## 1 Introduction  
The interest of the scientific community in remotely measuring Sea Surface Salinity (SSS) has been increasing in recent years and much effort has been spent in that direction by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) with the MIRAS/SMOS [1] and the AQUARIUS/SAC-D [2] missions, respectively. In principle, a number of radiometer architectures are suitable to be used for SSS retrieval applications, but the radiometric accuracy and sensitivity requirements are very challenging constraints (\(\Delta\)T\({}_{\text{B}}\)\(\sim\) 0.1 K). In this regard, one of the major concerns of a radiometer design is its radiometric sensitivity that can be improved using low noise devices or even operating in a cool temperature environment. Furthermore, to achieve these tough requirements, several auxiliary techniques can be used depending of the sensor type, not necessarily related to the radiometer design. For ground-based measurements, the radiometric resolution can be improved simply by increasing the integration time as long as needed, due to the stationary nature of the measurements. This, however, is not possible for airborne or spaceborne sensors. In this case, one option is to acquire multi-angular measurements of the same scene, taking advantage of the well-known dependence of the emissivity on the polarization and the incidence angle [3] (Figure 1).  
Knowing this angular and polarization dependence, it is possible to merge these measurements to retrieve geophysical parameters with a reduced standard deviation. This can be done using theoretical or semi-empirical models and minimizing a cost function following a non-linear least-squares estimation algorithm, such as the Levenberg-Marquardt algorithm [4]. If the measurements' noise is uncorrelated, the standard deviation of the estimated observable is reduced to \(\sqrt{N}\), where \(N\) is the number of independent measurements. If measurements are not uncorrelated, the standard deviation decreases less rapidly following different trends [5].  
Typically, brightness temperature measurements at different incidence angles can be obtained simultaneously using different receivers, or with the same receiver acquiring data at different times [6,7].  
Figure 1: Vertical and horizontal emissivities _vs._ incidence angle. PAU-RAD antenna boresight pointed at 45 \({}^{\circ}\) and with the digital beamforming signal processing technique creates beams from 30 \({}^{\circ}\) to 60 \({}^{\circ}\) incidence angles in 5 \({}^{\circ}\) steps. The emissivity curve corresponds to a calm sea with T\({}_{\text{sea}}\) = 293 K, SSS = 36 psu and at f\({}_{\text{GPS-L1}}\) = 1,575.42 MHz.  
Furthermore, multi-angular measurements are inherent in the image formation process in synthetic aperture radiometers such as SMOS/MIRAS [8] or PAU-Synthetic Aperture (PAU-SA) [9]. This paper presents a new option to obtain multi-angular measurements combining an L-band radiometer with the concept of digital beamformer (DBF), which is able to steer multiple and simultaneous beams at different incidence angles without mechanical scan. This work focuses on the design, first results and performance of using digital beamforming techniques applied to an L-band radiometer.