A method of determining low frequency or static loads on a system, the system comprising a test-object or structure operating in a first mode of operation, the method comprising: measuring indicators representing results of first loads exerted on at least one element of the test-object or structure during a first time interval when the system or structure is operating in the first mode of operation, wherein the first mode of operation is a natural operation condition, and deriving by measurement a relation between the first loads and the indicators valid for the first mode of operation in a controlled way during a second time interval, characterized in that at least the part of the test-object, which is outside the interfaces where the loads are to be identified, is removed, wherein during the second time interval the.test-object or structure is operated in a second mode of operation in which the test object or structure is conditioned to allow measurement of responses to applied known, or measurable, second loads, the environment of the test-object or structure and the way the test-object or structure is supported, are changed compared to the first mode of operation, the relation being determined by applying artificial second loads as dynamic excitations and measuring responses thereto at frequencies which are higher than the frequencies of the first loads applied in the first mode of operation, wherein the second mode of operation is a calibration condition, extrapolating, or completing down in frequency the relation to cover the frequency range relevant for the first mode of operation, the extrapolation being made either by assuming a frequency independent relation below the measured frequency range or based on a modal model of the test-object or structure; and the relation_being applied in an inverse analysis using the measured indicators in order to calculate the first loads at other frequencies in the time domain or in the frequency domain. measuring indicators representing results of first loads exerted on at least one element of the test-object or structure during a first time interval when the system or structure is operating in the first mode of operation, wherein the first mode of operation is a natural operation condition, and deriving by measurement a relation between the first loads and the indicators valid for the first mode of operation in a controlled way during a second time interval, characterized in that at least the part of the test-object, which is outside the interfaces where the loads are to be identified, is removed, wherein during the second time interval the.test-object or structure is operated in a second mode of operation in which the test object or structure is conditioned to allow measurement of responses to applied known, or measurable, second loads, the environment of the test-object or structure and the way the test-object or structure is supported, are changed compared to the first mode of operation, the relation being determined by applying artificial second loads as dynamic excitations and measuring responses thereto at frequencies which are higher than the frequencies of the first loads applied in the first mode of operation, wherein the second mode of operation is a calibration condition, extrapolating, or completing down in frequency the relation to cover the frequency range relevant for the first mode of operation, the extrapolation being made either by assuming a frequency independent relation below the measured frequency range or based on a modal model of the test-object or structure; and the relation_being applied in an inverse analysis using the measured indicators in order to calculate the first loads at other frequencies in the time domain or in the frequency domain.