Schottky mass spectrometry is a novel method of precision nuclear mass spectrometry based on the measurement of the revolution frequencies of cooled ions in storage rings performed by non-destructive frequency analysis of the beam noise, the well-established Schottky diagnosis technique. The method was applied for the first time at the Experimental Storage Ring ESR at GSI observing electron cooled highly charged ions up to bare nuclei at relativistic energies around several hundred MeV/u. To demonstrate the performance and feasibility of the method at the ESR, experimental tests have been carried out using beams of nuclear fragments produced in the ring itself by the interaction of different primary beams with the internal gas jet target. Futhermore, first Schottky mass measurements of secondary nuclear beams produced by projectile fragmentation of Au and Bi primary beams in a thick Be-target were carried out in order to determine the masses for numerous heavy neutron deficient nuclei which had not been measured before. Relative accuracies for the measured mass values in the order of 1 × 10^-6 and below can be achieved. The method is briefly discussed and some early results are presented.