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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 _at(self, t):
 """Compute this satellite's GCRS position and velocity at time `t`."""

        rITRF, vITRF, error = self.ITRF_position_velocity_error(t)
        rGCRS, vGCRS = ITRF_to_GCRS2(t, rITRF, vITRF)
        return rGCRS, vGCRS, rGCRS, error