WO2024003909A1 - Method and system for determining a reference frame of a north finder - Google Patents

Abstract

A method and system for transferring the north reference from a north finder to a destination platform typically, but not necessarily, uses a north finder that requires a very stable platform to find a very accurate north reference. First, the north finder operates to find the north reference. Then, using an inertial measurement unit, the north reference is transferred to a destination platform that does not need to, and often is unable to, maintain itself a degree of stability to the extent the north finder might need.

Description

METHOD AND SYSTEM FOR DETERMINING A REFERENCE FRAME OF A NORTH FINDER

RELATED APPLICATION

This application claims benefit under 35 U.S.C. § 119(e) of the June 28, 2022 filing of U.S. Provisional Application No. 63/356,043, which is hereby incorporated by reference in its entirety. In the event that terminology in the provisional application might differ somewhat from terminology in the present application, the terminology in the present application applies.

BACKGROUND

A simple conventional compass operates by allowing a magnetic needle to rotate in response to the earth’s natural magnetic field in such a way that it indicates the direction of north. That is, the compass is a “north finder,” and it provides a “north reference.” From the north reference, the directions east, south, and west are quickly known.

This conventional compass has been in use for centuries in fields such as navigation and cartography, and it is still in use today. However, a compass of this type is susceptible to errors due to factors such as proximity to local magnetic fields and/or the presence of nearby iron or steel. Also, the earth’s north geomagnetic pole is not exactly at the earth’s true north pole (at the earth’s axis of rotation, that is, at latitude 90 degrees north) but instead at Ellesmere Island, Canada, which is at latitude 80.8 degrees north. That is, the earth’s geomagnetic pole is offset somewhat from the earth’s axis of rotation and hence from true north. Accordingly, as a compass approaches these locations, or as it approaches the polar opposite locations in the south, the accuracy of the compass declines. Thus, where greater accuracy is needed, it becomes desirable to find the north reference without needing to detect the earth’ s magnetic field.

A more sophisticated north finder, known as an inertial north finder, is a system that uses gyroscopes to find the north reference, and this system is not affected by the presence of magnetic fields or magnetically-attracted metals. This system is simply set on a stable surface, such as the ground, and then, as the earth rotates, the gyroscope system rotates. The gyroscopes sense the western horizon rising as a rate of fifteen degrees per hour and the eastern horizon correspondingly dropping at the same rate. The axis of rotation of this gyroscope-based north reference finding system is parallel to the axis of rotation of the earth, and hence this axis of the system points north (using the right-hand rule to distinguish from pointing south).

Such is an effective way to find the north reference, when this north finder can remain stationary with respect to the surface of the earth during operation. After activating this north finder, the user needs only to wait a sufficient amount of time for the gyroscopes to sense enough of the rotation of the earth to determine the rotation direction. Indeed, the sophisticated tactical North Seeker model NS40 (the terminology “north seeker” being synonymous with “north finder”) manufactured by Condor Pacific Ltd. of Jerusalem, Israel, initially calculates the north azimuth (the north direction) in less than sixty seconds and calculates it with full accuracy in 300 seconds.

Although the technology to find the north reference has improved significantly since the advent of the conventional magnet-based compass, more improvements in the technology are nonetheless desired. For example, a gyroscope-based north finder of the type described above cannot work on a moving platform, such as a transportation vehicle. Even if the vehicle is a land vehicle, as opposed to a water or air vehicle, and stopped momentarily to remain relatively stationary with respect to the earth, such a platform is not necessarily stable enough for the gyroscopic system, and thus this particular north finder cannot operate as effectively in conditions not providing sufficient stability.

The operation of a telescope is an example where an accurate north reference is desired, both to locate images having known coordinates and to assist in providing coordinates of objects of which the coordinates are not previously known. Building a telescope with a gyroscope-based north finder, though, would require a very stable platform for the north finder/telescope assembly. Even though the mounting assembly for a telescope is designed for stability, stability sufficient for image detection and location, such mounting is not necessarily stable enough for conventional gyroscopebased north finders to operate well. A tripod-mounted infrared thermal observation device for rescue brigades, security systems, wildlife viewing, and the like is another example apparatus that, in many uses, requires an accurate north reference, but its tripod mount is generally not stable enough for an inertial north finder to operate effectively. A third example apparatus needing an accurate north reference is a mobile missile launcher in which the platform supporting the launcher is typically not stable enough for the conventional inertial north finder to operate effectively.

