Method for improving the hourly coverage of a channel-forming sonar
By dynamically adjusting sonar speed and transitioning between coherent and insensitive antenna treatments based on seabed evaluations, the hourly coverage of sonar systems is optimized, addressing the speed limitations of traditional synthetic aperture sonar systems.
Patent Information
- Application Number
- FR2023010629
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-10-05
- Publication Date
- 2025-04-11
AI Technical Summary
Existing sonar technologies with synthetic aperture treatments are limited by the physical length of the antenna, restricting the carrier's speed and thereby limiting the hourly coverage of the sonar without effective software solutions and with hardware solutions being costly and incompatible with existing sonars.
A process that improves the hourly coverage of a sonar with track training by dynamically adjusting the sonar speed and transitioning between coherent and insensitive antenna treatments based on early seabed evaluations and correlation coefficients, allowing for increased coverage without physical hardware modifications.
This approach optimizes hourly coverage by automatically regulating sonar speed and switching between treatment modes, effectively overcoming the speed limitations of traditional synthetic aperture sonar systems, thereby enhancing imaging capabilities without requiring new hardware.
Smart Images
Figure 00000000_0000_ABST
Abstract
Description
Title of the invention: Method for improving the hourly coverage of a channel-forming sonar Technical field
[0001] The invention relates to a method for improving the hourly coverage of a beamforming sonar, as well as to a system capable of implementing the method. The invention is particularly, but not exclusively, suitable for sonars embedded in autonomous drones, in the context of mine warfare.
[0002] Underwater imaging systems mainly use synthetic aperture processing in order to benefit from high longitudinal resolution. The antenna is fixed on a lateral face of a sonar 1 (for example an underwater drone, see [Fig.l]). Since the sonar 1 (more precisely the sonar carrier) moves in a longitudinal movement direction 2, the same point is illuminated several times (swath 3), which makes it possible to obtain a series of data for each point under the sonar. By combining the variation in amplitude and phase of these returns, synthetic aperture processing makes it possible to obtain images of the observed areas as if a large antenna with very high resolution were used.
[0003] The hourly coverage can be defined as the area covered by the sonar in a given time. [Fig.2] illustrates an area 4 (a seabed) covered by a sonar 1. The sonar path is segmented into a plurality of predetermined corridors 5 (or rails), in order to map the entire area 4.
[0004] A disadvantage of synthetic antenna treatments is that they impose a physical limit on the carrier's advancement. Indeed, for an antenna of length L, the carrier cannot move more than L / 2 between two pings (sound pulses). The following demonstrates that the hourly coverage depends almost exclusively on the length of the physical antenna, for a synthetic antenna treatment.
[0005] Typically, and as illustrated in [Fig. 3], for a SAS (Synthetic Aperture Antenna) sonar with two sides 1, i.e. illuminating on both sides of the longitudinal direction of movement, with overlap of the coverage hole under the sonar, the hourly coverage ACR (for “Area Coverage Rate” according to English terminology) is:
[0006] ACR = (3P-Am).V / 2
[0007] A corresponds to the altitude of the sonar; P corresponds to the range of the sonar; m corresponds to the overlap margin to take into account positioning errors; V corresponds to the forward speed of the carrier.
[0008] SAS processing imposes a limit on the carrier's forward speed. As a first approximation, it can be considered that this speed is:
[0009] V maxSAS L / (2.Tr)
[0010] L corresponds to the length of the receiving antenna; Tr corresponds to the interping duration, linked to the range P by the following relation: P=c.Tr / 2 (and c, speed of sound in water).
[0011] The hourly coverage is then written: AC R — ( - A - m )
[0012] Neglecting the margin, as well as the altitude (very low-level and / or long-range case), we obtain:
[0013] ACR = ^ O
[0014] This simplified formula shows that the hourly coverage, in the case of conventional synthetic antenna processing, is exclusively dependent on the length of the physical receiving antenna. Without modification of the sonar, it is therefore not possible to increase the hourly coverage (or only very marginally).
[0015] Several solutions exist to increase hourly coverage.
