Your difficulty is not going to be your overall low of 15 degrees but the high the next day and the duration of the cold spells. My trees have sustained 16 degrees, they were young (2 to 3 years) and had some significant damage, but we got to 35/36 the next day before plunging back into freezing temps again. The cold spell was also only 3 days straight. Satsumas are not very large trees, you could easily build a protective shelter for one if not more than one.