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A method for determining heave and heave rate for a vessel is described. The vessel includes an inertial navigation system (INS), and sensors for the INS being located at a point A of the vessel, the vessel having a zero heave reference point B, and the described method provides heave and heave rate

A method for determining heave and heave rate for a vessel is described. The vessel includes an inertial navigation system (INS), and sensors for the INS being located at a point A of the vessel, the vessel having a zero heave reference point B, and the described method provides heave and heave rate at a point C of the vessel. The method includes determining reference coordinates for points A, B, and C of the vessel, generating a velocity signal representative of a velocity at a point on the vessel, generating a heave rate signal based upon the velocity signal, and generating a heave based upon the heave rate signal.

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1. A method for determining heave and heave rate at a point C on a vessel, the vessel having an inertial navigation system (INS), sensors for the INS being located at a point A on the vessel, the vessel having a zero heave reference point B, said method comprising:determining body frame coordinates

1. A method for determining heave and heave rate at a point C on a vessel, the vessel having an inertial navigation system (INS), sensors for the INS being located at a point A on the vessel, the vessel having a zero heave reference point B, said method comprising:determining body frame coordinates for points A, B, and C;defining a transformation direction cosine matrix C B L , which redefines a vector expressed in body frame coordinates into a vector expressed in local level coordinates;determining local level coordinates for the vessel at points A, B, and C;determining a lever arm from point A to point C, R AC ;calculating a velocity signal representative of a velocity at point C on the vessel, using a known velocity at point A, based on the body frame coordinates and INS sensor measurements for point A, the transformation direction cosine matrix C B L , and the lever arm according tois the local level velocity at point A, V C L is the local level velocity at point C, and ω B is the body frame angular rate of the vessel;generating a heave rate signal based on the velocity signal; andgenerating a heave signal based upon the heave rate signal. 2. A method according to claim 1 wherein generating a heave rate signal based on the velocity signal comprises filtering a third element of V C L , V C L (3), with a high pass filter to determine a heave rate {circumflex over (V)} C L (3). 3. A method according to claim 2 wherein generating a heave signal based upon the heave rate signal comprises calculating a heave according torepresents heave rate integration, and C B L (3) t0 , is an initial condition on the integration, the initial condition being a local level vertical distance between point B and point C, which was valid at time t 0 , the starting time of the integration. 4. A method according to claim 1, wherein generating a heave rate signal based on the velocity signal comprises determining heave rate of the vessel according tois a filtered third element of local level velocity at point A, V C L (3) is the third element of local level velocity at point C, ω B is the body frame angular rate of the vessel, and the subscript t k indicates that the term is updated every iteration. 5. A method according to claim 4 wherein generating a heave signal based upon the heave rate signal comprises determining a heave according towhere the subscript t k indicates that the term is updated at every iteration. 6. A method according to claim 1 wherein generating heave rate signal based on the velocity signal comprises filtering the velocity signal according to the transfer function [(1+z −1 )(1+z −2 )/4]×[4/(33-48z −1 +19z −2 )]=[1−((1+z −4 )(1+z −8 )(1+z −16 )/(15,500-30,750z −16 +15,254z −32 )(125-123z −16 ))]. 7. A method according to claim 1 wherein generating a heave signal comprises filtering the heave rate signal according to the transfer function (1/200)/(1−z −1 ). 8. A method according to claim 7 further comprising filtering the heave according to 1−[((1+z −4 )(1+z −8 )(1+z −16 )/(15,500-30,750z −16 +15,254z −32 ) (125-123z −16 )]. 9. An inertial navigation system (INS) for determining a heave and a heave rate for a vessel, said system comprising:a main unit;a user interface unit;a global positioning satellite (GPS) receiver; anda sensor unit comprising an inertial sensor assembly (ISA), said ISA located at a point A of the vessel, the vessel having a zero heave reference point B, said main unit comprising interfaces for communication of inertial navigation information to other systems on the vessel, said user interface unit comprising a display and keypad, said system configured with body frame coordinates for points A, B, and C, point C being a location at which it is desired to be provided heave and heave rate data, said system con figured to define a transformation direction cosine matrix C B L , which redefines a vector expressed in body frame coordinates into a vector expressed in local level coordinates, determine a lever arm from point A to point C, R AC , said system configured to determine local level coordinates for the vessel at points A, B, and C and determine a velocity of the vessel at point C, using a known velocity at point A, based on the body frame coordinates and data received from said ISA, the transformation direction cosine matrix, and the lever arm, velocity being determined according to according tois the local level velocity at point A, V C L is the local level velocity at point C, and ω B is the body frame angular rate of the vessel, filter the velocity signal to provide a heave rate signal, and integrate the heave rate signal to determine a heave. 10. An INS system according to claim 9 wherein to filter the velocity signal to provide a heave rate signal, said system is configured to filter a third element of V C L , V C L (3), with a high pass filter to determine a heave rate signal, {circumflex over (V)} C L (3). 11. An INS system according to claim 10, wherein to integrate the heave rate signal to determine a heave, said system is configured to calculate a heave according torepresents heave rate integration, and C B L R BC (3) t0 , is an initial condition on the integration, the initial condition being a local level vertical distance between point B and point C, which was valid at time t 0 , the starting time of the integration. 12. An INS system according to claim 9 wherein said system is provided a velocity of the vessel at point A, and wherein to filter the velocity signal to provide a heave rate signal, said system is configured to determine heave rate of the vessel according tois a filtered third element of local level velocity at point A, V C L (3) is the third element of local level velocity at point C, ω B is the body frame angular rate of the vessel, and the subscript t k indicates that the term is updated every iteration. 13. An INS system according to claim 12 wherein to integrate the heave rate signal to determine a heave, said system is configured to determine a heave according towhere the subscript t 0 indicates that the term is an initial condition and is only calculated once at the start of integration. 14. A filter comprising:a first stage configured to provide a heave rate signal based upon a velocity signal input according to the transfer function [(1+z −1 )(1+z −2 )/4]×[4/(33-48z −1 +19z −2 )]=[1−((1+z −4 )(1+z −8 )(1+z −16 )/(15,500-30,750z −16 +15,254z −32 )(125-123z −16 ))]; anda second stage configured to provide a heave signal by filtering the heave rate signal output by said first stage according to the transfer function (1/200)/(1−z −1 ). 15. A filter according to claim 14 wherein said second stage is configured to filter the heave signal according to according to the transfer function 1−[(1+z −4 )(1+z −8 )(1+z −16 )/(15,500-30,750z −16 +15,254z −32 )(125-123z −16 )], providing a filtered heave distance signal.