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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _find_navigation_coefs(self):
 """Find navigation coefficients for the current time The navigation Chebyshev coefficients are only valid for a certain time interval. The header entry SatelliteStatus/Orbit/OrbitPolynomial contains multiple coefficients for multiple time intervals. Find the coefficients which are valid for the nominal timestamp of the scan. Returns: Corresponding index in the coefficient list. """

        # Find index of interval enclosing the nominal timestamp of the scan
        time = np.datetime64(self.prologue['ImageAcquisition']['PlannedAcquisitionTime']['TrueRepeatCycleStart'])
        intervals_tstart = self.prologue['SatelliteStatus']['Orbit']['OrbitPolynomial']['StartTime'][0].astype(
            'datetime64[us]')
        intervals_tend = self.prologue['SatelliteStatus']['Orbit']['OrbitPolynomial']['EndTime'][0].astype(
            'datetime64[us]')
        try:
            return np.where(np.logical_and(time >= intervals_tstart, time < intervals_tend))[0][0]
        except IndexError:
            raise NoValidNavigationCoefs('Unable to find navigation coefficients valid for {}'.format(time))