[0016] A first solution consists of increasing the number of carriers in order to mechanically increase the hourly coverage, but with a proportional increase in the cost of the solution. Furthermore, this solution requires handling a significant number of sonars, as well as complex management of interference between sonars.
[0017] A second solution consists of applying a so-called "Fast SAS" processing, such as those described in [1] or [2]. This algorithm is also called "under-sampled SAS Data" or "sparse SAS data". It consists of increasing the sonar's displacement speed beyond the limit speed VmaxSAs; the synthetic antenna becomes under-sampled, causing grating lobes to appear, which degrade the image quality. "Fast SAS" algorithms therefore aim to reduce these grating lobes. However, the reduction of grating lobes is not effective enough in the prior art solutions.
[0018] Thus, no software solution works correctly. Only hardware solutions would solve the problem related to exceeding the VmaxsAs speed limit • But these solutions are not compatible with existing sonars, are expensive and bulky. Their compatibility with underwater drones with a reduced carrying volume is very uncertain.
[0019] Thus, there is a need to provide a method for improving the hourly coverage of a channel-forming sonar, which does not require hardware modifications to existing sonars, and which overcomes the physical limit of the forward speed of the carrier. Summary of the invention
[0020] An object of the invention is therefore a method for improving the hourly coverage of a channel-forming sonar and configured to be embarked on a carrier in an area to be imaged in order to characterize at least one target, the method comprising the steps of: - providing imaging, by coherent synthetic antenna aperture processing, of a seabed in the area, along a lateral line of sight which is substantially perpendicular to the direction of movement of the carrier; - provision of an advance assessment of the seabed, along a line of sight offset from the lateral line of sight; - selection of an order from: — a reduction in the carrier velocity below a threshold value specific to the coherent antenna synthetic aperture processing, based on the anticipated seabed assessment, in order to maintain the coherent antenna synthetic aperture processing, or — a transition from coherent processing to antenna processing insensitive to carrier speed, without reducing the speed below the threshold value specific to coherent synthetic antenna aperture processing.
[0021] Advantageously, the antenna processing insensitive to the carrier speed comprises so-called incoherent processing, the incoherent processing comprising: - an estimation of the wearer's movement by a positioning module; - compensation of each reconstructed image based on the wearer's movement, so as to obtain compensated images; - an amplitude summation of the compensated images.
[0022] Advantageously, the transition from coherent processing to incoherent processing is conditioned by the passage of a correlation coefficient below a threshold value, the correlation coefficient being obtained by the correlation of the antenna signals from one ping to another.
[0023] Advantageously, the provision of an advance assessment of the seabed comprises the steps of: - acquisition of measurements along the line of sight offset from the lateral line of sight, so as to obtain a so-called offset image; - detection of the target in the defocused image; - classification of the target according to at least one classification criterion, in order to determine whether the target is an object of interest; and in which, if the classification is considered to be of good quality, the coherent processing switches to antenna processing insensitive to the carrier speed, and If the classification is considered to be of poor quality, the carrier velocity is reduced below the threshold value specific to the coherent antenna synthetic aperture processing.
[0024] Advantageously, the classification of the object comprises an allocation of a quality score, the quality score being calculated by the product of a first probability and a second probability, the first probability being specific to the algorithm used for the classification, the second probability depending on the resolution of the defocused image and the dimensions of the target.
[0025] Advantageously, the method comprises a calculation of a fusion of quality scores, for a plurality of targets detected during the anticipated evaluation of the seabed, the command being selected according to the result of the fusion.
[0026] Advantageously, the carrier velocity insensitive antenna processing is performed provided that the anticipated seabed assessment determines that a target is located at a distance less than the resolution of the coherent synthetic aperture antenna processing.
[0027] Advantageously, the carrier velocity insensitive antenna processing is performed if the anticipated seabed assessment includes detecting a target that has already been imaged by coherent antenna synthetic aperture processing in a rail separate from the current rail in which the carrier is located.
