This article presents the design, fabrication, and testing of a modulated metasurface (MTS) antenna for monopulse radar tracking at Ka-band. The antenna consists of a circular, thin grounded dielectric layer printed by a texture of metallic patches modulated in shape and size. The patch layer can be modeled as a spatially variable capacitive impedance sheet, which together with the grounded slab contribution provides an overall, modulated, inductive boundary condition (BC). The antenna aperture is divided into four identical angular quadrants each of them radiating independent broadside beams when excited by an individual monopole launcher. Each of the four launchers excites a TM cylindrical surface wave (SW), which is progressively converted into a leaky-wave (LW) by the MTS. The four subapertures are virtually separated by properly designing the MTS modulation. To this end, the LW attenuation constant is calibrated for dumping sufficiently each individual SW, thus preventing the interaction between the adjacent regions. Therefore, the printed structure is not delimited by any physical separation, but only by a continuous change of the equivalent BCs. The monopulse-type linearly polarized Sigma, Delta Delta, Delta(E), Delta(H) beams are obtained by combining the source excitation with a simple phasing scheme. Notably, this solution does not affect the overall lightness, low profile, feed simplicity, and low fabrication cost of the structure, which constitutes an inherent advantage with respect to more traditional waveguide-based solutions.