This paper describes results of laboratory experiments and theoretical simulations of the signal-to-noise ratio using a Geiger-mode avalanche photodiode (GM-APD) array-based detector. The results include dark count rate (DCR), afterpulsing, and photon detection efficiency (PDE). The device has been operated over a range of temperature and operating modes, varying the gate width, hold-off time, overbias, and duty cycle. To simulate radiation damage for a long space mission, the device was exposed to a total of 50 krad (Si), 10x an equivalent mission lifetime of 11 years at an L2 orbit. Preand post-radiation results, including projected SNR, are compared. The simulated SNR is evaluated for both low and high flux imaging. These simulations include shot noise of the signal, DCR, afterpulsing, PDE, and dead time; there is no read noise because the photon signal is digitized in the pixel.