A method and system of time-to-intercept determination for a radiation source using passively-sensed irradiance data. The invention provides a plurality of noise reduction features to reduce the noise present in the data and improve the accuracy of the time-to-intercept computation. The method includes reducing data noise by defining an acceptable noise level and eliminating any excessively noisy data from the time-to-intercept computation. The method further includes constantly updating the time-to-intercept computation by using irradiance values that are advanced in time. Other features of the present invention includes averaging of irradiance values over a time interval, filtering of the irradiance data received by the method, and triggering at a predetermined time-to-intercept. The invention also includes a time-to-intercept system and processor implementing the above method.