L-band radiometry is widely considered the best technique for Earth observing satellites to measure sea surface salinity (SSS). Interferometric aperture synthesis is a new technology applicable in spaceborne remote sensing at low frequencies. The challenge of the technology comes with decreased radiometric resolution and complexity in calibration compared to conventional radiometer systems. Due to these issues and the overall newness of the concept, validation of the technology for salinity retrieval purposes is desired. In this paper, we describe an intense measurement campaign carried out with the complete interferometric aperture synthesis radiometer system HUT-2-D, designed and operated by the Helsinki University of Technology. The campaign aimed at the detection of a changing salinity level in the Baltic Sea, in the coastal areas of Finland. We describe the campaign comprising details of the ground truth collection, sea surface emission modeling, and radiometric data analysis. We have a special emphasis on the assessment of the impact of the sea state on the radiometric measurements, which is considered one of the major obstacles for SSS retrieval at the L-band. For this purpose, we present a new correlation between sea roughness information collected with the Global Navigation Satellite System reflectometer and radiometric data measured by an L-band radiometer system.