To be sure, even more sophisticated north finders have been developed to provide an accurate north reference on unstable platforms, such as not ships. However, such systems are much more expensive than the inertial north finder described above.

In view of the unmet needs, the present inventors endeavored to find a way to provide a highly-accurate north reference to a platform of limited stability in a relatively cost-conscious fashion.

SUMMARY

Embodiments of the present invention facilitate the transfer of the north reference from a north finder to a destination platform. The embodiments exploit the advantage of using a north reference on a stable surface to make the north reference available by transferring the north reference and/or inertial data using an inertial measurement unit.

Specifically, the invention may be embodied as a method for transferring a north reference to a destination platform. The method includes: operating a north finder to find a north reference in the reference frame of the north finder; after the north reference is found, storing the orientation of an inertial measurement unit that is coupled to a mechanical interface of the north finder; de-coupling the inertial measurement unit from the mechanical interface of the north finder and coupling the inertial measurement unit to a mechanical interface of a destination platform; storing the orientation of the inertial measurement unit while still coupled to the mechanical interface of the destination platform; determining the difference between the orientation of the inertial measurement unit when coupled to the destination platform and the orientation of the inertial measurement unit when coupled to the north finder; determining the north reference in the reference frame of the destination platform from the difference in the orientations and the north reference in the reference frame of the north finder; and transmitting the north reference in the reference frame of the destination platform to the destination platform.

The invention may also be embodied as a system for transferring a north reference to a destination platform. The system has a north finder, an inertial measurement unit, and first and second communication links. The north finder operates to generate a north reference in a reference frame of the north finder. The inertial measurement unit operates to mechanically couple alternately to the north finder and to a destination platform, each at known relative alignments, and the inertial measurement unit further operates to generate orientation data indicating its orientation as it moves. The first communication link operates to relay the orientation data from the inertial measurement unit to the north finder. The second communication link operates to relay the north reference in a reference frame of the destination platform from the north finder to the destination platform. The north finder determines the north reference in the reference frame of the destination platform using the north reference in a reference frame of the north finder, the orientation data when the inertial measurement unit is coupled to the north finder, and the orientation data when the inertial measurement unit is coupled to the destination platform.

The invention may further be embodied as a method for transferring a north reference to a destination platform. The method includes: operating a north finder to find a north reference; coupling the north finder to an inertial measurement unit; sending the north reference from the north finder to the inertial measurement unit; maintaining the north reference within the inertial measurement unit; de-coupling the inertial measurement unit from the north finder; coupling the inertial measurement unit to a destination platform; and sending the north reference from the inertial measurement unit to the destination platform.

The invention may alternatively be embodied as an inertial measurement unit for transferring a north reference from a north finder to a destination platform. The inertial measurement unit has an interface assembly and circuity. The interface assembly is operative to couple to a north finder to receive therefrom a north reference. The circuitry is operative to maintain the north reference after de-coupling the inertial measurement unit from the north finder. The interface assembly is further operative to couple the inertial measurement unit to a destination platform and to provide the north reference thereto.

The invention may also be embodied as a system for transferring a north reference to a destination platform. The system has a north finder and an inertial measurement unit. The north finder has an interface assembly and is operative to generate a north reference and to send it through the interface assembly. The inertial measurement unit has an interface assembly that is operative to couple to the interface assembly of the north finder and to receive through the coupled interface assemblies the north reference; the inertial measurement unit being operative to maintain the north reference after de-coupling from the north finder. The interface assembly of the inertial measurement unit is further operative to couple to a destination platform and to provide the north reference thereto.

The invention may additionally be embodied as a method for transferring a north reference to a destination platform. The method includes: operating a north finder to find a north reference in the reference frame of the north finder; after the north reference is found, storing a first orientation, which is the orientation of an inertial measurement unit that is rigidly coupled to the north finder; rigidly coupling the inertial measurement unit to a destination platform; storing a second orientation, which is the orientation of the inertial measurement unit while coupled to the destination platform; determining the difference between the first and second orientations of the inertial measurement unit; determining the north reference in the reference frame of the destination platform from the north reference in the reference frame of the north finder and the difference between the first and second orientations; and providing the north reference in the reference frame of the destination platform to the destination platform.

