PEACC -- Precision Emitter for 21 cm Array Coherent Calibration

Foreground mitigation remains a central challenge for 21 cm intensity mapping experiments, which require precise, wideband calibration of telescope beams and gains. We present the Precision Emitter for 21 cm Array Coherent Calibration (PEACC), a digitally synthesized calibration source that generates Gaussian noise across a 1.2 GHz bandwidth, time-synchronized to a 1 pulse-per-second output from a GPS-disciplined oscillator, and optimized for aerial deployment. PEACC uses a dual-source architecture with one unit mounted on an aerial platform and a second reference unit connected directly to the radio data acquisition system; this configuration enables improved sensitivity in the low-SNR regime and direct phase measurement. The system further supports configurable band selection, allowing adaptation to various 21 cm intensity mapping telescopes. We validated PEACC through anechoic chamber measurements and by integrating the source on a drone flown over a local radio dish testbed. In both settings, the correlated channel substantially outperformed the auto-correlation channel across all signal-to-noise regimes of interest, confirming the key advantage of the dual-source architecture. To our knowledge, this is the first published demonstration of a free-space coherent calibration signal synchronized only by clocks, the first deployment of such a source on a drone, and the first published beam measurements made with such a source. Given the growing interest in drone-based calibration for 21 cm arrays, this work establishes the feasibility of high-fidelity digital calibration for next-generation 21 cm instruments, and provides a practical path towards improved foreground control and beam calibration in future arrays.