[0028] Advantageously, the method comprises the steps of: - receipt of a mission plan defining at least a first rail and a second rail to be taken by the carrier; - the anticipated assessment including a determination of the distance between the sonar and the target; - the command being executed in the first rail depending on the distance between the sonar and the target in the first rail and in the second rail.
[0029] The invention also relates to a system configured to improve the hourly coverage of a channel-forming sonar and configured to be embarked on a carrier in an area to be imaged in order to characterize at least one target, the system comprising: - an imaging device, by coherent processing of synthetic antenna aperture, of a seabed of the area, along a lateral line of sight which is substantially perpendicular to the direction of movement of the carrier; - a device for early assessment of the seabed, along a line of sight offset from the lateral line of sight; - a device for selecting an order from: — a reduction in carrier velocity below a threshold value specific to coherent antenna synthetic aperture processing, depending on the evaluation anticipated from the seabed, in order to maintain consistent antenna synthetic aperture processing, or — a transition from coherent processing to antenna processing insensitive to carrier speed, without reducing the speed below the threshold value specific to coherent synthetic antenna aperture processing. Description of figures
[0030] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example.
[0031] [Fig.l], already described, illustrates the movement and swath of a synthetic aperture antenna sonar.
[0032] [Fig.2], already described, illustrates the trajectory of a synthetic aperture antenna sonar in a predetermined area.
[0033] [Fig. 3], already described, illustrates a geometric representation of the range and altitude of the sonar.
[0034] [Fig.4] illustrates the lateral line of sight as well as the depointed line of sight of the sonar.
[0035] [Fig.5] illustrates different embodiments of providing an advance assessment of the seabed.
[0036] [Fig.6] illustrates the system according to the invention.
[0037] [Fig.7] illustrates one embodiment of target classification.
[0038] Figures 8 to 11 illustrate different embodiments making it possible to implement antenna processing insensitive to the carrier speed.
[0039] In [Fig. 4], a synthetic antenna side-scan sonar 1 is mounted on an underwater carrier. In addition to its conventional lateral SAS imaging capability 6, the sonar is capable of providing a forward 7 or "anticipatory" view of the bottom that will be seen later by the side-scan imaging. Thus, the sonar is capable of providing imaging, by coherent antenna synthetic aperture processing, of a seabed of the area, along a lateral line of sight 8 that is substantially perpendicular to the direction of movement 10 of the carrier 1. In addition, it can provide an anticipated assessment of the seabed, along an offset line of sight 9 relative to the lateral line of sight.
[0040] In [Fig.4], the sonar is of the “single-board” type (the antenna is located only on one side of the carrier); preferably, the sonar can be of the “double-board” type (one antenna on each side of the carrier), which makes it possible to increase the hourly coverage of the area.
[0041] Depending on the result of the front view 7, the method provides either a reduction of the carrier speed below the limit speed VmaxSAs (characteristic of the coherent processing), which makes it possible to maintain the coherent processing of synthetic antenna aperture, or a transition from the coherent processing to an antenna processing insensitive to the carrier speed, without reducing the speed below the limit speed VmaxSAs-
[0042] Thus, the method makes it possible to optimize the hourly coverage, by automatically regulating the speed of the sonar.
[0043] As illustrated in [Fig.5], the anticipated assessment of the environment can be carried out using one or more sonar channels pointed forward. Several embodiments are possible for achieving this forward view.
[0044] According to a first embodiment, illustrated in the left part of [Fig. 5], a single transmitter 16 having an electronic depointing capability can be used. This embodiment has the advantage of being compatible with electronic scanning sonars available on the market. As a variant, illustrated by the central part of [Fig. 5], the sonar comprises an additional transmitter 12 mechanically depointed forward (fixed angle depointing). According to another variant, illustrated by the right part of [Fig. 5], a very wide emission transmitter 13 can be used. A first sub-sector 14 perpendicular to the direction of movement of the carrier is used for imaging by coherent processing, and a second sub-sector 15, directed towards the front of the carrier, is used for the anticipated evaluation of the seabed.