Embodiments of the present invention are described in detail below with reference to the accompanying drawings, which are briefly described as follows:

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in the appended claims, which are read in view of the accompanying description including the following drawings, wherein:

Fig. 1 provides as a block diagram a logical representation of a system for transferring a north reference to a destination platform in accordance with one embodiment of the invention;

Fig. 2 illustrates the coupling of two of the elements of the system of Fig. 1 ;

Fig. 3 provides as a block diagram a logical representation of the IMU component of Figs. 1 and 2 transferring the north reference to a destination platform;

Figs. 4A - 4C illustrate physically the angular relationships between a north finder, an IMU, and a destination platform in accordance with embodiments of the invention;

Fig. 5 provides a flowchart representative of a method of transferring a north reference to a destination platform in accordance with an alternate embodiment of the invention;

Fig 6 provides a block diagram of a logical representation of a system for transferring a north reference to a destination platform in accordance with another embodiment of the invention;

Fig 7 provides a block diagram of a logical representation of the system of Fig. 6 coupled to a destination platform;

Fig 8 provides a flowchart representative of a method for transferring a north reference to a destination platform in accordance with yet another embodiment of the invention; and Fig 9 provides a flowchart representative of a method for transferring a north reference to a destination platform in accordance with an additional embodiment of the invention.

DETAILED DESCRIPTION

The inventive concepts described herein can be applied to situations in which relatively unstable, or “moderately” stable, platforms need the north reference. Described are both systems and methods for providing the north reference.

The block diagram in Fig. 1 provides a logical representation of an example of the invention embodied as a system for transferring a north reference to a “destination platform.” The terminology “destination platform” refers to an apparatus that uses a north reference. Examples of destination platforms include telescopes, thermal observation devices, and missile launchers, and the terminology is not limited to those examples. As shown in Fig. 1, the system 10 includes a north finder 12 and an inertial measurement unit (IMU) 14.

The north finder 12 has a set of sensors 16 that operates with circuitry 18 to find the north reference 20 in a conventional manner or in a manner specifically-engineered to meet particular needs of an individual implementation. Non-limiting example sets of sensors in various implementations include three gyroscopes, one gyroscope and two accelerometers, and three gyroscopes and three accelerometers. The term “circuitry” as used herein is a comprehensive term, which includes, but is not limited to, processors, storage devices, data buses, hardware, software, firmware, and the like.

The north finder 12 has an interface assembly 22 attached securely thereto and constrained to remain stationary with respect to the sensors 16. The interface assembly 22 enables the north finder 12 to couple to the IMU 14 at its own mating interface assembly 24. When coupled as such, the interface assembly 24 of the IMU 14 is also constrained to remain stationary with respect to the sensors 16 of the north finder 12. The coupled interface assemblies 22, 24 also provide the communication channel for the north finder 12 to send 26 signals representing the north reference 20 to the IMU 14.

Except and unless specifically noted otherwise, the terms “interface” and “interface assembly” in the present disclosure are comprehensive terms. The terms cover, but are not limited to, implementations in which signals representing the north reference flow through the mechanical interfaces between the north finder 12 and the IMU 14 in a non-wireless fashion. The terms also cover implementations in which the mechanical interface constraining the interface assembly 24 of the IMU 14 to remain stationary with respect to the sensors 16 of the north finder 12 is a separate interface module from a distinct wired or wireless electrical connection allowing the north reference 20 signals to flow through from the north finder 12 to the IMU 14. The terms further cover implementations in which the mechanical interface module of the IMU 14 coupling to the north finder 12 is distinctly separate from another mechanical interface module coupling the IMU 14 to a destination platform.

With reference to Fig. 2, the interface assemblies 22, 24 couple to each other mechanically using mating sets of protrusions 28 and recesses 30. In this embodiment, for an accurate transfer of the north reference 20 from the north finder 12 to the IMU 14, the pattern of protrusions 28 and recesses 30 is designed so that interface assembly 22 can only couple to interface assembly 24 in one relative orientation. (Such contrasts with a two-prong electrical plug able to connect to a two-hole socket in either of two orientations, one offset for the other by 180-degrees.) Further, both interface assemblies 22, 24 extend substantially in planes normal to the drawing page, and this “plane-to- plane” contact further ensures that the electrical signals representing the north reference 20 entering the IMU 14 through the interface assembly 24 and flowing further to circuitry 32 in the IMU 14 continue to represent the north reference 20 in the reference frame of the north finder 12 or in another reference frame offset from the first reference frame by a known constant. In other words, knowing the north reference in the reference frame of the north finder 12 enables knowing the north reference in the reference frame of the interface assembly 24 of the IMU 14.

