  Low Earth Orbit (LEO) satellite constellations are bringing the Internet of
Things (IoT) to the space arena, also known as non-terrestrial networks.
Several IoT satellite applications for tracking ships and cargo can be seen as
exemplary cases of intermittent transmission of updates whose main performance
parameter is the information freshness. This paper analyzes the Age of
Information (AoI) of a satellite network with multiple sources and destinations
that are very distant and therefore require several consecutive multi-hop
transmissions. A packet erasure channel and different queueing policies are
considered. We provide closed-form bounds and tight approximations of the
average AoI, as well as an upper bound of the Peak Age of Information (PAoI)
distribution as a worst-case metric for the system design. The performance
evaluation reveals complex trade-offs among age, load, and packet losses. The
optimal operational point is found when the combination of arrival rates and
packet losses is such that the system load can ensure fresh information at the
receiver; nevertheless, achieving this is highly dependent on the mesh
topology. Moreover, the potential of an age-aware scheduling strategy is
investigated and the fairness among users discussed. The results show the need
to identify the bottleneck nodes for the age, as improving the rate and
reliability of those critical links will highly impact on the overall
performance. The model is general enough to represent other multi-hop mesh
networks.
