the message is used to ensonify each of “i” fans with “j” beams. Thereafter, in step406, returns are processed and velocities, for example Doppler radial velocities DRVi,jassociated with the beams, are determined. And, in a subsequent step408, the Doppler radial velocities DRVi,jalong with known variables such as fan and beam geometry are used to determine a velocity vector that is representative of Vsource.

Notably, where each Doppler radial velocity DRVi,jis a projection of Vsourcealong beam j, each of the Doppler radial velocities can be said to be indicative of Vsource.

In light of the multimode Doppler operations discussed above, it will be appreciated thatFIG. 4Aillustrates operation of an MBES in a DVL bottom-tracking mode where the MBES is moving and the reflector(s) are stationary. However, if the backscatter originates from a layer of the water column, the DVL operates in water-tracking mode. Additionally, if the MBES ofFIG. 4Ais made stationary and directed to target scattering centers in the water column, the MBES operates in an ADCP mode.

FIGS. 4B-Cshow a moving source400B-C. As seen inFIG. 4B, a boat403carries a hull mounted MBES405. The MBES405has a heading416. A heading coordinate system with orthogonal axes x′, y′, z′ is shown inFIG. 4C. The heading coordinate system has an origin centered on the source414and an x′ axis aligned with the heading416. Message beams emanating from the source are directed with fan angles in the x′z′ plane FAiand beam projection angles in the x′y′ plane BAi. Multiple beams within a fan do not need to be equally spaced within the sector they span, nor do fans need to be evenly distributed fore and aft. There is no requirement to use a fan at nadir, although in an embodiment where three fans cover a 40 degree arc in the x′z′ plane, fan angles measured from the z′ axis in the x′z′ plane are −20, 0 and +20 degrees.

FIGS. 4D-Fshow a course coordinate system400D-F. This course coordinate system is centered on the source414and is described by orthogonal axes x, y, z. As seen inFIG. 4D, motion of the source may include one or more of rolling about the x axis, pitching about the y axis, and yawing about the z axis.

InFIG. 4E, a yaw angle YA describes the angular offset measured in the xy plane. The yaw angle, also commonly referred to as crab angle, is the angle between the course direction along the x axis and the heading direction along the x′ axis. Here, the x axis lies along the line that is the course or instantaneous course of the boat412. As skilled artisans will appreciate, in the absence of external forces on the boat, the course and heading may not differ. However, when an external force, e.g., wind, waves, or current, acts on the boat, the heading will be offset from the course to compensate for the forces which tend to move the boat off course.

InFIG. 4F, a pitch angle PA describes an angular offset measured in the xz plane. The pitch angle is measured from the z axis and indicates the pitch of the boat. Notably, the location of a fan on a waterbody bottom varies with fan angle FA setting and with boat pitch angle PA.

As mentioned above, a multiplicity of Doppler velocities such as a multiplicity of Doppler radial velocities DRVi,jmay be used to estimate and/or determine a relative velocity between an MBES source Vsourceand a reflector. Notably, the reflector may be a waterbody bottom, scattering centers entrained in the water column such as bubbles or particulate, and reflectors otherwise located in the water column. Notably, where the MBES is mounted on a vessel for DVL bottom tracking, Vsourceis a source velocity relative to a stationary waterbody bottom target. Where the MBES is mounted on a vessel for DVL water tracking, Vsourceis a source velocity relative to a water layer that may be moving in the water column. And, where the MBES is mounted on a stationary vessel or waterbody bottom for ADCP water column measurements, Vsourceis water column target velocity relative to a stationary MBES.

FIGS. 4G-Ishow exemplary equations400G-I to solve for Vsourceusing a plurality of Doppler radial velocities, each Doppler radial velocity being indicative of Vsource. Notably, applicants have found that such estimates of Vsourcemay be more accurate than estimates based on a single or only a few beams and/or may be more accurate than estimates based on returns from a single or only a few waterbody bottom or water column locations.