As discussed above, when the north finder 12 is coupled to the IMU 14, the north finder 12 sends to the IMU 14 the north reference 20 through the interface assemblies 22, 24 to the IMU 14, which stores the north reference 20 in its circuitry 32. The IMU 14 has a set of sensors 34 that can maintain the north reference 20 even if the IMU 14 when de-coupled from the north finder 12. The sensors may be selected by one skilled in the art to meet the needs of the particular task. Typically, an IMU has gyroscopes, accelerometers, and can even have magnetometers. Implementations of the present embodiment have at least one gyroscope and may also include an additional two gyroscopes, two accelerometers, and/or two more gyroscopes and three accelerometers, as non-limiting examples.

When the IMU 14 is de-coupled from the north finder 12 after receiving the north reference 20, it becomes available to provide the north reference to a destination platform. With reference to Fig. 3, the IMU 14 is coupled to a destination platform 36 at its interface assembly 38. In this embodiment, the interface assembly 24 of the IMU 14 couples to the interface assembly of the destination platform 36, but other embodiments may have an IMU that has multiple interfaces adding versatility for use with many types of destination platforms. As indicated above, all interfaces of an element are referred to globally as interface assemblies.

The interface assembly 38 of the destination platform 36 is fixed with respect to the reference frame of the destination platform 36 in the same way that the interface assembly 24 of the IMU 14 is fixed with respect to the reference frame of the IMU 14 and that the interface assembly 22 of the north finder 12 is fixed with respect to the reference frame of north finder 12. The interface assembly 38 of the destination platform 36 in this embodiment is designed similarly to the interface assembly 22 of the north finder 12 so that the firm connection of the interface assemblies 24, 38 enables the representation of the north reference 20 in the reference frame of the destination platform 36 to be the same as, or offset by a known constant, from the representation of the north reference in the reference frame of the IMU 14. Accordingly, the IMU 14 can send 40 signals representing the north reference 20 through the interface assemblies 24, 38 to provide the north reference 20 to the destination platform 36.

Figs. 4A - 4C illustrate physically the angular relationships between a north finder, an IMU, and a destination platform in accordance with embodiments of the invention. For simplicity, movement in one plane only is discussed. The north reference is depicted by 42, and the north finder, and IMU, and destination platform are depicted as shapes 44, 46, 48, respectively. The mechanical interfaces of the north finder 42 and destination platform 48 are labeled 50, 52, respectively, when the IMU 46 is not coupled thereto.

In Fig. 4A, the IMU 46 is coupled to the north finder 44, which determines the north reference 42. The north reference 42 is offset by an angle A from an arbitrary zero-degree reference 54 in the reference frame of the north finder 44.

In Fig. 4B, the IMU 46 is de-coupled from the north finder 48 and moved to the destination platform 48. As the IMU 46 moves and rotates, an angle B increases. As illustrated in Fig. 4B, angle B measures the angle between the reference 54 of the north finder 48 and an arbitrary IMU zero-degree reference 56, which was aligned with the zero-degree reference 54 of the north finder 48 when the IMU 46 was coupled thereto.

In Fig. 4C, the IMU 46 is coupled to the destination platform 48, which has its own arbitrary zero-degree reference 58. At this point, the angle B, which had changed when the IMU 46 was moved from the north finder 44 to the destination platform 48, is now labeled angle C. The north reference 42 is determined by subtracting angle C from angle A, and the difference is indicated as angle D.

In this simplified discussion, the zero-degree references 54, 58 of the north finder 48 and the destination platform 48, respectively, were aligned with the zero-degree reference 56 of the IMU 46 when the IMU was coupled to the respective element. In practical situations, where the precision of alignment needed is greater than can be machined, an offset between the zero-degree reference 56 of the IMU 46 and the zero-degree references 54, 58 of the north finder 44 and the destination platform 48, respectively, would need to be determined and applied as discussed above.

In addition to the embodiments discussed above, the invention may also be embodied as a method for transferring a north reference to a destination platform, and this method may be executed using the system 10 illustrated in Fig. 1. The individual steps of the method are discussed with reference to Fig. 5 as follows:

The first step of the method is to operate a north finder to find a north reference. (Step SI.) The example north finder 12 in Fig. 1 may be used. Alternatively, a north finder that is not an inertial north finder may be used.

