## 1 Introduction  
The use of signals from the GNSS satellites in scatterometry constitutes a relatively new scientific area. Determination of parameters such as sea surface roughness, winds, ocean wave-heights, spectra, and tilts can be extracted from a spectral analysis of sea surface GNSS reflection measurements. A number of measurement campaigns from mountain tops, airplanes, and satellites have shown results leading to some of these parameters [1; 2; 3]. International satellite missions, such as TDS-1, CYGNSS, COSMIC-2, the European satellite project proposals GEROS and G-TERN, and the ESA Scout mission proposal HydroGNSS, have underlined the need for simulation studies highlighting the assumptions for the data retrievals and the precision and the accuracy of such measurements [4; 5; 6; 7; 8; 9; 10; 11]. Ocean reflected signals from the GNSS satellites (received at low Earth orbiting satellites, airplanes, and fixed mountain locations) describe the ocean surface mean height, waves, and sea surface roughness. The estimated accuracy of the average surface height is on the order of 10 cm for smooth and stable conditions. Thus, global observations could be an important contribution to long-term variations of the ocean mean height, as well as the monitoring of ocean mesoscale eddies (with spatial features of 100 km and temporal variability of the order of several days), which result in sea height changes much larger than the accuracy of the GNSS technique. The ocean reflected signals can be divided into two set of measurements, (1) high-elevation measurements (equal to low incidence angles) and (2) low-elevation grazing angle measurements. For the first type, the ocean reflection cross-section has a limited extent. The reflected signal is coherent with smaller errors due to ocean waves, sampling rate, and the internal processing method of the receiver [6; 11; 12]. For low elevations, the signal revealsthe incoherent scatter process in the reflection zone. Using open-loop high-precision GNSS receivers, it is possible to provide the in-phase \(l\) and quadrature \(Q\) components of the signal at high sample rates, which enables investigation of the spectral signatures of the received reflected GNSS signals. The reflected signal is able to establish experimental knowledge on the influence of signal multipath interference and signal disturbances caused by the atmosphere and the ocean reflection. To quantify the potential, we performed a series of high-altitude ocean reflection measurements from the Haleakala Summit on Maui, Hawaii in 2004, revealing the spectral characteristics of the direct satellite signal and the ocean reflected signal for grazing elevation angles. The experimental setup was a bistatic scattering system, consisting of a GPS satellite emitting electromagnetic waves, which are reflected by the ocean surface, and a GPS receiver at a low elevation angle. Both the direct wave and the reflected wave are collected by the receiver, as depicted in Figure 1.  
The color-coded ocean surface heights in Figure 1 originate a retrieval of ocean and atmosphere conditions from a numerical weather prediction model (NWP). Large positive heights are colored red. While blue heights have lower surface altitudes compared to a mean ocean surface model for the area. Bands of higher wave heights are easy to identify with rather long scale sizes in the vertical plane containing Tx and Rx. Due to the resolution in the NWP calculation shorter wave lengths are smeared out in the plot. The bands of positive wave heights extend almost perpendicular to the plane containing the surface reflection observations, which could indicate a uniform atmosphere wind field in the region. The characteristics of the reflected signal depend on the scattering properties of the sea surface and the footprint of the reflection zone. The footprint size and shape in turn depend on the geometry of the measurements and the relative velocities of transmitter and receiver with respect to the reflection zone.