  The balloon-borne very long baseline interferometry (VLBI) experiment is a
technical feasibility study for performing radio interferometry in the
stratosphere. The flight model has been developed. A balloon-borne VLBI station
will be launched to establish interferometric fringes with ground-based VLBI
stations distributed over the Japanese islands at an observing frequency of
approximately 20 GHz as the first step. This paper describes the system design
and development of a series of observing instruments and bus systems. In
addition to the advantages of avoiding the atmospheric effects of absorption
and fluctuation in high frequency radio observation, the mobility of a station
can improve the sampling coverage (`uv-coverage') by increasing the number of
baselines by the number of ground-based counterparts for each observation day.
This benefit cannot be obtained with conventional arrays that solely comprise
ground-based stations. The balloon-borne VLBI can contribute to a future
progress of research fields such as black holes by direct imaging.