The next step is to couple the north finder to an IMU. (Step S2.) The example IMU 14 in Fig. 1 may be used. The north finder and the IMU should have mating interface assemblies with a known physical transformation constant to enable a north reference represented in the reference frame of the IMU to be quickly determined from knowledge of the north reference in the reference frame of the north finder. Note that Steps S 1 and S2 can be performed in an order with either step executed first.

The next step is to send the north reference from the north finder to the IMU. (Step S3.) The north reference may be sent through interface assemblies in the manner described above with respect to the other embodiments of the invention.

At this point, the IMU maintains the north reference. (Step S4.) In this embodiment, the IMU maintains the north reference using circuity and sensors located within the IMU, but in alternate implementations some of the circuitry and/or sensors could be located elsewhere, such as on the outer casing of the IMU. The term “within” is used broadly here to cover circuity and sensors associated with IMU and operating to maintain the north reference. The next step is to de-couple the IMU from the north finder. (Step S5.) As discussed above, the IMU in this embodiment continues to maintain the north reference by virtue of its circuitry and sensors.

At this point, the IMU is coupled to the destination platform. (Step S6.) Such may be executed as discussed above with respect to the diagram in Fig. 3. In a manner analogous that of Step S2, the IMU and the destination platform should have mating interface assemblies with a known physical transformation constant to enable a north reference represented in the destination platform to be quickly determined from knowledge of the north reference in the reference frame of the IMU.

The final step of the method in this embodiment is to send the north reference from the IMU to the destination platform. (Step S7.) The north reference may be sent through interface assemblies in the manner described above with respect to the other embodiments of the invention.

The block diagram in Fig. 6 provides a logical representation of the invention embodied as an alternate system for transferring a north reference to a destination platform. As shown in Fig. 6, the system 60 includes a north finder 62 and an IMU 64.

The north finder 62 has a set of sensors 66 that operates with circuitry 68 to find the north reference 70 in a conventional manner or in a manner specifically-engineered to meet particular needs of an individual implementation. Non-limiting example sets of sensors in various implementations include three gyroscopes, one gyroscope and two accelerometers, and three gyroscopes and three accelerometers. The north finder 62 has its own reference frame and generates the north reference 70 in that reference frame.

The IMU 64 has a set of sensors 72 working with circuitry 74, for example, in the manner described above with respect to other embodiments. The IMU 64 uses the sensors 72 and circuitry 74 to generate orientation data to indicate the orientation of the IMU 64 as it moves. With reference also to Fig. 7, the IMU 64 can mechanically couple to both the north finder 62 and to a destination platform 76 (not simultaneously). The mechanical coupling of the components is facilitated by a mechanical interface 78 of the north finder 62, a mechanical interface 80 of the IMU 64, and a mechanical interface 82 of the destination platform 76. The mechanical interface 78 of the north finder 62 is stationary with respect to the reference frame of the north finder 62. The mechanical interface 82 of the destination platform 76 is stationary with respect to the reference frame of the destination platform 76. The orientation data of the IMU 64 is indicative of the orientation of the mechanical interface 80 of the IMU 64. The north finder 62 and the IMU 64 couple at their respective mechanical interfaces 78, 80, and the destination platform 76 and the IMU 64 couple at their respective mechanical interfaces 82, 80. Accordingly, the IMU 64 mechanically couples to the north finder 62 and to the destination platform 76 at known relative alignments.

The components in this embodiment also have communication interfaces, separate from the mechanical interfaces 78, 80 ,82, facilitating communication between the circuitry of the components. Specifically, the north finder 62 first and second communication interfaces 84, 86 connected to circuitry 68, the IMU 64 has a communication interface 88 connected to circuitry 74, and the destination platform 76 has a communication interface 90 connected to internal circuit elements that use the north reference.

A first communication link 92 connects the first communication interface 84 of the north finder 62 to the communication interface 88 of the IMU 64. Through this communication link 92, the orientation data of the IMU 64 is relayed from the IMU 64 to the north finder 62.

