  Low energy ground-based cosmic ray air shower experiments generally have
energy threshold in the range of a few tens to a few hundreds of TeV. The
shower observables are measured indirectly with an array of detectors. The
atmospheric absorption of low energy secondaries limits their detection
frequencies at the Earth's surface. However, due to selection effects, a tiny
fraction of low energy showers, which are produced in the lower atmosphere can
reach the observational level. But, due to less information of shower
observables, the reconstruction of these showers are arduous. Hence, it is
believed that direct measurements by experiments aboard on satellites and
balloon flights are more reliable at low energies. Despite having very small
efficiency ($\sim$0.1%) at low energies, the large acceptance ($\sim$5 m$^2$sr)
of GRAPES-3 experiment allows observing primary cosmic rays down below to
$\sim$1 TeV and opens up the possibility to measure primary energy spectrum
spanning from a few TeV to beyond cosmic ray knee (up to 10$^{16}$ eV),
covering five orders of magnitude. The GRAPES-3 energy threshold for primary
protons through Monte Carlo simulations are calculated, which gives reasonably
good agreement with data. Furthermore, the total efficiencies and acceptance
are also calculated for protons primaries. The ability of GRAPES-3 experiment
to cover such a broader energy range may provide a unique handle to bridge the
energy spectrum between direct measurements at low energies and indirect
measurements at ultra-high energies.
