I. ABSTRACTFor the determination of the solar absorptance αs and the thermal emittance ε optical methods are preferred. These optical measuring procedures are relatively exact. They also deliver information about any potential spectral selective behaviour of surfaces. However, special measuring equipment is required. Particularly if space simulation facilities are already available, the method presented in  appears as a suitable enrichment of the measurement methods for the determination of the thermooptical properties αs and ε. It is possible with slight technical expense to expand the usage potential of such facilities.For this paper the same test rig as in  is used. In a common space simulation chamber (vacuum; cryogenic shroud; solar simulator) a test item (target) is arranged. In different measuring phases the target is heated up by an integrated electrical heater or by solar irradiation. The arrangement (target / shroud) is regarded as a two node model. Its (integrated) nodal equation describes the behaviour T(t) of the target. A proper Kalman filter-algorithm can estimate the parameters for the heat transfer between target and cryogenic shroud from the transient measured target temperature. Some Improvements compared with  are discussed.The method used for parameter estimation by a Kalman filter is also the first application of a real measured nodal model and is so far the first verification step for a complex correction procedure of incorrect mathematical models of measured nodal temperatures.