Recent developments on Single Photon Avalanche Diodes (SPADs) have opened the way to the design of single-photon time of flight systems based on very large arrays of detectors. In particular, the exploitation of 3D stacking now allows the use of different technologies to optimize both the detector and the electronics. Very high performance in terms of Photon Detection Efficiency, Dark Count Noise and Afterpulsing probability can be achieved with a dedicated custom technology fabrication process, as the one developed by Politecnico di Milano. Custom SPADs require external high-performance electronics to be properly operated. In 2019, an active quenching circuit able to operate an external custom SPADs with a dead time as short as 6ns has been developed. These results open the way to the exploitation of these detectors in many applications as spaceborne remote sensing. The very short dead time, indeed, means having a quick recovery, that is paramount to investigate the layers below a very bright surface, e.g. to measure the backscatter from plankton immediately below the ocean surface. Targeting the exploitation of a 256x256 SPAD array, we designed a fully integrated front end and processing circuit able to provide the number of impinging photons during time windows as short as 8ns.