  Experiments deploying large arrays of transition-edge sensors (TESs) often
require a robust method to monitor gain variations with minimal loss of
observing time. We propose a sensitive and non-intrusive method for monitoring
variations in TES responsivity using small square waves applied to the TES
bias. We construct an estimator for a TES's small-signal power response from
its electrical response that is exact in the limit of strong electrothermal
feedback. We discuss the application and validation of this method using flight
data from SPIDER, a balloon-borne telescope that observes the polarization of
the cosmic microwave background with more than 2000 TESs. This method may prove
useful for future balloon- and space-based instruments, where observing time
and ground control bandwidth are limited.
