Reads strange. Hypersonic = 3500 MPH, low altitude = heat from high drag and also lowers endurance of flight time. A front facing window of IR transparent stuff, will soon get too hot and it's emitted IR will overload the sensors. It must front face = that is where it is going. They might have snap open/close windows that the sensor looks through for 1-5 ms while open so the cooled sensor can see the target. Repeated 1-5 ms snapshots are used for targeting - must be close, target drift is fast at 3500 MPH, snap you have passed it...