A sonobuoy

Abstract

A sonobuoy 10 comprising an upper unit 12 and a lower unit 14, the lower unit, in use, arranged vertically below the upper unit, in which the upper and lower units are releasably retained to each othe

Description

The present invention relates to a sonobuoy. Sonobuoys are buoys which contain sonar equipment for the detection and localisation of underwater objects such as submarines. The sonobuoys are ejected from fixed or rotary wing aircraft in canisters which comprise an upper and a lower unit, and deploy upon impact with water. The upper unit includes an inflatable surface float with a transmitter which remains on the water surface for communication with the aircraft, while the lower unit containing sonar equipment, typically active and / or passive transducers, descends below the water surface to a selected depth. Once activated, either remotely or after a pre-determined time, the sonobuoy transmits acoustic information detected by the transducers to the aircraft for further processing, for example, processing to determine the positional information such as classification and positional information of the detected underwater object. The upper U and lower L units of known sonobuoys are housed within an outer canister A (Figures 1 and 2). The outer canister A retains the upper U and lower L units together with further assemblies and components contained within those upper and lower units until the sonobuoy is deployed, at which point in time, the outer canister A is jettisoned, thereby exposing the upper and lower units to the surrounding environment. In Figure 2, the upper U and lower L units have an outer diameter Dupper unit, and the outer canister A has an outer diameter Douter canister. It can be seen from Figure 2 that the outermost circumferential surface of the sonobuoy is defined by the circumferential outer surface of the outer canister A. There is a continuous drive to improve the performance of sonobuoys. To improve the performance, for example, to provide a greater number of transducers or a larger battery, the size of the sonobuoy will correspondingly increase. Furthermore, and counter to the increasing size requirement to house a greater number of transducers or a larger battery, there is a drive to reduce the sonobuoy size to enable transportation and ejection from lighter aircraft, for example, ejection from unmanned drones. An object of the present invention therefore is to provide a sonobuoy with improved performance without significantly increasing the sonobuoy size, or to provide sonobuoys with a reduced size with at least comparable performance. According to the present invention there is provided a sonobuoy comprising an upper unit and a lower unit, arranged, in use, vertically below the upper unit, and a retention feature, in which the upper unit is realisably retained to the lower unit by the retention feature, the sonobuoy further comprising a printed flexible electronic circuit board, in which the printed electronic circuit board comprises one or more of a radio frequency identification (RFID) chip, an antenna, a user display, a user input interface and a seawater switch. Advantageously, it is the retention feature which retains the upper and lower units together, as opposed to the outer canister of prior art sonobuoys, which therefore frees up space for larger components within the same overall sonobuoy diameter, or enables the same components to reduce the overall sonobuoy diameter. Preferably, the retention feature is housed within a radially inwardly projecting indent provided on an outer circumferential surface of the upper and / or lower units. More preferably, an outermost surface of the retention feature is positioned substantially flush or radially inwardly of the outermost circumferential surface of the upper and / or lower units. Advantageously, the retention feature does not add to the diameter of the sonobuoy unlike prior art sonobuoys with outer canisters, enabling the use of large components such as batteries or transducer assemblies to improve the performance of the sonobuoy. Alternatively, the same size components can be used without the need for an outer canister, thereby reducing the diameter of the sonobuoy. Preferably, the upper unit includes an upper projection which releasably retains the retention feature on the upper unit. Preferably, the lower unit includes a lower projection which releasably retains the retention feature on the lower unit. Including the projections on the respective upper and lower units enable the retention feature to retain those upper and lower units together, and any other assemblies housed within those upper and lower units, for example, a transducer assembly, a battery assembly, a float, and a cable pack, until deployment is required. Preferably, the radially inwardly projecting indent is provided on the transducer assembly. Advantageously, the indent enables the retention feature to not contribute to the overall diameter of the sonobuoy. If the outer circumferential surface of the transducer assembly defines an outermost circumferential surface of the sonobuoy then the overall diameter of the sonobuoy is defined by the transducers rather than the retention feature, enabling larger transducer assemblies to be used. Preferably, a or the lower projection is provided on the battery assembly, the lower most assembly of the sonobuoy, thereby retaining the other assemblies between the battery assembly and the upper unit, for example, the float in the upper unit. Preferably, the retention feature is an elongated strap which locates radially within the indent on the upper and lower units, thereby enabling the upper and lower units to be retained together without compromising the size of the assemblies within those upper and lower units any more than the size required by indent to house the elongated strap. Preferably, the elongated strap is a first elongated strap and a second elongated strap, the first elongated strap positioned radially opposite the second elongated strap. Preferably, the upper unit includes a gas canister configured to inflate an inflation mechanism to float the upper unit at water level, in which the gas canister is inclined relative to a vertical axis defined by a longitudinal axis of the sonobuoy. Inclining the gas canister further enables the sonobuoy size to be reduced. Preferably, the upper unit is configured to detach from the lower unit on contact with the water surface, by configuring the upper projection to enable the upper unit to be detached from the lower unit. More preferably, the upper projection detaches when the inflation mechanism fires and pressure builds up in the float of the surface unit. Preferably a radio frequency identification (RFID) chip is provided on a surface unit housing of the sonobuoy to enable remote programming of the sonobuoy which is advantageous when the sonobuoy is deployed from unmanned aircraft. Additionally, or alternatively, the sonobuoy is programmed via a user input interface, typically, switches, and a display, both positioned on an outside surface of the sonobuoy to provide additional space on the inside of the sonobuoy enabling the use of large components, or the same size components can be used thereby reducing the diameter of the sonobuoy. Preferably the printed electronic circuit board is of sufficient flexibility to enable it to be mounted on and substantially take the form of the outer circumferential surface of a or the surface unit housing of the upper unit thereby freeing up room inside the sonobuoy to house larger components. Preferably the printed electronic circuit board is a single assembly. Preferably the upper unit of the sonobuoy comprises a cable pack assembly. Preferably the cable pack assembly includes a cable pack housing and a compliant rubber housed therein and connected to a transducer assembly of the lower unit to enable relative movement between the cable pack assembly and the transducer assembly allowing the transducer housing to descend below the cable pack assembly, in which the cable pack housing includes a retention guide to retain the compliant rubber in the cable pack assembly until the transducer assembly descends below the cable pack assembly. This has the advantage that the upper unit is able to relative to the lower unit, specifically, the cable pack assembly is able to move due to the sea wave motion relative to the transducer assembly without translating this wave motion to the transducers located within the transducer assembly, which movement would otherwise adversely affect the performance of the transducers. Preferably, the compliant rubber is less than one metre in length to enable the use of a smaller cable pack assembly, and hence smaller sonobuoy. Prior art cable pack assemblies are typically larger, and house longer cables. The cable of the present invention has a length which can be spirally wound within the cable pack assembly but still provides sufficient movement when unwound to prevent translation of wave movement to the transducers. The smaller cable pack assembly has the consequence that it is more difficult to pack and retain the cable therein. The retention guide both retains the cable during packing enabling easier packing, and allows the cable to be efficiently stored within the cable pack assembly so as to minimise the size of the cable pack assembly. Preferably the cable pack housing is manufactured by 3D printing. Preferably the lower unit comprises lower unit electronics including a substantially circular printed circuit board comprising a plurality of capacitors used to provide a high drive to the transducers, in which a proportion, preferably great than 50%, of the plurality of capacitors are arranged around a circumference of the printed circuit board. Compared to prior art sonobuoys with single capacitors, the plurality of capacitors enables shorter capacitors to be used, thereby reducing the overall height required in the lower unit. A plurality of sonobuoys forms part of a sonar system comprising an aircraft to both deploy the sonobuoys and containing electronics to communicate with the sonobuoy to enable the transmission of data between the sonobuoy and the aircraft. Where an unmanned aircraft is used to deploy the sonobuoy, such as a drone, the electronics can be remote from the drone, such as in a ship, or a land-based location. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a prior art sonobuoy. Figure 2 is a front sectional view of part of the prior art sonobuoy of Figure 1, Figure 3 is a top perspective view of part of the prior art sonobuoy of Figure 1, Figure 4 is perspective view of a sonobuoy according to the present invention, Figures 5A and 5B are front sectional views of the sonobuoy of Figure 4, Figures 6 and 7 are top views of the sonobuoy of Figure 4, Figure 8 is a side view of the sonobuoy of Figure 4, Figure 9 is a top perspective view of part of the sonobuoy of Figure 4, Figure 10 is a front perspective sectional view of the sonobuoy of Figure 4, Figure 11 is a front perspective sectional view of part of the sonobuoy of Figure 4, Figure 12 is an exploded view of the sonobuoy of Figure 4, Figure 13 is a schematic view of the components of the sonobuoy of Figure 4, Figure 14 is a front view of part of the sonobuoy of Figure 4, Figure 15 is a rear view of part of the sonobuoy of Figure 4, Figure 16 is a top view of part of the sonobuoy of Figure 4, Figure 17 is a perspective view of part of the sonobuoy of Figure 4, Figure 18 is a top view of part of the sonobuoy of Figure 4, and Figures 19a to 19e are schematic views of the deployment stages of the sonobuoy of Figure 4. In Figures 4 to 18, a sonobuoy 10, preferably cylindrical in shape, comprises an upper unit 12, a lower unit 14, positioned vertically below (in normal use) the upper unit 12, and a retention feature (best seen in Figures 4 to 8 and 12) in the form of an elongated strap 54. By elongated it is meant that the strap is longer than it is wide to enable the upper (12) and lower (14) units to be releasably retained to each other. Figures 12 and 13 identify the various modules and elements which comprise the upper 12 and lower 14 units of the sonobuoy 10, not all of which are referenced in the other figures. In Figures 12 and 13, the upper unit 12 comprises a surface unit deployment system 34 and a surface unit 18. The surface unit deployment system 34 comprises a windflap assembly 44 comprising a windflap 41 and a windflap release mechanism 46, a parachute assembly 23 comprising a parachute housing 17 and a parachute 43, a float assembly 24 comprising a float 15, an antenna and a scuttling system (both only shown schematically in Figure 13). The windflap 41 comprises a circular disc-like top portion 25 and an elongated downwardly extending portion 27. The downwardly extending portion 27 includes an aperture 29 into which the windflap release mechanism 46 locates so as to retain the windflap 41 on the upper unit 12 until the sonobuoy 10 is deployed as will be described below. The surface unit 18 comprises a surface unit housing 26 and a cable pack assembly 28. The surface unit housing 26 comprises an inflation mechanism 30, a depth selection device 31, a seawater switch 33, surface unit electronics 32, and a surface unit battery 35. The surface unit housing 26 includes a Remote Function Select (RFS) transceiver 49 with an RFID chip 37 and an antenna 75 to enable remote programming of the sonobuoy, and / or an Autonomous Function Select (AFS) for manual programming. In Figures 14 to 16, a single assembly flexible printed electronic circuit board (PEC) 45 is provided on an outer circumferential surface 57 of the surface unit housing 26. The PEC 45 has an inside surface 89 and an outside surface 91. It will be understood the PEC 45 is sufficiently flexible to take the form (cylindrical) of the outer surface 57 of the surface unit housing 26. The RFID chip 37 is located on an inside surface 89 of the PEC 45. The seawater switch 33 comprises anode 85 and cathode 87 plates also mounted on the inside surface 89 of the PEC 45. The PEC 45 further comprises a user display in the form of two screens 81 and two switches 79 mounted on the outside surface 91 to enable access by a user when programming the sonobuoy, in addition to, or as an alternative to remote programming via the RFID chip 37. In Figures 10 and 11, the surface unit housing 26 further comprises a gas canister 21 configured to inflate the inflation mechanism 30. It can be seen from Figure 11 that the gas canister 21 is inclined along an axis A relative to a vertical axis V defined by a longitudinal axis L of the sonobuoy 10. The inflation mechanism 30 includes a piercing mechanism (not shown) configured to