 Its pulse-Doppler radar (PD) has a range of more than 250 mi (400 km) for low-flying targets at its operating altitude, and the pulse (BTH) radar has a range of approximately 400 mi (650 km) for aircraft flying at medium to high altitudes.The radar, combined with a secondary surveillance radar (SSR) and electronic support measures (ESM), provides a look down capability, to detect, identify, and track low-flying aircraft, while eliminating ground clutter (radar) returns.Starting in 1987, USAF E-3s were upgraded under the "Block 30/35 Modification Program" to enhance the E-3's capabilities.