[0045] In [Fig.4], the depointing angle, which corresponds to the angle between the lateral line of sight 8 and the depointed line of sight 9, can be adjustable. A large angle allows for better anticipation of detections and more reaction time. On the other hand, the image will be of lower quality. An angle between 30° and 45° is a good compromise for good anticipation of detections and good image quality.
[0046] [Fig. 6] illustrates the system according to the invention. The system capable of implementing the method according to the invention comprises a sonar channel formation block 17, which comprises a “conventional” SAS channel processing unit 18, and an imaging unit 19 which makes it possible to estimate the forward environment in real time.
[0047] The “classic” SAS channel processing unit 18 can be configured, for example, to carry out a mission of detecting and classifying targets of interest, in particular in a mine warfare context.
[0048] The track forming block 17 is capable of providing two solutions which are not or only slightly sensitive to speed.
[0049] The first solution consists of carrying out so-called “natural” channel processing, which only uses the length of the physical antenna.
[0050] Natural processing is a classic channel formation. This formation is carried out, for each desired observation direction (i.e. for each channel) by summing the signals received by the sensors / hydrophones. The signals are previously delayed according to their position in space in order to put them in phase before summing in the desired listening direction.
[0051] A second solution consists of applying a so-called incoherent treatment, for the “broadside” sector (substantially perpendicular to the axis of movement) and / or for the sector front view.
[0052] For this, a positioning module 21 estimates the movement of the carrier 1. Advantageously, the positioning module 21 can be an inertial unit. Inertial units are capable of providing measurements without depending on external sources, and with a high refresh rate.
[0053] The sonar channel formation block 17 performs compensation of each reconstructed image according to the movement of the carrier, so as to obtain compensated images.
[0054] For incoherent SAS processing, the images acquired by the sensor 23 are amplitude integrated. These images must therefore be recalibrated with a precision close to the resolution of the images (generally fixed by the frequency band). To estimate the movements allowing this recalibration, the channel formation block 17 can use the information extracted from the positioning module 21, as described in particular in [3]. Thus, the positioning module 21 is used to estimate the movements necessary for the recalibration of the incoherent processing.
[0055] The “incoherent SAS” processing is particularly advantageous because, in addition to its performance, it is almost native to “classic” SAS processing, namely that it is sufficient to replace a sum of complex images (coherent SAS) by a sum of amplitude images (incoherent SAS).
[0056] The transition from coherent processing to incoherent processing can advantageously be conditioned by the passage of a correlation coefficient H below a threshold value, the correlation coefficient being obtained by the correlation of the antenna signals from one ping to another. The correlation coefficient l{ is therefore calculated by calculating the intercorrelation between a ping and the following ping. Indeed, when the speed of the carrier increases beyond the limit speed VmaxSAs, no sensor correlates with the other sensors, and the correlation coefficient drops significantly. In this case, the images are summed in amplitude.
[0057] As a reminder, for coherent SAS processing, the images must be integrated (summed) in phase. This means that the images must be extremely precisely aligned with each other. For example, for a 300 kHz sonar, the wavelength is 5 mm. To phase align the images, the motion must be compensated with an accuracy that is a fraction of this wavelength, for example 0.5 mm (1 / 10th of a wavelength). This alignment is carried out using a ping-to-ping cross-correlation method (or "P2C2" for "ping-to-ping cross-correlation").
[0058] Thus, it is sufficient to base oneself on the correlation coefficient to pass from one mode to another. The channel formation processing will automatically and systematically place itself in the best possible mode (coherent or incoherent). Thus, a speed-robust imaging mode is advantageously created.
[0059] An example of anticipated assessment of the seabed is illustrated by Figure 7. On this “before” image 25, a detection processing is then carried out. The objective of this detection processing may be, for example, to detect all objects 26 of mine size. This detection may be carried out by an artificial intelligence type processing, or by a shadow detection processing, as illustrated by Figure 7. The dimensions of these shadows provide direct information on the width l and the height h of the object, which makes it possible to then verify that these dimensions are compatible with the type of object sought.
