Abstract: Compact antenna test range (CATR) systems for indoor radar cross section (RCS) measurements show clear limitations at low frequencies, as achieving a sufficiently large and phase-flat quiet zone requires bulky reflectors and shielded chambers, leading to significant system size and cost, which makes high-accuracy measurements in compact indoor spaces difficult. To address this problem, we proposed a new indoor RCS measurement method based on a planar array. The method used a transmit-receive partitioned plane wave generator (PWG) as the core and adopted an interleaved transmit-and-receive row layout on the same aperture, which enhanced transmit-receive isolation and end-to-end amplitude-phase consistency. We developed a 16×16 S-band planar array system with a 1.7 m×1.7 m aperture and carried out indoor measurements of a scaled X47B model from 2 GHz to 4 GHz, then compared the results with far-field reference data obtained in a CATR. The results show that the RCS curves from the two systems agree very well in main lobes, side lobes, and deep nulls, with the average error over all frequency bands remaining below 0.1 dB, which confirms the measurement accuracy and stability of the proposed planar-array system in a non-ideal anechoic environment. Compared with CATR, the proposed system achieves higher aperture utilization, a more compact physical configuration, and clear potential for electronic beam scanning, thereby offering a practical route to high-accuracy and scalable RCS measurements at low frequencies and in space-constrained indoor environments.