A second communication link 94 connects the second communication interface 86 of the north finder 62 to the communication interface 90 of the destination platform 76. Through this communication link 94, the north reference in the reference frame of the destination platform 76 is relayed from the north finder 62 to the destination platform 76. Using for example relationships and options discussed above, the north finder 62 in this embodiment is able to determine the north reference in the reference frame of the destination platform 76 by using the north reference 70 in the reference frame of the north finder 62, the orientation data when the IMU 64 is coupled to the north finder 62, and the orientation data when the IMU 64 is coupled to the destination platform 76. (In alternate embodiments, the north reference in the reference frame of the destination platform may be determined in the destination platform instead of in the north finder.)

In the present embodiment, the communication links 92, 94 are cables that enable wired communication. As non-limiting examples, The RS-485 or RS-422 standard protocol may be employed for data transmission, but any suitable digital or analog protocol may be used. In alternate embodiments, the communication links may be wireless, such as according to Bluetooth, such as the Wi-Fi, near-field communication (NFC) protocols, as non-limiting examples.

The invention may also be embodied as another type of method for transferring a north reference to a destination platform. This method may be executed using the system 60 illustrated in Figs. 6 and 7. The individual steps of the method are discussed with reference to Fig. 8 as follows:

The first step of the method is to operate a north finder to find a north reference in the reference frame of the north finder. (Step SI.) The example north finder 62 in Figs. 6 and 7 may be used.

After the north reference is found, the next step is to store the orientation of an IMU that is coupled to a mechanical interface of the north finder. (Step S2.) The IMU 64 in Figs. 6 and 7 may be used. If north finder 62 of Figs. 6 and 7 is used, the orientation may be stored in circuitry 68. If using both IMU 64 and north finder 62, the IMU orientation data could be transmitted through cable 92.

Then, the IMU is de-coupled from the mechanical interface of the north finder and coupled to a mechanical interface of a destination platform. (Step S3). The IMU senses its movement and produces data indicative of how its orientation may have changed.

The next step is to store the orientation of the IMU while it is still coupled to the mechanical interface of the destination platform. (Step S4.) Again, the orientation may be stored in the north finder’s circuitry. In alternate embodiments, the orientation may be stored in the destination platform or in the IMU itself.

The following step is to determine the difference between the orientation of the IMU when coupled to the destination platform and the orientation of the IMU when coupled to the north finder. (Step S5.) Depending on the particular implementation, the data indicating the orientations at the two locations may be retrieved from storage and a simple subtraction may be performed. In this embodiment, the computation is performed in the north finder, but in alternate embodiments the computation may be performed in the destination platform or in the IMU.

Then, the north reference in the reference frame of the destination platform is determined from the difference in the orientations determined in Step S5 above and the north reference in the reference frame of the north finder. (Step S6.) In this embodiment, the computation is performed in the north finder, but in alternate embodiments the computation may be performed in the destination platform or in the IMU.

The last step of the present method is to transmit the north reference in the reference frame of the destination platform to the destination platform. (Step S7.) If using the system 60 of Fig. 6 and 7, the north reference would be transmitted through cable 94.

In applications where a north reference is sufficiently light in weight, with respect to the strength of the mounting apparatus of the destination platform, it may not be necessary to decouple the IMU from the north finder before coupling it to the destination platform. Thus, in some embodiments, the north finder and the IMU may be manufactured as a single unit. A firm mechanical interface is still needed for coupling to the destination platform.

Accordingly, the invention may also be embodied as a method for transferring a north reference to a destination platform such that the method does not require decoupling the IMU from the north finder before coupling it to the destination platform. Decoupling the IMU may be desired though in applications in which the north finder is not sufficiently light with respect to the strength of the mounting apparatus of the destination platform. The method applies regardless of whether the north finder and the IMU are manufactured as a single unit or separately. The individual steps of the method are discussed with reference to Fig. 9 as follows:

The first step of the method is to operate a north finder to find a north reference in the reference frame of the north finder. (Step SI.) The example north finder 62 in Figs. 6 and 7 may be used, or alternatively a north finder with a built-in IMU may be used.

After the north reference is found, the next step is to store a first orientation, which is the orientation of an IMU that is rigidly coupled to the north finder. (Step S2.) The IMU 64 in Figs. 6 and 7 may be used. If north finder 62 of Figs. 6 and 7 is used, the orientation may be stored in circuitry 68. If using both IMU 64 and north finder 62, the IMU orientation data could be transmitted through cable 92. Alternately, the north finder and the IMU may be manufactured as a single unit and have internal circuity for transferring the IMU orientation data.

