  We investigated the detection rates and early warning parameters of binary
neutron star (BNS) populations with decihertz gravitational-wave observatories
in a realistic detecting strategy. Assuming 4 years' operation of B-DECIGO, we
based on parameter precision to classify the detectable BNSs into three
categories: (a) sources that merge within 1 year, which could be localized with
an uncertainty of $\Delta\Omega \sim 10^{0}$ deg$^2$; (b) sources that merge in
1-4 years, which take up three quarters of the total events and yield the most
precise angular resolution with $\Delta \Omega\sim 10^{-2}$ deg$^2$ and
time-of-merger accuracy with $\Delta t_c\sim 10^{-1}$ s; and (c) sources that
do not merge during the 4-yr mission window, which enable possible early
warnings, with $\Delta \Omega\sim 10^{-1}$ deg$^2$ and $\Delta t_c\sim 10^{0}$
s. Furthermore, we compared the pros and cons of B-DECIGO with the
third-generation ground-based detectors, and explored the prospects of
detections using 3 other decihertz observatories and 4 BNS population models.
In realistic observing scenarios, we found that decihertz detectors could even
provide early-warning alerts to a source decades before its merger while their
localizations are still as accurate as ground-based facilities. Finally we
found a decrease of events when considering the confusion noise, but this could
be partially solved by a proper noise subtraction.

High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology