Fajr End Time
Times and practice of prayer come from Sunnah. Quran sometimes mentions them. Rise of the sun is when the horn of the sun appears. this is what we call the rise of sun in every language. Also the sunnah is that as soon as the horn of the sun appears the time of Fajr elapses. You need not to worry about detecting when that happens, as it is practically very difficult in cities to find the accurate time of sun rise and sun set. Timings of sun are well recorded and can be easily followed. Just follow them.