Then, the IMU is rigidly coupled to a destination platform. (Step S3). The IMU senses its movement and produces data indicative of how its orientation changes.

The next step is to store a second orientation, which is the orientation of the IMU while it is still coupled to the destination platform. (Step S4.) Again, the orientation may be stored in the north finder’s circuitry. In alternate embodiments, the orientation could be stored in the IMU’s circuitry, if distinct from the north finder’s circuitry, or in the destination platform.

The following step is to determine the difference between the first and second stored orientations of the inertial measurement unit (Step S5), which is the difference between the orientation of the IMU when coupled to the destination platform and the orientation of the IMU when the north finder was found the north reference.

Then, the north reference in the reference frame of the destination platform is determined from the north reference in the reference frame of the north finder and the difference between the first and second stored orientations of the IMU. (Step S6.) The computation may be performed in the north finder, the destination platform, or, if distinct from the north finder, in the IMU.

The last step of the present method is to provide the north reference in the reference frame of the destination platform to the destination platform. (Step S7.) If using the system 60 of Fig. 6 and 7, the north reference would be transmitted through cable 94. If the computation of Step S6 is performed in the destination platform, the north reference would be forwarded internally where desired within the north reference’s circuitry.

Having thus described exemplary embodiments of the invention, it will be apparent that various alterations, modifications, and improvements will readily occur to those skilled in the art. Alternations, modifications, and improvements of the disclosed invention, though not expressly described above, are nonetheless intended and implied to be within spirit and scope of the invention. For example, although the north finder of embodiments described above are inertial north finders, other embodiments of the invention may include or use north finders that are not inertial north finders. Also, interface assemblies do not need to be built to have large planar surfaces contacting each other, as long as some means are available to maintain a known relationship between the reference frames of both components involved in transferring the north reference, IMU orientation data, or related information.

Accordingly, the foregoing discussion is intended to be illustrative only; the invention is limited and defined only by the following claims and equivalents thereto.

Claims

CLAIMS We claim:

1. A method for transferring a north reference to a destination platform, the method comprising: operating a north finder to find a north reference in the reference frame of the north finder; after the north reference is found, storing the orientation of an inertial measurement unit that is coupled to a mechanical interface of the north finder; de-coupling the inertial measurement unit from the mechanical interface of the north finder and coupling the inertial measurement unit to a mechanical interface of a destination platform; storing the orientation of the inertial measurement unit while still coupled to the mechanical interface of the destination platform; determining the difference between the orientation of the inertial measurement unit when coupled to the destination platform and the orientation of the inertial measurement unit when coupled to the north finder; determining the north reference in the reference frame of the destination platform from the difference in the orientations and the north reference in the reference frame of the north finder; and transmitting the north reference in the reference frame of the destination platform to the destination platform.

2. The method of claim 1, wherein the north finder is an inertial north finder.

3. The method of either claim 1 or claim 2, wherein the inertial measurement unit includes at least one gyroscope.

4. The method of either claim 1 or claim 2, wherein the inertial measurement unit includes at least one accelerometer.

5. The method of either claim 1 or claim 2, wherein the inertial measurement unit includes at least three gyroscopes and at least three accelerometers.

6. The method of any of claims 1-5, wherein the destination platform is one of a telescope, a thermal observation device, and a missile launcher.

7. A system for transferring a north reference to a destination platform, the system comprising: a north finder operative to generate a north reference in a reference frame of the north finder; an inertial measurement unit operative to mechanically couple alternately to the north finder and to a destination platform, each at known relative alignments, and the inertial measurement unit further operative to generate orientation data indicating its orientation as it moves; a first communication link operative to relay the orientation data from the inertial measurement unit to the north finder; and a second communication link operative to relay the north reference in a reference frame of the destination platform from the north finder to the destination platform; wherein the north finder determines the north reference in the reference frame of the destination platform using the north reference in a reference frame of the north finder, the orientation data when the inertial measurement unit is coupled to the north finder, and the orientation data when the inertial measurement unit is coupled to the destination platform.