Determination of DRVi,jmay utilize the pulse-pair method, an efficient computational algorithm known to skilled artisans, to process data from each fan i and beam j individually (See e.g. U.S. Pat. No. 5,483,499). A complex representation (angle and magnitude) of the autocorrelation of beam data at a time lag equal to one pulse length T is calculated for all range cells k, one of which is selected, depending on the operating mode of the MBES, to provide angle information CSANGLEi,jto the calculation of DRVi,jas follows:
DRVi,j=CSANGLEi,j*(c/(2*pi*fc*T))
where c=speed of sound (m/s)fc=center frequency of transmitted pulse (Hz)T=pulse length (s)

As seen, Doppler radial velocity DRVi,jvaries with fan i and beam j, and there are a total of (i*j) DRV estimates. In various embodiments, all of these DRV estimates contribute to an estimate of Vsource. And, in various embodiments only selected ones of these DRV estimates contribute to an estimate of Vsource. For example, in a beam skipping embodiment, pairs of selected beams in a particular fan may be separated by an integer quantity of p beams that are not selected such that where p=2, beams1,4,7. . . are selected and beams2,3,5,6are not selected. For example, in a first reduced beam count embodiment a source velocity is estimated using a quantity r1<(i*j) of the DRVi,jestimates and r1 is determined in part by the processing capacity of a digital processing section of the receiver. For example, in a second reduced beam count embodiment, the value of r1 is automatically determined by the acoustic Doppler system as a function of equipment variables, environmental variables, and/or mission requirements.

The quantity Vsourceis estimated via minimization of a cost equation400H such as the cost equation ofFIG. 4H.FIG. 4Ishows cost equation variables400I that are knowns DRVi,j, BAj, FAiand cost equation variables that are unknown Vsource, YA, and PA. As skilled artisans will appreciate, the hypothesized and/or estimated values for the unknowns Vsource, YA, and PA that minimize cost and/or result in the smallest cost are declared the best estimates for those values, together describing a source velocity vector in polar coordinates. Cost equation minimization methods include brute force searches, Nelder-Mead type methods, Newton's method, and other suitable minimization methods known to skilled artisans.

Summed for a plurality of beams j in each of a plurality of fans i, the cost equation is

Notably, constant kx1recognizes that for typical DVL modes the source is in motion and therefore its echo is Doppler shifted twice, once on transmit and once on receive. Hence the value kx1=2 for DVL mode and kx1=1 for ADCP mode.

Exemplary DVL Process

FIG. 4Jshows a flowchart400J of an exemplary DVL process using pulse pairs to determine source velocity. Here, a source for emitting an acoustic message with consecutive components is provided482. The source, such as a vessel mounted source405, may be subjected to pitch PA and yaw YA while moving along a course484. As skilled artisans will appreciate, a source heading may differ from the course by a yaw angle YA486.

As the source moves along the course, it ensonifies a waterbody bottom with multiple fans Fiat fan angles FAiwith multiple beams Bijat beam angles BAij488. In a step490that follows, returns from the ensonified fans are received, time gated, and associated with range cells along each beam in each fan RCi,j,k. See for example the multibeam echo sounder systems ofFIGS. 1B-F.

In a step492that follows, application of the pulse-pair method provides a complex autocorrelation value with an angle CSANGLEi,j,kand magnitude CSMAGi,j,kfor each range cell RCi,j,k(see. e.g., U.S. Pat. No. 5,483,499).

In a step494that follows, the range cell in each beam that corresponds to the waterbody bottom is determined. For example, the maximum correlation magnitude CSMAGi,j,kin each beam may be used to identify a waterbody bottom range cell WBBRCi, jin each beam.

In a step496that follows, Doppler radial velocities are calculated for each range cell corresponding to the waterbody bottom. For example, CSANGLEi,j,kmay be used to calculate Doppler radial velocity DRVi,jfor each WBBRCi,j.

In a step498that follows, source velocity is estimated. For example, the cost equation mentioned above, with kx1=2 for DVL mode, may be minimized to estimate unknowns Vsource, YA, and PA given knowns DRVi,j, BAj, and FAi.

Exemplary ADCP Process

FIG. 4Kshows a flowchart400K of an exemplary ADCP process using pulse pairs to determine source velocity. Here, a source for emitting an acoustic message with consecutive components is provided482. The source is considered stationary485, such as one mounted on the sea floor or to a non-moving vessel. Water layers above or below the source, respectively, may be in motion relative to the source, and each layer may have a velocity independent of the other layers487. The stationary source ensonifies a water column with multiple fans Fiat fan angles FAiwith multiple beams Bijat beam angles BAij488. In a step490that follows, returns from the ensonified fans are received, time gated, and associated with range cells along each beam in each fan RCi,j,k. See for example the multibeam echo sounder systems ofFIGS. 1B-F.

