Argus is an experimental antenna array system designed to demonstrate all-sky monitoring for transient signals in the frequency range 1200-1700 MHz. It currently consists of 22 broadband spiral antennas (expandable to 32) which are individually instrumented, digitized, and analyzed in an attempt to detect and localize both broadband and narrowband astronomical transients. In this paper, we describe the design of the instrument. Notable features include a novel array data aggregation architecture, a detection algorithm which does not require accurate calibration or detailed knowledge of the array manifold, and very low per-element cost of about US1 k/element. A sensitivity of at least 6.6times10 -22 Wm -2 Hz -1 = 66 kJy (zenith at 1700 MHz for a 209 ms observation with 60 kHz bandwidth) is achieved for the system as implemented. Performance is demonstrated in an experiment in which the Sun is detected, localized, and tracked as it moves across the sky. Other experiments confirming the functionality of Argus as an all-sky surveillance system are summarized.