  High frequency quasi-periodic oscillations (HF QPOs) appear in the X-ray
variability of several accreting low-mass binaries. In a series of works it was
suggested that these QPOs may have connection to inhomogeneities orbiting close
to an inner edge of the accretion disc. In this paper we explore the appearance
of an observable signal generated by small radiating circular hot spots moving
along quasi-elliptic trajectories close to the innermost stable circular orbit
in the Schwarzschild spacetime. Our consideration takes into account the
capabilities of observatories that have been operating in the past two decades
represented by the Rossi X-ray Timing Explorer (RXTE) and the proposed future
instruments represented by the Large Observatory for X-ray Timing (LOFT). For
these purposes we choose such model parameters that lead to lightcurves
comparable to those observed in Galactic black hole sources, in particular the
microquasar GRS 1915+105. We find that when a weak signal corresponding to the
hot-spot Keplerian frequency is around the limits of the RXTE detectability,
the LOFT observations can clearly reveal its first and second harmonics.
Moreover, in some specific situations the radial epicyclic frequency of the
spot can be detected as well. Finally, we also compare the signal produced by
the spots to the signal produced by axisymmetric epicyclic disc-oscillation
modes and discuss the key differences that could be identified via the proposed
future technology. We conclude that the ability to recognize the harmonic
content of the signal can help to distinguish between the different proposed
physical models.