In a step492that follows, application of the pulse pair method provides a complex autocorrelation value with an angle CSANGLEi,j,kand magnitude CSMAGi,j,kfor each range cell RCi,j,k. In a step495that follows, for each beam in each fan a water column range cell at a depth of interest WCRCi,jis selected. These range cells likely capture part of the water column or a water volume instead of the waterbody bottom. Further, although only one depth of interest may be indicated, the process can be repeated for numerous depths to collectively form a vertical profile of velocity estimates and/or an average velocity estimate.

In a step497that follows, Doppler radial velocities DRVi,jare calculated for each range cell corresponding to the depth of interest. For example, CSANGLEi,j,kmay be used to calculate DRVi,jfor each WCRCi,j.

In a step499that follows, source velocity is estimated. For example, the cost equation mentioned above, with kx1=1 for ADCP mode, may be minimized to estimate unknowns Vsource, YA, and PA given knowns DRVi,j, BAj, and FAi.

FIGS. 5A-Fbelow show exemplary message constructs500A-F. One or more of these message constructs may be propagated through a liquid medium to generate returns from one or more fans to estimate a velocity of the sonar system sending the message and/or the velocity of a vessel that carries the sonar system.

FIG. 5Ashows a single fan message including a pulse pair500A. A first message component includes message code1transmitted during a timespan tx1and a second message component includes the same message code1transmitted during a timespan tx3. These two timespans and similar timespans discussed below may be separated by a timespan tx2or they may be contiguous such that tx2=0. The timespan between the beginning of the first message component and the end of the second message component is tx4. In various embodiments, pulses in one or more pulse pairs do not overlap in time.

As shown, the message components may be sent using a particular transmitter frequency band. This transmitter frequency band may be less than (as shown) or substantially equal to an available frequency band of a multibeam echo sounder transmitter150and/or a multibeam echo sounder receiver170.

FIG. 5Bshows a multi-fan message including three pulse pairs for ensonifying three fans simultaneously or substantially simultaneously500B. Here, the message includes pulse pairs transmitted in parallel and serially transmitted pulses within the each pulse pair. The message may therefore be referred to as a serial-parallel message or a serial-parallel message using single pulse pairs insofar as the pulses of the pulse pairs are transmitted serially and the fans are ensonified simultaneously.

For a first fan (Fan1), message components include message code1and message code1transmitted serially in a first receiver frequency band A. For a second fan (Fan2), message components include message code2and message code2transmitted serially in a second receiver frequency band B. For a third fan (Fan3), message components include message code3and message code3transmitted serially in a third receiver frequency band C.

In some embodiments, message codes1,2,3of the first message component for each of the fans are transmitted simultaneously in each of three non-overlapping and/or contiguous receiver frequency bands. And, in some embodiments message codes1,2,3of the second message component for each of the fans are transmitted simultaneously in each of the three non-overlapping and/or contiguous receiver frequency bands.

FIG. 5Cshows a multi-fan message including three pulse pairs for ensonifying three fans in sequence500C. Here, the message includes serially transmitted pulse pairs and serially transmitted pulses within the each pulse pair. The message may therefore be referred to as a serial-serial message using single pulse pairs insofar as the pulses of pulse pairs are transmitted serially and the fans are ensonified serially.

Skilled artisans will recognize serial messages may increase the strength of the signal ensonifying each fan because transmitter signal strength is not shared among fans as may happen with messages having components transmitted in parallel.

For a first fan, message components include message code1and message code1transmitted serially in a first receiver frequency band A. For a second fan, message components include message code2and message code2transmitted serially in a second receiver frequency band B. For a third fan, message components include message code3and message code3transmitted serially in a third receiver frequency band C.

As seen, each of the fans is ensonified in sequence as a fan1pulse pair transmitted in band A is followed by a fan2pulse pair transmitted in band B which is followed by a fan3pulse pair transmitted in band C. In various embodiments, the fan ensonifying messages do not overlap and in various embodiments the fan ensonifying messages are contiguous. The transmission frequency bands may be non-overlapping and/or contiguous receiver frequency bands. The messages may be transmit in one or more message cycles.

Disambiguation

Where a single pulse pair is used in each frequency band as shown inFIG. 5B, source velocity accuracy and/or ambiguity issues may arise. For example, it is known that longer pulses can provide greater accuracy when using e.g. pulse pair correlation methods. However, longer pulses may result in ambiguous source velocity estimates because longer pulses may experience phase shifts exceeding a full 2 pi rotation. This phase wrapping ambiguity may result in erroneous source velocity estimates.

