The CSAT Rear Sight for the AR family of weapons is a concept born from the mind of Paul Howe of Combat Shooting and Tactics (CSAT). It adds a sighting notch above the small aperture of a same plane A2 sight. This notch serves as an aiming reference that compensates for mechanical offset for closer shots where speed is of the essence. It is a more formalized approach to the technique of holding the front sight above the aperture for closer shots.
Until recently, this sight was only available for sights that used the A2 aperture which left most folding back up iron sights out in the cold. Now XS Sight Systems makes a version that will work with the premier folding sight on the market – the Troy Folding Battle Sight. It should also work on the newer Troy Fixed Battle Sights as well.
You can learn more about the CSAT sights at the XS Sight Systems website.