Abstract: In order to achieve and maintain high resolution imaging performance in space-based earth-observing system,extended-beacon(extended-scene) and H-S wavefront sensors are employed to estimate the pupil wavefront for compensating the aberrations.Taking this model into consideration,sufficient illuminance of the extended-beacon is required,thus directly constrains the application of extended-beacon to the space-based earth-observing system.By the derivation and analysis of the signal to noise ratio(SNR) formula of the sub-aperture image on the detector array,two methods are proposed to raise the SNR: decreasing the lateral magnification of the wavefront sensing system and increasing the number of measurement.The feasibility of these two methods is shown by a practical example.