  The integration of Non-Terrestrial Networks (NTNs) with Low Earth Orbit (LEO)
satellite constellations into 5G and Beyond is essential to achieve truly
global connectivity. A distinctive characteristic of LEO mega-constellations is
that they constitute a global infrastructure with predictable dynamics, which
enables the pre-planned allocation of the radio resources. However, the
different bands that can be used for ground-to-satellite communication are
affected differently by atmospheric conditions such as precipitation, which
introduces uncertainty on the attenuation of the communication links at high
frequencies. Based on this, we present a compelling case for applying
integrated sensing and communications (ISAC) in heterogeneous and multi-layer
LEO satellite constellations over wide areas. Specifically, we present an ISAC
framework and frame structure to accurately estimate the attenuation in the
communication links due to precipitation, with the aim of finding the optimal
serving satellites and resource allocation for downlink communication with
users on ground. The results show that, by dedicating an adequate amount of
resources for sensing and solving the association and resource allocation
problems jointly, it is feasible to increase the average throughput by 59% and
the fairness by 600% when compared to solving these problems separately.