[0060] The detections then pass into a classification stage, using ATR (Automatic Target Recognition) or AuDC (Automatic Detection and Classification) type algorithms, well known to those skilled in the art. The classification stage, implemented by the channel formation block 17, associates the classification (for example mine / non-mine) with the position of the detection, obtained using the positioning module 21 and the sonar channel pointing information calculated in the channel formation block 17.
[0061] The system according to the invention, illustrated by [Fig. 6], also comprises a decision-making autonomy block 20, which has, as input, the detection / classification information provided by the lane formation block 17, and mission plan and / or mission directive information, provided by a command, control and communications system 22 (system called “C3”). The decision-making autonomy block 20 provides the carrier, as output, with a speed instruction, a time of application of the instruction (to manage the acceleration / deceleration constraints of the vehicle), and a duration of application of the instruction.
[0062] To simplify the description, two possible speed instructions are defined: - Vhigh: speed instruction higher than the VmaxSAs limit imposed by the coherent SAS processing. The effective speed depends on the vehicle's capabilities and the limits imposed by the opening of the lobe on transmission. - Viow: speed instruction lower than the VmaxSAs limit imposed by coherent SAS processing.
[0063] According to one embodiment, the decision-making autonomy block 20 determines whether the classification is considered to be of “good” or “bad” quality. The “good” or “bad” character can advantageously be set according to a quality score nq which is calculated by the product of a first probability PI and a second probability P2. The first probability is specific to the algorithm used for the classification. Thus, the first probability PI depends on the algorithm used for the classification (signal-to-noise contrast, complexity of the shape of the object, size of the object, statistical approach, etc.).
[0064] The second probability P2 depends on the resolution of the defocused image and the di dimensions of the target. Thus, a two-meter-long cylinder that would be seen from the side does not need very fine resolution to be correctly classified. According to one embodiment, the second probability P2 can be based on the Johnson criterion, described in more detail in [4].
[0065] Thus, using the second probability P2, in the calculation of the quality score nq, makes it possible to take into account the “physical” characteristics of the target, as seen in the anticipated assessment of the seabed.
[0066] The quality score nq can be compared to a threshold sq: - if nq > sq, then speed setpoint = Vhigh - otherwise speed instruction = Viow
[0067] The decision-making autonomy block 20 can also take into account, in making the speed instruction decision, the mission directives from the C3 system 22.
[0068] In particular, the C3 system 22 can transmit, to the decision-making autonomy block 20, a DirA directive to minimize the duration of the mission, at the cost of greater risk (which can be useful in a well-known area, for example a harbor of a port in which it is certain that no mines are located there), or a DirB directive to minimize the risk, at the cost of greater mission duration (which can be useful in a war zone).
[0069] Thus, two thresholds can be defined, a first threshold sA associated with the directive DirA, and a second threshold sB associated with the directive DirB, with the following condition: sA < sB.
[0070] The hourly coverage is therefore higher in the case of a DirA directive compared to a DirB directive.
[0071] Figures 8 to 11 illustrate different embodiments of the invention.
[0072] According to a first embodiment, illustrated by [Fig.8], the antenna processing insensitive to the carrier speed is executed provided that the anticipated assessment of the seabed determines that a target is located at a distance less than the resolution of the coherent synthetic aperture antenna processing.
[0073] Indeed, depending on the sizing of the sonar, it is possible that at the start of the range, the resolution of the incoherent SAS processing, or of the natural SAS processing, is equal to the resolution of the coherent SAS processing. This occurs when a single ping is used at the start of the range. On the bottom portion 27, which corresponds to a dip distance between the sonar 1 and the bottom, the coherent SAS processing does not provide any gain in resolution. Thus, for any detection which occurs at a distance less than dip, the speed setpoint can be the higher speed setpoint Vhigh, greater than the limit VmaxSAS.
[0074] According to another embodiment, illustrated by [Fig.9], the antenna processing insensitive to the carrier speed is executed if the anticipated assessment of the seabed includes detecting a target that has already been imaged by coherent antenna synthetic aperture processing in a rail separate from the current rail in which the wearer is located.
