Analysis of dynamic performances of hasi temperature sensor during the entry in the Titan atmosphere

The performances of the temperature sensors of the HASI instrument (Fulchignoni et al., 1996) have been analyzed using a numerical model, validated by comparison with the results of various experimental tests. ea of implementing a numerical model of the sensors arose when the tests for the dynamic characterization of the thermometer (Angrilli et al., 1996a) showed that the behavior was quite different from that of a first order system. ng the sensor only by means of the time constant of the sensing wire does not account for the effect of the mechanical structures thermal inertia. A more accurate dynamic characterization of the sensor requires three parameters, that depend on the thermo-fluid environment. Actually the sensor during the descent in Titan atmosphere changes its behavior dramatically according to the changes in velocity and atmospheric conditions. The most relevant effect is the change of the frequency bandwidth of the sensor, leading to a variation of the actual spatial resolution in the measured temperature profile. Using the numerical model of the sensors together with the profile of temperature, pressure and composition given in literature (Lellouch et al., 1989) for the Titan atmosphere, the dynamic characteristics of the sensor at various altitude have been determined. oice of using numerical simulation instead of experimental tests on the unit depends on the consideration that to accomplish similitude, both Re and Nu numbers must be tuned. A complete experimental characterization of the sensor would require a series of tests in a wind tunnel with various fluid velocities and thermal conductivity. st relevant results are related to the large difference between the frequency bandwidth of the sensors at the beginning of the measurements, where the speed is high, but the density is low with respect to those at the end. the changes in density, the response of the sensor is slow when the velocity of the probe is high, at the beginning of the measurements, in this case the sensor could detect only very large atmospheric structures. The maximum in thermometer response speed is obtained during the last part of the descent, when the velocity is low but the foreseen density is high, therefore in this phase the sensor will be able to detect structures of small dimension. ng from the results of these dynamic performances a new sensor has been devised, the dynamic performances seem to be promising, showing an improvement in the frequency bandwidth of one decade, and though its realization is only at the beginning, it can be considered as a candidate in the planning of future missions.