pierce an opening of the gas canister 21 to enable gas to gaseously communicate with the float 15. The piercing mechanism describes an arc-like movement when piercing the opening to prevent the opening from icing up, the consequences of which would be to inhibit gas flow and therefore prevent or delay float inflation. The cable pack assembly 28 comprises a depth cord 39, cable pack housing 13 and a compliant rubber or snubber 72. In Figure 17, the cable pack housing 13 is manufactured by 3D printing, and includes a plurality of integrated retention guides in the form of downward (towards the sea) projections or tongues 77 which retain the compliant rubber (snubber) 72 within the housing 13 and therefore within the cable pack assembly 28. The tongues 77 are radially spaced. The spacing between adjacent tongues 77 is configured such that the snubber 72 is retained between adjacent tongues 77 when the snubber 72 is being packed into the housing 13. When the transducer assembly 36 is released, the spacing allows the snubber 72 to pull free of the tongues 77 and allow the transducer assembly 36 to take the snatch load during descend and high sea state levels. The lower unit 14 comprises a transducer assembly 36 and a battery assembly 38. The transducer assembly 36 comprises a transducer housing 40, within which is housed a plurality of transducers 47, in this embodiment, six transducers. The transducers 47 are active transducers. In alternative embodiments, passive transducers, or a combination of both active and passive transducers can be used. Furthermore, different numbers of transducers can be used. The battery assembly 38 comprises lower unit electronics 42, a lower unit battery 44 and magnetics 48 comprising a matching transformer and inductor. The transformer steps up the battery voltage to the required voltage / currents to drive the transducers as is known in the art. In Figure 18, the lower unit electronics 42 includes a circular printed circuit board PCB 42 comprising a plurality of capacitors 69. It can be seen that 7 of the 12, that is more than 50% of the capacitors are arranged around the circumference C of the PCB 42. It will be understood that using a plurality of shorter capacitors, as opposed to a single taller capacitor, enables a shorter (in height) lower unit 14 to be used. The sonobuoy 10 further comprises two radially opposite retaining straps 54. In an alternative embodiment, the retaining straps need not be radially opposite, that is, angularly spaced by more or less than 180 degrees. One end of each strap 54 is pivotally and releasably attached to a bulkhead 50 of the lower unit 14 via a lower projection in the form of lugs 63 (Figures 4 and 12), and the other, opposite end of the strap 54 includes two holes 56 which each locate around an upper projection 58 of a deformable plate 19 of the upper unit 12 to releasably retain the retaining straps 54 on the upper projections 58 and therefore on the upper unit 12. The transducer housing 40, cable pack assembly 28, surface unit housing 26, and the suspension system 16 include a pair of oppositely located radially inwardly projecting indents 60 into which the retention straps 54 locate (best seen in Figures 4 to 7). It can be seen from Figure 5A (with retention straps 54 in place) and Figure 5B (with retention straps 54 removed) that the indents 60 have a radial depth Dr and the retention straps 54 a thickness Tr which allows an outer circumferential surface 55 of the retention strips 54 to be flush with or radially inward of an outer radial surface 57 of one or both of the upper 12 and lower 14 units. That is, the retention straps 54 do not define the outer diameter of the outer canister. Specifically, the retention strips 54 are flush with or radially inward of the outer radial surface 57 of at least one of the transducer housing 40, the cable pack assembly 28, the surface unit housing 26, and the surface unit deployment system 34. That is, the outermost circumferential surface of the sonobuoy is defined by at least one of the transducer housing 40, cable pack assembly 28, surface unit housing 26, and the suspension system 16. A comparison of the prior art sonobuoy of Figure 3 and the sonobuoy of the present invention in Figure 9 shows that removal of the outer canister A results in the diameter of the outer canister being defined by the diameter of the upper unit D’upper unit. For the same outer canister diameter, the upper unit D’upper unit can be greater, thereby enabling larger sub-assembly components such as transducers or batteries, with an associated performance increase. Alternatively, the outer diameter of the canister can be reduced without sacrificing performance. The sonobuoy 10 is deployed in the same way as prior art sonobuoys other than with reference to how the upper 12 and lower units 14 are retained together prior to separation as will be described below. Prior to release from