The Pierre Auger Observatory, located in western Argentina, is the world's largest cosmic-ray observatory. While it was originally built to study the cosmic-ray flux above $10^{18.5}~$eV, several enhancements have reduced this energy threshold. One such enhancement is a surface array composed of a triangular grid of $19$ water-Cherenkov detectors separated by $433~$m (SD-433) to explore the energies down to about $10^{16}~$eV. We are developing two research lines employing the SD-433. Firstly, we will measure the energy spectrum in a region where previous experiments have shown evidence of the second knee. Secondly, we will search for ultra-high energy photons to study PeV cosmic-ray sources residing in the Galactic center. In this work, we introduce the SD-433 and we show that it is fully efficient above $5\times 10^{16}~$eV for hadronic primaries with $\theta < 45^\circ$. Using seven years of data, we present the parametrization of the lateral distribution function of measured signals. Finally, we show that an angular resolution of $1.8^\circ$ ($0.5^\circ$) can be attained at the lowest (highest) primary energies.
Our study lays the goundmark for measurements in the energy range above $10^{16}~$eV by utilizing the SD-433 and thus expanding the scientific output of the Auger surface detector.