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High frequency quasi-periodic oscillations (HF QPOs) appear in the X-rayvariability of several accreting low-mass binaries. In a series of works it wassuggested that these QPOs may have connection to inhomogeneities orbiting closeto an inner edge of the accretion disc. In this paper we explore the appearanceof an observable signal generated by small radiating circular hot spots movingalong quasi-elliptic trajectories close to the innermost stable circular orbitin the Schwarzschild spacetime. Our consideration takes into account thecapabilities of observatories that have been operating in the past two decadesrepresented by the Rossi X-ray Timing Explorer (RXTE) and the proposed futureinstruments represented by the Large Observatory for X-ray Timing (LOFT). Forthese purposes we choose such model parameters that lead to lightcurvescomparable to those observed in Galactic black hole sources, in particular themicroquasar GRS 1915+105. We find that when a weak signal corresponding to thehot-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 thespot can be detected as well. Finally, we also compare the signal produced bythe spots to the signal produced by axisymmetric epicyclic disc-oscillationmodes and discuss the key differences that could be identified via the proposedfuture technology. We conclude that the ability to recognize the harmoniccontent of the signal can help to distinguish between the different proposedphysical models.

Power density spectra of modes of orbital motion in strongly curved spacetime: obtaining the observable signal