the aircraft (not shown), the release latch mechanism 46 is activated into a free position on loading the sonobuoys into a launcher (not shown) on the aircraft. The sonobuoy can be programmed prior to release as is known in the art using the user switches 81. Alternatively, the sonobuoy can be programmed remotely using the RFID chip 37. It will be understood that the sonobuoy can be programmed using the switches prior to release, and later remotely via the RFID chip 37. In stage 1 (Figure 19a), the sonobuoy 10 is shown shortly after release from an aircraft (not shown). Once the sonobuoy 10 is released from the launcher, and descends in the air, the windflap separates by breaking the frangible release mechanism 46 (shown in Figure 4) to free it from the aperture 29 to enable the windflap 40 to detach from the upper unit 12 and release and then open the parachute 43. In stage 2 (Figure 19b), the sonobuoy 10 impacts with the sea and descends below the surface S, causing the seawater switch 33, depth selection device 31 and the inflation mechanism 30 to activate. In stage 3 (Figure 19c), activation of the inflation mechanism 30 deploys the float 15 which raises the upper unit 12 such that it floats on the surface S of the sea. At the same time, when the inflation mechanism activates, and pressure builds up in the float 15, the deformable plate 19 deforms which releases the retention straps 54 from the upper unit 12 to enable the lower unit 14 to detach from the upper unit 12 and descend to an initial pre-determined depth. All the elements of the depth selection device 31 and the cable pack housing 13 are retained as part of the upper unit 12 when deployed in the water, in contrast to known sonobuoys where the cable pack and selected depth pins descend with the lower unit. Furthermore, depth select pins (not shown) are designed to release when under load from the suspended lower unit. In stage 4 (Figure 19d), the transducers 47 are initially deployed, and the retention straps 54 are released from the lower unit 14. In stage 5 (Figure 19e), the transducers 47 are released to a lower depth D according to the command given to enable the detection of underwater objects. The command given can be predetermined to a given depth prior to launch, or changed remotely from a processor on board the aircraft after initial deployment in the water such that the depth can be varied. The processor can also perform other remote functions such as changing the ping type and scuttling the buoy after use. Once the transducers 47 have been released to their commanded depth, they are activated, either remotely or after a pre-determined time, to enable the detection of underwater objects. Communication of acoustic information detected by the transducers is transmitted between the transducers 47 and the surface unit 18 (in upper unit 12) in the same way as prior art sonobuoys. Similarly, communication between the surface unit 18 and processing electronics on the aircraft, and processing of the detected acoustic information is identical to prior art sonobuoys. It will be appreciated that in prior art sonobuoys, the outer canister is released and exposes the upper and lower units, whereas in the present invention, the retention straps retain the upper and lower units together until the seawater switch is activated, and then releases the lower unit from the surface unit. The following clauses are intended to provide further understanding of the invention. 1. A sonobuoy (10) comprising an upper unit (12) and a lower unit (14), the lower unit (14), in use, arranged vertically below the upper unit (12), in which the upper (12) and lower units (14) are releasably retained to each other by a retention feature (54). 2. A sonobuoy (10) according to clause 1 in which the retention feature (54) is housed within a radially inwardly projecting indent (60) provided on an outer circumferential surface (57) of the upper (12) and / or lower units (14). 3. A sonobuoy (10) according to clause 2 in which an outermost surface (55) of the retention feature (54) is positioned substantially flush or radially inwardly of the outermost circumferential surface (57) of the upper (12) and / or lower (14) units. 4. A sonobuoy (10) according to any preceding clause in which the upper unit (12) includes an upper projection (58) which releasably retains the retention feature (54) on the upper unit (12). 5. A sonobuoy (10) according to any preceding clause in which the lower unit (14) includes a lower projection (63) which releasably retains the retention feature (54) on the lower unit (14). 6. A sonobuoy (10) according to any preceding clause in which the lower unit (14) includes a transducer assembly (36) and a battery assembly (38). 7. A sonobuoy (10) according to clause 6 in which the radially inwardly projecting indent (60) is provided on the transducer assembly (36). 8. A sonobuoy (10) according to clause 6 or 7 when dependent on clause 5 in which the lower projection (63) is provided on the battery assembly (38). 9. A sonobuoy (10) according to any preceding clause in which a or the lower projection (63) releasably retains the retention feature (54) on the lower unit (14) until the retention feature (54) has been released from the upper unit (12). 10.A sonobuoy (10) according to any preceding clause in which the retention feature (54) is an elongated strap. 11. A sonobuoy (10) according to clause 10 in which the elongated strap (54) is a first elongated strap and a second elongated strap, the first elongated strap positioned radially opposite the second elongated strap. 12. A sonobuoy according to any preceding clause in which the upper unit (12) is configured to detach from the lower unit (14) on contact with the water surface. 13. A sonobuoy according to clause 12 when dependent on clause 4 or clauses 5 to 12 when dependent on clause 4 in which the upper projection (58) is configured to enable the upper unit (12) to detach from the lower unit (14) on contact with the water surface. 14.A sonobuoy according to any preceding clause in which the upper unit (12) includes a float assembly (24). 15.A sonobuoy according to clause 6 or any one of clauses 7 to 14 when dependent on clause 6 in which, prior to deployment in the water, an outer circumferential surface of the transducer assembly (36) defines an outermost circumferential surface of the sonobuoy (10). 16.A sonobuoy according to any preceding clause in which the upper unit (12) includes a gas canister (21) configured to inflate an inflation mechanism (30) to retain the upper unit (12) at water level, in which the gas canister (30) is inclined relative to a vertical axis (V) defined by a longitudinal axis (L) of the sonobuoy (10). 17. A sonobuoy according to any preceding clause comprising a radio frequency identification (RFID) chip (37) to enable remote programming of the sonobuoy. 18. A sonobuoy according to clause 17 in which the RFID chip (37) is provided on the upper unit (12), preferably on a surface unit housing (26) of the upper unit (12). 19. A sonobuoy according to any preceding clause further comprising a printed flexible electronic circuit board (45). 20. A sonobuoy according to clause 19 in which the printed electronic circuit board (45) comprises one or more of the RFID chip (37), an antenna (75), a user display (79), a user input interface (81) and a seawater switch (83). 21 .A sonobuoy according to clause 19 or 20 in which the printed electronic circuit board (45) is of sufficient flexibility to enable it to be mounted on and substantially take the form of the outer circumferential surface (57) of a or the surface unit housing (26) of the upper unit (12). 22. A sonobuoy according to clause any one of clauses 19 to 21 in which the printed electronic circuit board (45) is a single assembly. 23.A sonobuoy according to any preceding clause in which the upper unit (12) comprises a cable pack assembly (28). 24. A sonobuoy according to clause 23 in which the cable pack assembly (28) includes a cable pack housing (13) and a compliant rubber (72), preferably less than one metre in length, housed therein and connected to a transducer assembly (36) of the lower unit (14) to enable relative movement between the cable pack assembly (28) and the transducer assembly (36) allowing the transducer assembly (36) to descend below the cable pack assembly (28), in which the cable pack housing (13) includes a retention guide (77) to retain the compliant rubber (72) in the cable pack assembly (28) until the transducer assembly (36) descends below the cable pack assembly (28). 25. A sonobuoy according to clause 24 in which the retention guide (77) comprises a plurality of radially spaced downwardly projecting projections (77) between which the compliant rubber (72) locates so as to retain the compliant rubber (72) in the cable pack assembly (28) until the transducer housing (40) descends below the cable pack assembly (28). 26. A sonobuoy according to any one of clauses 23 to 25 in which the cable pack housing (13) is manufactured by 3D printing. 27. A sonobuoy assembly according to any preceding clause, the lower unit (14) comprises lower unit electronics (42), the lower unit electronics (42) comprising a plurality of capacitors (69). 28. A sonobuoy assembly according to clause 27 in which the lower unit electronics (42) comprises a substantially circular printed circuit board (71), in which a proportion, preferably great than 50%, of the plurality of capacitors (69) are arranged around a circumference of the printed circuit board (42). 29. A sonar system (100) comprising an aircraft and electronics, and a plurality of sonobuoys (10) according to any preceding clause, in which the sonobuoys (10) are configured to communicate with the electronics to enable the transmission of data between the sonobuoys (10) and the aircraft.

