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A rapid, in-drive technique for the measurement of the thermal decay rate of the magnetic readback signal amplitude has been developed. The adaptive variable-gain-amplifier (VGA) of the read channel is used to measure the signal amplitude eliminating the need for a high-speed digital oscilloscope. Individual VGA measurements include a noise component significantly larger than the amplitude change due to thermal decay. It was found that by using interleaved sectors of aged reference data, the noise could be dramatically reduced. This report will detail techniques to reduce the noise level of the measurement and techniques to acquire the final thermal decay rate as rapidly as feasible.