[0075] Indeed, the route planned for the sonar, composed of rails generally parallel to each other, provides for overlaps between the rails, in order to fill coverage holes. The decision-making autonomy block 20 having knowledge of the mission plan via the C3 block 22, it is possible to exploit this plan to improve the hourly coverage.
[0076] Thus, the embodiment consists of not taking into account a detection already processed during a previous rail. In [Fig.9], if the target 28 has already been observed on rail 1, it is not necessary to slow down during rail 2. Thus, even if it is established that a target 28 is located in rail 1, and requires coherent SAS processing, the decision autonomy block 20 can transmit a command to switch from coherent processing to antenna processing insensitive to the carrier speed, without reducing the speed below the threshold value specific to coherent antenna synthetic aperture processing.
[0077] [Fig. 10] illustrates another embodiment. Assuming that, with the anticipated assessment of the seabed, the target 28 was detected at the end of the range during a first pass (first rail 29) with insufficient quality (nq < sq), it would therefore be necessary to slow down the vehicle (speed setpoint = Viow), in order to provide imaging by coherent SAS processing.
[0078] However, the decision-making autonomy block 20, which has the mission plan information transmitted by the C3 block 22, can determine that the target 28 will be reviewed at the start of the range during the next pass (2nd pass 30), and therefore that it will be faster to carry out imaging there by coherent SAS processing. Thus, during the first pass, the decision-making autonomy block 20 can decide not to slow down, and to keep the speed setpoint at maximum Vhigh, higher than the limit speed V maxSAS*
[0079] Thus, the reception of a mission plan defining at least two rails, and a determination of the distance between the sonar and the target during the anticipated evaluation, makes it possible to increase the hourly coverage of the sonar.
[0080] [Fig. 11] illustrates another embodiment of the method according to the invention. In this figure, targets 31-35 are detected and classified as being of good quality, and target 36 is detected and classified as being of poor quality.
[0081] The method advantageously comprises calculating a fusion of quality scores, for the plurality of targets detected during the early assessment of the seabed. The control (speed reduction or switching from coherent processing to antenna processing insensitive to the carrier speed) is then selected as a function of the result of the fusion. The fusion of notes thus corresponds to a risk associated with the group of targets seen in a time slot by the anticipated assessment of the seabed.
[0082] This embodiment is more “risky”, because it is not necessarily the lower speed setpoint Viow which prevails over the higher speed setpoint Vhigh. However, the embodiment makes it possible to increase the hourly coverage.
[0083] With N detections (targets 31-36 in [Fig.l 1]), the overall quality is expressed by the following relationship:
[0084]
[0085] nq(x) corresponds to the quality score nq for detection “x”, which is calculated by the product of a first probability PI and a second probability P2, as described previously.
[0086] The command selection is expressed as follows:
[0087] if n / TT „ s , then speed setpoint = Vhigh
[0088] otherwise speed setpoint = Viow
[0089] is a predefined threshold.
[0090] The aforementioned embodiments can be combined, in order to increase the hourly coverage of the sonar.
[0091] The method as described makes it possible to optimize the hourly coverage of a system existing by means of decision-making autonomy, while controlling overall performance. The process has the advantage that it does not require developing a new, specific, expensive and complex sonar system.