Claims

22 07 251. A sonobuoy (10) comprising an upper unit (12) and a lower unit (14), the lower unit (14), in use, arranged vertically below the upper unit (12), in which the upper (12) and lower units (14) are releasably retained to each other by a retention feature (54), the sonobuoy further comprising a printed flexible electronic circuit board (45), in which the printed electronic circuit board (45) comprises one or more of a radio frequency identification (RFID) chip (37), an antenna (75), a user display (79), a user input interface (81) and a seawater switch (83).

2. k sonobuoy (10) according to claim 1 in which the retention feature (54) is housed within a radially inwardly projecting indent (60) provided on an outer circumferential surface (57) of the upper (12) and / or lower units (14).

3. A sonobuoy (10) according to claim 2 in which an outermost surface (55) of the retention feature (54) is positioned substantially flush or radially inwardly of the outermost circumferential surface (57) of the upper (12) and / or lower (14) units.

4. A sonobuoy (10) according to any preceding claim in which the upper unit (12) includes an upper projection (58) which releasably retains the retention feature (54) on the upper unit (12).

5. A sonobuoy (10) according to any preceding claim in which the lower unit (14) includes a lower projection (63) which releasably retains the retention feature (54) on the lower unit (14).

6. A sonobuoy (10) according to any preceding claim in which the lower unit (14) includes a transducer assembly (36) and a battery assembly (38).

7. A sonobuoy (10) according to claim 6, when dependent on claim 2, in which the radially inwardly projecting indent (60) is provided on the transducer assembly (36).22 07 258. A sonobuoy (10) according to claim 6 or 7 when dependent on claim 5 in which the lower projection (63) is provided on the battery assembly (38).

9. A sonobuoy (10) according to any preceding claim in which a or the lower projection (63) releasably retains the retention feature (54) on the lower unit (14) until the retention feature (54) has been released from the upper unit (12).

10. A sonobuoy (10) according to any preceding claim in which the retention feature (54) is an elongated strap.

11. A sonobuoy (10) according to claim 10 in which the elongated strap (54) is a first elongated strap and a second elongated strap, the first elongated strap positioned radially opposite the second elongated strap.

12. A sonobuoy according to any preceding claim in which the upper unit (12) is configured to detach from the lower unit (14) on contact with the water surface.

13. A sonobuoy according to claim 12 when dependent on claim 4 or claims 5 to 12 when dependent on claim 4 in which the upper projection (58) is configured to enable the upper unit (12) to detach from the lower unit (14) on contact with the water surface.14.A sonobuoy according to any preceding claim in which the upper unit (12) includes a float assembly (24).

15. A sonobuoy according to claim 6 or any one of claims 7 to 14 when dependent on claim 6 in which, prior to deployment in the water, an outer circumferential surface of the transducer assembly (36) defines an outermost circumferential surface of the sonobuoy (10).16.A sonobuoy according to any preceding claim in which the upper unit (12) includes a gas canister (21) configured to inflate an inflation mechanism (30) to retain the upper unit (12) at water level, in which the gas canister (30) is inclined22 07 25relative to a vertical axis (V) defined by a longitudinal axis (L) of the sonobuoy (10).

17. A sonobuoy according to any preceding claim, in which the RFID chip (37) is provided on the upper unit (12) on a surface unit housing (26) of the upper unit (12).

18. A sonobuoy according to any preceding claim in which the printed electronic circuit board (45) is of sufficient flexibility to enable it to be mounted on and substantially take the form of the outer circumferential surface (57) of a or the surface unit housing (26) of the upper unit (12).

19. A sonobuoy according to any preceding claim in which the printed electronic circuit board (45) is a single assembly.20.A sonobuoy according to any preceding claim in which the upper unit (12) comprises a cable pack assembly (28).

21. A sonobuoy according to claim 20 when dependent upon claim 6in which the cable pack assembly (28) includes a cable pack housing (13) and a compliant rubber (72) less than one metre in length, housed therein and connected to the transducer assembly (36) of the lower unit (14) to enable relative movement between the cable pack assembly (28) and the transducer assembly (36) allowing the transducer assembly (36) to descend below the cable pack assembly (28), in which the cable pack housing (13) includes a retention guide (77) to retain the compliant rubber (72) in the cable pack assembly (28) until the transducer assembly (36) descends below the cable pack assembly (28).

22. A sonobuoy according to claim 21 in which the retention guide (77) comprises a plurality of radially spaced downwardly projecting projections (77) between which the compliant rubber (72) locates so as to retain the compliant rubber (72) in the cable pack assembly (28) until the transducer housing (40) descends below the cable pack assembly (28).22 07 2523. A sonobuoy according to claims 21 or 22 in which the cable pack housing (13) is manufactured by 3D printing.24.A sonobuoy according to any preceding claim, in which the lower unit (14) comprises lower unit electronics (42), the lower unit electronics (42) comprising a plurality of capacitors (69).25.A sonobuoy according to claim 24 in which the lower unit electronics (42) comprises a substantially circular printed circuit board (71), in which a proportion great than 50%, of the plurality of capacitors (69) are arranged around a circumference of the printed circuit board (42).

26. A sonar system (100) comprising an aircraft and electronics, and a plurality of sonobuoys (10) according to any preceding claim, in which the sonobuoys (10) are configured to communicate with the electronics to enable the transmission of data between the sonobuoys (10) and the aircraft.

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