A computer-implemented method of estimating a heeling angle of a marine body from a roll angle measurement, by decomposing a roll angle signal into components in frequency spectrum in real-time n, the method comprising the following steps: (a) receiving a first signal representative of the composite roll angle signal, the first signal comprising at least of three frequency components H, W and N, two of which ( H and W) are adjacent in the frequency domain, the H component representing a heel angle arising from the marine body load, the W component representing motion of the marine body resulting from wave action, and the N component representing a signal noise component; (b) transmitting the first composite roll angle signal to a high pass filter with a cut-off frequency designed to separate the lower frequency component, H, from the other components (W and N), the high pass filter producing a first intermediate signal (S 1) representative of a phase-delayed and amplitude-distorted W component (W̃) and the N component, simultaneously using a reference impulse signal to find a second intermediate signal (S 2) representative of the phase-delay and amplitude-distortion characteristics of the same high pass filter; (c) transmitting the first composite roll angle signal to a reconstructing processor, the reconstructing processor detecting and reconstructing in time domain the W component that has been distorted by the application of the high pass filter in the relevant time window, to form a third intermediate signal (S 3); (d) transmitting the second intermediate signal (S 2) and the third intermediate signal (S 3) to an adjustment processor, the adjustment processor computing from these the second intermediate signal (S 2) and the third intermediate signal (S 3), a fourth intermediate signal (S 4) representative of an estimated undistorted W component in the recent past, and computing a fifth intermediate signal (S 5) representative of a predicted W component at current time instance; (e) transmitting the first composite roll angle signal, the first intermediate signal (S 1), the fourth intermediate signal ( S 4), and the fifth intermediate signal (S 5) to an estimating module, the estimating module adding the fourth intermediate signal (S 4) to the first composite roll angle signal, and then subtracting each of the first intermediate signal (S 1) and the fifth intermediate signal (S 5), to produce a first estimation of the H frequency component (Ḣ); (f) refining the first estimation of the H frequency component (Ḣ), obtained from steps (a) to (e), by taking into account the direction and magnitude of change from the past time steps to produce the final estimate of H, H ⃛; and (g) repeating steps (a) through (f) at a pre-determined time step frequency to obtain a stepwise series of values for the first estimation of the H frequency component H ⃛ to thereby provide an estimate of the heeling angle of the marine body. (a) receiving a first signal representative of the composite roll angle signal, the first signal comprising at least of three frequency components H, W and N, two of which ( H and W) are adjacent in the frequency domain, the H component representing a heel angle arising from the marine body load, the W component representing motion of the marine body resulting from wave action, and the N component representing a signal noise component; (b) transmitting the first composite roll angle signal to a high pass filter with a cut-off frequency designed to separate the lower frequency component, H, from the other components (W and N), the high pass filter producing a first intermediate signal (S 1) representative of a phase-delayed and amplitude-distorted W component (W̃) and the N component, simultaneously using a reference impulse signal to find a second intermediate signal (S 2) representative of the phase-delay and amplitude-distortion characteristics of the same high pass filter; (c) transmitting the first composite roll angle signal to a reconstructing processor, the reconstructing processor detecting and reconstructing in time domain the W component that has been distorted by the application of the high pass filter in the relevant time window, to form a third intermediate signal (S 3); (d) transmitting the second intermediate signal (S 2) and the third intermediate signal (S 3) to an adjustment processor, the adjustment processor computing from these the second intermediate signal (S 2) and the third intermediate signal (S 3), a fourth intermediate signal (S 4) representative of an estimated undistorted W component in the recent past, and computing a fifth intermediate signal (S 5) representative of a predicted W component at current time instance; (e) transmitting the first composite roll angle signal, the first intermediate signal (S 1), the fourth intermediate signal ( S 4), and the fifth intermediate signal (S 5) to an estimating module, the estimating module adding the fourth intermediate signal (S 4) to the first composite roll angle signal, and then subtracting each of the first intermediate signal (S 1) and the fifth intermediate signal (S 5), to produce a first estimation of the H frequency component (Ḣ); (f) refining the first estimation of the H frequency component (Ḣ), obtained from steps (a) to (e), by taking into account the direction and magnitude of change from the past time steps to produce the final estimate of H, H ⃛; and (g) repeating steps (a) through (f) at a pre-determined time step frequency to obtain a stepwise series of values for the first estimation of the H frequency component H ⃛ to thereby provide an estimate of the heeling angle of the marine body.