Its a trailing cone generally used in testbed aircraft to calibrate the static pressure error (which reports the altitude) of the pitot-static system. It trails the aircraft in the free stream which gives the most accurate pressure. As the sensors that will be used on real life aircraft are on the skin of the aicraft there is some error in the readings due to the airflow over the aircraft surface resulting in different values given to the true ones. The trailing cone provides the real values, and the difference between the sensors and the trailing cone gives the correction factor.
Credit for this answer to delta_xray on the "Aviation & Aircraft" forum on boards.ie