  This paper investigates the potential of non-terrestrial and terrestrial
signals of opportunity (SOOP) for navigation applications. Non-terrestrial SOOP
analysis employs modified Cram\`er-Rao lower bound (MCRLB) to establish a
relationship between SOOP characteristics and the accuracy of ranging
information. This approach evaluates hybrid navigation module performance
without direct signal simulation. The MCRLB is computed for ranging accuracy,
considering factors like propagation delay, frequency offset, phase offset, and
angle-of-arrival (AOA), across diverse non-terrestrial SOOP candidates.
Additionally, Geometric Dilution of Precision (GDOP) and low earth orbit (LEO)
SOOP availability are assessed. Validation involves comparing MCRLB predictions
with actual ranging measurements obtained in a realistic simulated scenario.
Furthermore, a qualitative evaluation examines terrestrial SOOP, considering
signal availability, accuracy attainability, and infrastructure demands.
