## 1 Introduction and State of the Art  
GNSS (Global Navigation Satellite System) signals reflected from the surface of the sea can be used to infer its physical properties, such as the sea-surface roughness [1] and altimetry [2]. This measurement concept is known as GNSS Reflectometry (GNSS-R) or as PAssive Reflectometry and Interferometry System (PARIS). In a multistatic configuration with many GNSS transmitters, one single receiver can observe multiple reflection points simultaneously, providing a synoptic view of the sea surface. Despite its limited altimetric precision compared to conventional radar altimeters, GNSS-R is a measurement concept for synoptic mesoscale altimetry that has been taken into consideration by the European Space Agency [3, 4].  
Several GNSS-R signal processing techniques have been proposed, and interferometric GNSS-R (iGNSS-R) [3] is among the most promising for altimetric applications as it yields an enhanced ranging precision. This technique consists of acquiring the direct line-of-sight and the sea-surface reflected GNSS transmitter signals from an airborne or space-borne platform and computing the delay and Doppler compensated cross-correlation function between both signals. From the resulting waveform, the altimetric delay is estimated, e.g., [5]. By using the direct cross-correlation between the direct and reflected signals, the whole bandwidth of the GNSS signals is exploited, including those of the non-public signals. However, the required separability between different GNSS transmitter signals has to be achieved by Doppler filtering and by using narrow beam antennas at the receiver pointing towards the transmitter and the specular reflection point. This is especially the case in airborne platforms, where the Doppler differences between different reflected signals are too close to be used as a means of separation. In contrast, the clean-replica correlation technique [6] uses code-correlation to achieve separability between different transmitters, and can be used with non-directive antennas. However, it suffers from a very reduced bandwidth since it is limited to civilian signals.  
In 2011, we first demonstrated the iGNSS-R concept for altimetry on board an aircraft by using two fixed, high-gain zenith and nadir pointing antennas, and by selecting a proper acquisition time window with a single GPS (Global Positioning System) transmitter at high elevation [7; 8]. In spite of the success, this scheme only allowed us to obtain single iGNSS-R observations at a time.  
To demonstrate synoptic altimetric measurements with the iGNSS-R technique, we have developed the Software PARIS Interferometric Receiver (SPIR). This instrument collects raw data from all individual elements of two antenna arrays of eight elements at a high rate (320 MBytes/s) and saves them to solid state disks, and all data samples are recorded with precise time-tagging with respect to UTC (Coordinated Universal Time) time. Data processing is completed by an off-line software signal processor. This approach allows for pointing of the antennas in post-processing, thus enabling the possibility of selecting different GNSS signals at the same time. It is also possible to reprocess exactly the same data using different algorithms to compare their performance. Previous work in this line has been reported in [9; 10].  
This paper first describes the instrument hardware and its software processor. Then, it shows instrument validation measurements for significant subsystems and presents a selection of observables (waveforms) using the clean-replica and interferometric techniques which were acquired during an airborne campaign over the Baltic Sea. The paper ends with some concluding remarks.