A spacecraft's guidance, navigation, and control system needs 3D imagery of proximity objects or surfaces in the targeted space segment. Ideally, this data is obtained using sensors with low size, weight, and power (SWAP). The spatial- and depth-resolution capabilities that can be achieved using laser detection and ranging (LADAR) sensors in a small-sized detector without requiring large processing resources make LADAR a suitable choice for hazard detection, absolute-state estimation, landing/sampling site selection, and/or body-shape characterization. To address this opportunity, hardened silicon (Si) Geiger-mode (Gm) LADAR focal plane arrays (FPAs) will be developed. In Phase I, spot-scan, line-scan, area-scan, and 2D-flash LADAR formats will be considered. They will specifically take into account platform SWAP, calibration, deep-space environmental conditions, rigors of landing on planetary bodies both with and without atmospheres, and planetary protection requirements. With a radiometric budget, end-to-end physics-based system models combined with Monte Carlo detector models and numeric receiver circuit simulations will be performed to provide accurate range and range-precision predictions. Candidate silicon single-photon avalanche photodiode (SPAD) detectors and readout integrated circuits (ROICs)integrating time-of-flight (TOF), time-to-digital converters (TDC), time-to-amplitude converters (TAC), and active quenching circuits (AQDs)will be demonstrated in a benchtop LADAR testbed.