As seen below, both short and long pulse pairs in each frequency band may be used to resolve ambiguous long pulse measurements. Here, a short pulse pair provides an initial estimate within 2 pi of the long pair phase shift, and this initial estimate is used to resolve any ambiguity in a corresponding long pulse-pair estimate.

InFIG. 5D, a multifan message utilizes dual pulse pairs transmitted in parallel500D. In particular, for each of n=3 fans, a dual pulse message is used to ensonify the fan such that 2n=6 pulses are transmitted in the message. Here, the3dual pulse pairs are transmitted simultaneously while the message components within each dual pulse pair are transmitted serially. The message may therefore be referred to as a serial-parallel message or a dual pulse pair serial-parallel message insofar as the pulses of the dual pulse pairs are transmitted serially and the fans are ensonified simultaneously.

For a first fan, message components for a short pulse pair include message code11and message code11while message components for a long pulse pair include message code12and message code12. As such, the first fan may be ensonified by a short pulse pair and a long pulse pair. These pulse pairs may be transmitted in band A.

For a third fan, message components for a short pulse pair include message code31and message code31while message components for a long pulse pair include message code32and message code32. As such, the third fan may be ensonified by a short pulse pair and a long pulse pair. These pulse pairs may be transmitted in band C which may or may not be contiguous with band B.

As seen, there are n fans and 2n pulse pairs including i) n short pulse pairs, each pair including first and second message components and ii) n long pulse pairs, each pair including third and fourth message components.

In some embodiments, short pulse message codes11,21,31of the first message component for each of the fans are transmitted simultaneously in each of three non-overlapping and/or contiguous receiver frequency bands. And, in some embodiments, short pulse message codes11,21,31of the second message component for each of the fans are transmitted simultaneously in each of the three non-overlapping and/or contiguous receiver frequency bands. In some embodiments, long pulse message codes12,22,32of the third message component for each of the fans are transmitted simultaneously in each of three non-overlapping and/or contiguous receiver frequency bands. And in some embodiments, long pulse message codes12,22,32of the fourth message component for each of the fans are transmitted simultaneously in each of the three non-overlapping and/or contiguous receiver frequency bands.

InFIG. 5Dand inFIG. 5Ebelow, embodiments include those where i) message codes11,21,31are g bit codes and each bit is u samples long and message codes12,22,32are g bit codes and each bit is an integer multiple of u samples long, for example 4u samples long. For example, when u=1 and where 13-bit Barker codes are used with no time delay between codes, 130 samples (13+13+52+52) at a sample rate of 68,400 Hz results in a message that is 1.9 milliseconds long.

Applicant notes this and other examples may suggest use of a limited number of fans, for example three fans. However, no such limitation is intended. Rather, survey system hardware100B may support larger fan arrays such as arrays of 5, 7, 10, 20, 40, or more fans.

While various codes known to skilled artisans might be used in constructing message components, the inventor's experience suggest that Barker codes and Orthogonal Spread Spectrum (“OSS”) codes are suitable alternatives in many applications of interest.

InFIG. 5E, a multifan message utilizes dual pulse pairs500E transmitted in sequence. In particular, for each of n=3 fans, a dual pulse message is used to ensonify the fan such that 2n=6 pulses are transmitted in the message. Here, the3dual pulse pairs are transmitted sequentially and the message components within each dual pulse pair are transmitted serially. The message may therefore be referred to as a serial-serial message or a dual pulse pair serial message insofar as the pulses of dual pulse pairs are transmitted serially and the fans are ensonified serially.

For a first fan, message components for a short pulse pair include message code11and message code11while message components for a long pulse pair include message code12and message code12. As such, the first fan may be ensonified by a short pulse pair and a long pulse pair. These pulse pairs may be transmitted in band A.

For a third fan, message components for a short pulse pair include message code31and message code31while message components for a long pulse pair include message code32and message code32. As such, the third fan may be ensonified by a short pulse pair and a long pulse pair. These pulse pairs may be transmitted in band C which may or may not be contiguous with band B.

As seen, there are n fans and 2n pulse pairs including n short pulse pairs and n long pulse pairs.