8. A system of claim 7, wherein the north finder, the inertial measurement unit, and the destination platform each has a mechanical interface such that: the mechanical interface of the north finder is stationary with respect to the reference frame of the north finder, the mechanical interface of the destination platform is stationary with respect to the reference frame of the destination platform, and the orientation data of the inertial measurement unit is indicative of the orientation of the mechanical interface of the inertial measurement unit; and wherein: the north finder and the inertial measurement unit couple at their respective mechanical interfaces, and the destination platform and the inertial measurement unit couple at their respective mechanical interfaces.

9. The system of either claim 7 or claim 8, wherein the north finder is an inertial north finder.

10. The system of any of claims 7-9, wherein the inertial measurement unit includes at least one gyroscope.

11. The system of any of claims 7-9, wherein the inertial measurement unit includes at least one accelerometer.

12. The system of any of claims 7-9, wherein the inertial measurement unit includes at least three gyroscopes and at least three accelerometers.

13. The system of any of claims 7-12, wherein the destination platform is one of a telescope, a thermal observation device, and a missile launcher.

14. A method for transferring a north reference to a destination platform, the method comprising: operating a north finder to find a north reference; coupling the north finder to an inertial measurement unit; sending the north reference from the north finder to the inertial measurement unit; maintaining the north reference within the inertial measurement unit; de-coupling the inertial measurement unit from the north finder; coupling the inertial measurement unit to a destination platform; and sending the north reference from the inertial measurement unit to the destination platform.

15. The method of claim 14, wherein the north finder is an inertial north finder.

16. The method of either claim 14 or claim 15, wherein the north reference is maintained within the inertial measurement unit by using at least one gyroscope.

17. The method of either claim 14 or claim 15, wherein the north reference is maintained within the inertial measurement unit by using at least one accelerometer.

18. The method of either claim 14 or claim 15, wherein the north reference is maintained within the inertial measurement unit by using at least three gyroscopes and at least three accelerometers.

19. The method of any of claims 14-18, wherein the destination platform is one of a telescope, a thermal observation device, and a missile launcher.

20. An inertial measurement unit for transferring a north reference from a north finder to a destination platform, the inertial measurement unit comprising: an interface assembly operative to couple to a north finder to receive therefrom a north reference; and circuitry operative to maintain the north reference after de-coupling from the north finder; wherein the interface assembly is further operative to couple to a destination platform and to provide the north reference thereto.

21. The inertial measurement unit of claim 20, wherein the north finder is an inertial north finder.

22. The inertial measurement unit of either claim 20 or claim 21, wherein the circuitry includes at least one gyroscope.

23. The inertial measurement unit of either claim 20 or claim 21, wherein the circuitry includes at least one accelerometer.

24. The inertial measurement unit of either claim 20 or claim 21, wherein the circuitry includes at least three gyroscopes and at least three accelerometers.

25. The method of any of claims 20-24, wherein the destination platform is one of a telescope, a thermal observation device, and a missile launcher.

26. A system for transferring a north reference to a destination platform, the system comprising: a north finder having an interface assembly and operative to generate a north reference and to send it through the interface assembly; and an inertial measurement unit having an interface assembly operative to couple to the interface assembly of the north finder and to receive through the coupled interface assemblies the north reference, the inertial measurement unit operative to maintain the north reference after de-coupling from the north finder; wherein the interface assembly of the inertial measurement unit is further operative to couple to a destination platform and to provide the north reference thereto.

27. The system of claim 26, wherein the north finder is an inertial north finder.

28. The system of either claim 26 or claim 27, wherein the inertial measurement unit includes at least one gyroscope.

29. The system of either claim 26 or claim 27, wherein the inertial measurement unit includes at least one accelerometer.

30. The system of either claim 26 or claim 27, wherein the inertial measurement unit includes at least three gyroscopes and at least three accelerometers.

31. The system of any of claims 26-30, wherein the destination platform is one of a telescope, a thermal observation device, and a missile launcher.

32. A method for transferring a north reference to a destination platform, the method comprising: operating a north finder to find a north reference in the reference frame of the north finder; after the north reference is found, storing a first orientation, which is the orientation of an inertial measurement unit that is rigidly coupled to the north finder; rigidly coupling the inertial measurement unit to a destination platform; storing a second orientation, which is the orientation of the inertial measurement unit while coupled to the destination platform; determining the difference between the first and second orientations of the inertial measurement unit; determining the north reference in the reference frame of the destination platform from the north reference in the reference frame of the north finder and the difference between the first and second orientations; and providing the north reference in the reference frame of the destination platform to the destination platform.