  In this work we study the problem of collecting protected data in ad-hoc
sensor network using a mobile entity called MULE. The objective is to increase
information survivability in the network. Sensors from all over the network,
route their sensing data through a data gathering tree, towards a particular
node, called the $sink$. In case of a failed sensor, all the aggregated data
from the sensor and from its children is lost. In order to retrieve the lost
data, the MULE is required to travel among all the children of the failed
sensor and to re-collect the data. There is a cost to travel between two points
in the plane. We aim to minimize the MULE traveling cost, given that any sensor
can fail. In order to reduce the traveling cost, it is necessary to find the
optimal data gathering tree and the MULE location. We are considering the
problem for the unit disk graphs (UDG) and Euclidean distance cost function. We
propose a primal-dual algorithm that produces a
$\left(20+\varepsilon\right)$-approximate solution for the problem, where
$\varepsilon\rightarrow0$ as the sensor network spreads over a larger area. The
algorithm requires $O\left(n^{3}\cdot\varDelta\left(G\right)\right)$ time to
construct a gathering tree and to place the MULE, where
$\varDelta\left(G\right)$ is the maximum degree in the graph and $n$ is the
number of nodes.
