This paper describes the design and development of the Georgia Tech Quadrotor (GTQ) Unmanned Aerial System (UAS). The GTQ is an autonomous quadrotor helicopter capable of exploring cluttered indoor areas without relying on external navigational aids such as GPS. It weights around 1600 grams, has a width of about 60 cm. The GTQ uses an off-the- shelf quadrotor platform and is equipped with off-the-shelf avionics and sensor packages using custom software and interface electronics. Similar platforms have previously used an off-board computer to achieve laser-aided inertial navigation due to the limited on- board computational power. The GTQ on the other hand, is capable of exploring indoor areas fully autonomously using only the processing power on-board the aircraft. The GTQ achieves this by using an elaborate navigation algorithm that fuses information from a laser range sensor, an inertial measurement unit, and a sonar altitude sensor to form accurate estimates of the vehicle attitude, velocity, and position relative to indoor structures. The overall architecture and hardware that make this possible are discussed in detail.