  During certain times of the year at middle and low latitudes, winds in the
upper stratosphere move in nearly the opposite direction than the wind in the
lower stratosphere. Here we present a method for maintaining a high-altitude
balloon platform in near station-keeping mode that utilizes this stratospheric
wind shear. The proposed method places a balloon-borne science platform high in
the stratosphere connected by a lightweight, high-strength tether to a "tug"
vehicle located in the lower or middle stratosphere. Using aerodynamic control
surfaces, wind-induced aerodynamic forces on the tug can be manipulated to
counter the wind drag acting on the higher altitude science vehicle, thus
controlling the upper vehicle's geographic location. We describe the general
framework of this station-keeping method, some important properties required
for the upper stratospheric science payload and lower tug platforms, and
compare this station-keeping approach with the capabilities of a high altitude
airship and conventional tethered aerostat approaches. We conclude by
discussing the advantages of such a platform for a variety of missions with
emphasis on astrophysical research.
