  This paper proposes a novel online data-driven adaptive control for unknown
linear time-varying systems. Initialized with an empirical feedback gain, the
algorithm periodically updates this gain based on the data collected over a
short time window before each update. Meanwhile, the stability of the
closed-loop system is analyzed in detail, which shows that under some mild
assumptions, the proposed online data-driven adaptive control scheme can
guarantee practical global exponential stability. Finally, the proposed
algorithm is demonstrated by numerical simulations and its performance is
compared with other control algorithms for unknown linear time-varying systems.