As seen, each of the fans is ensonified in sequence as a fan1dual pulse pair transmitted in band A is followed by a fan2dual pulse pair transmitted in band B which is followed by a fan3dual pulse pair transmitted in band C. In various embodiments, the fan ensonifying messages do not overlap and in various embodiments the fan ensonifying messages are contiguous. The transmission frequency bands may be non-overlapping and/or contiguous receiver frequency bands. The message may be transmit in one or more message cycles.

In some embodiments, Barker codes with lengths of one or more of 2, 3, 4, 5, 7, 11, and 13 bits are used to construct single pulse pair X-X messages and/or dual pulse pair messages X-X, Y-Y.

For example, where each of qty. xb bits in message code X are expressed with xs samples and each of qty. yb bits in message code Y are expressed with ys samples, an exemplary short message code X might be an xb=11 bit Barker code with xs=1 sample per bit while a long message code Y might be the same code with yb=11 and a greater number of samples ys=4 samples per bit. Note that in a dual pulse pair message X-X, Y-Y codes, the code in message code X may differ from the code in message code Y and the number of samples used to express each bit in message code X may differ from the number of samples used to express each bit in message code Y.

Yet another message may include short message code X using, for example, a 7 bit Barker code and a long message code Y using, for example, a 13 bit Barker code. In various embodiments, the number of samples used to express bits in message code X may be the same or different from the number of samples used to express bits in the message code Y so long as (xs*xb)<(ys*yb). In some embodiments, (xs*xb)>(ys*yb).

In some embodiments, the pulses of pulse pairs may be ordered such that short pulse pairs are transmitted first or such that long pulse pairs are transmitted first. In some embodiments pulses used to construct a pulse pair may be transmitted as concatenated pulses or transmitted with a time delay therebetween to the extent that the velocity estimates remain substantially concurrent in time.

FIG. 5Fshows various code selections for use in a three fan Doppler velocity measurement500F. For each of the fans, the message includes a short code pulse pair and a long code pulse pair.

Here, fan1message components include a Barker Code short pulse pair (Barker Code11, Barker Code11) and a Barker Code long pulse pair (Barker Code12, Barker Code12) in frequency band A. Fan2message components include a Barker Code short pulse pair (Barker Code21, Barker Code21) and a Barker Code long pulse pair (Barker Code22, Barker Code22) in frequency band B. Fan3message components include a Barker Code short pulse pair (Barker Code31, Barker Code31) and a Barker Code long pulse pair (Barker Code32, Barker Code32) in frequency band C.

Frequency bands A, B, C may be non-overlapping and/or contiguous. In some embodiments, for each fan, the short and long pulse pairs are contiguous. In some embodiments, these pulse pairs are not contiguous.

As skilled artisans will appreciate, when a particular fan (e.g., 1, 2, 3) and pulse-pair type (e.g., 1:short, 2:long) uses a particular Barker code (e.g., Barker Code (fan, type)=Barker Code11for fan1, type 1), signals returned from a particular fan may be distinguished by frequency while in-band signals (e.g., short pulse pair [Barker Code1, Barker Code1] and long pulse pair [Barker Code4, Barker Code4]) may be temporally non-overlapping and/or temporally separated.

In an embodiment, Orthogonal Spread Spectrum (“OSS”) codes are used. Here, an exemplary short message code might be an OSS code n samples long while a long message code might be an OSS code that is greater than n samples long.

Here fan1message components include an OSS short pulse pair (OSS Code11, OSS Code11) and an OSS long pulse pair (OSS Code12, OSS Code12) in frequency band A. Fan2message components include an OSS short pulse pair (OSS Code21, OSS Code21) and an OSS long pulse pair OSS Code22, OSS Code22) in frequency band B. Fan3message components include an OSS short pulse pair (OSS Code31, OSS Code31) and an OSS long pulse pair (OSS Code32, OSS Code32) in frequency band C.

In some embodiments, for each fan, the short and long pulse pairs are contiguous. In some embodiments, these pulse pairs are not contiguous.

As skilled artisans will appreciate, when a particular fan (e.g., 1, 2, 3) and pulse pair type (e.g., 1:short, 2:long) uses a particular OSS code (e.g., OSS Code (fan, type)=OSS Code11for fan1, type 1), signals returned from a particular fan may be distinguished by frequency. In various embodiments in-band signals (e.g., short pulse pair [Code X, Code X] and long pulse pair [Code Y, Code Y]) may be temporally non-overlapping and/or temporally separated.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to those skilled in the art that various changes in the form and details can be made without departing from the spirit and scope of the invention. As such, the breadth and scope of the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and equivalents thereof.