  Age of Information (AoI), namely the time that has elapsed since the most
recently delivered packet was generated, is receiving increasing attention with
the emergence of many real-time applications that rely on the exchange of
time-sensitive information. AoI captures the freshness of the information from
the perspective of the destination. The term "accuracy of information" is used
to assess how close the estimate at the destination is to the parameter value
measured by the sensor. In this paper, the mean square error (MSE) is used to
evaluate the accuracy of information. We focus on a single sensor that monitors
a time-sensitive physical process, which is modelled as a random walk. Whenever
the state of the random walk changes by more than a specified threshold, the
sensor generates a status update packet and transmits it to the destination.
When no update packet is received, the destination assumes that the state of
the process has not changed. We study the problem of finding the minimum update
rate under AoI and accuracy of information constraints. More specifically, we
derive analytical expressions for the update rate, the AoI, and the MSE.