[0092] References cited:
[0093] [1] A Putney, “Reconstruction of Under-sampled SAS Data Using the WIPE Algorithm”, OCEANS 2005
[0094] [2] Shi YI, “A Method of SAS with Sparse Aperture Based on Compressive Sensing”, IEEE 2020
[0095] [3] Michael P. Hayes et al., Synthetic Aperture Sonar: A Review of Current Status
[0096] [4] F. Langner et al., “Side Scan Sonar Image Resolution and Automatic Object Détection, Classification and Identification”, OCEANS 2009-EUROPE
Claims
Claims
1. Method for improving the hourly coverage of a sonar (1) with channel formation and configured to be embarked on a carrier in an area to be imaged in order to characterize at least one target, the method comprising the steps of: - providing imaging, by coherent antenna synthetic aperture processing, of a seabed of the area, along a lateral line of sight (8) which is substantially perpendicular to the direction of movement (10) of the carrier; - providing an anticipated evaluation of the seabed, along an offset line of sight (9) relative to the lateral line of sight (8);- selecting a command from: - reducing the carrier speed (24) below a threshold value (VmaxSAs) specific to the coherent antenna synthetic aperture processing, based on the anticipated assessment of the seabed, in order to maintain the coherent antenna synthetic aperture processing, or - switching from the coherent processing to an antenna processing insensitive to the carrier speed, without reducing the speed below the threshold value (VmaxSAs) specific to the coherent antenna synthetic aperture processing.;
2. 2. Method according to claim 1, in which the antenna processing insensitive to the speed of the carrier comprises a so-called incoherent processing, the incoherent processing comprising: - an estimation of the movement of the carrier by a positioning module (21); - a compensation of each reconstructed image as a function of the movement of the carrier (24), so as to obtain compensated images; - an amplitude summation of the compensated images.
3. Method according to one of the preceding claims, in which the transition from coherent processing to incoherent processing is conditioned by the passage of a correlation coefficient below a threshold value, the correlation coefficient being obtained by the correlation of the antenna signals from one ping to another.
4. 4. Method according to one of the preceding claims, in which the provision of an advance assessment of the seabed comprises the steps of: - acquiring measurements along the offset sighting axis (9) relative to the lateral line of sight, so as to obtain a so-called defocused image; - detection of the target in the defocused image; - classification of the target according to at least one classification criterion, in order to determine whether the target is an object of interest; and in which, if the classification is considered to be of good quality, the coherent processing switches to antenna processing insensitive to the carrier speed, and if the classification is considered to be of poor quality, the carrier speed is reduced below the threshold value specific to the coherent antenna synthetic aperture processing.
5. 5. Method according to claim 4, in which the classification of the object comprises an assignment of a quality score (nq), the quality score being calculated by the product of a first probability (PI) and a second probability (P2), the first probability (PI) being specific to the algorithm used for the classification, the second probability (P2) depending on the resolution of the defocused image and the dimensions of the target.
6. Method according to one of claims 4 or 5, comprising a calculation of a fusion of quality scores (nq), for a plurality of targets (31-36) detected during the early assessment of the seabed, the control being selected according to the result of the fusion.
7. 7. A method according to any preceding claim, wherein the carrier velocity insensitive antenna processing is performed subject to the anticipated seabed assessment determining that a target is located at a distance less than the resolution of the coherent antenna synthetic aperture (dip) processing.
8. A method according to one of the preceding claims, wherein the carrier velocity insensitive antenna processing is performed if the anticipated seabed assessment includes detecting a target that has already been imaged by coherent antenna synthetic aperture processing in a rail distinct from the current rail in which the carrier is located.
9. Method according to one of the preceding claims, comprising the steps of: - receiving a mission plan defining at least a first rail (RI) and a second rail (R2) to be taken by the carrier; - the anticipated evaluation comprising a determination of the distance between the sonar and the target; the command being executed in the first rail (RI) as a function of the distance between the sonar and the target in the first rail (RI) and in the second rail (R2).
10. System configured to improve the hourly coverage of a channel-forming sonar and configured to be embarked on a carrier in an area to be imaged in order to characterize at least one target, the system comprising: - an imaging device (18), by coherent processing of synthetic antenna aperture, of a seabed of the zone, along a lateral sighting axis which is substantially perpendicular to the direction of movement of the carrier; - a device for early assessment of the seabed (19), along a sighting axis offset from the lateral sighting axis; - a device for selecting a command (20) from: - a reduction of the carrier speed below a threshold value specific to the coherent antenna synthetic aperture processing, based on the anticipated assessment of the seabed, in order to maintain the coherent antenna synthetic aperture processing, or - a transition from coherent processing to antenna processing insensitive to the carrier speed, without reducing the speed below the threshold value specific to coherent synthetic antenna aperture processing.