  Let $P$ be a set of $n$ points in the plane where each point $p$ of $P$ is
associated with a radius $r_p>0$.The transmission graph $G=(P,E)$ of $P$ is
defined as the directed graph such that $E$ contains an edge from $p$ to $q$ if
and only if $|pq|\leq r_p$ for any two points $p$ and $q$ in $P$, where $|pq|$
denotes the Euclidean distance between $p$ and $q$. In this paper, we present a
data structure of size $O(n^{5/3})$ such that for any two points in $P$, we can
check in $O(n^{2/3})$ time if there is a path in $G$ between the two points.
This is the first data structure for answering reachability queries whose
performance depends only on $n$ but not on the number of edges.
