Measurements of Ocean Surface Waves Using Airborne GNSS Multistatic Radar

The characteristics of GNSS reflected signals, such as the shape of the correlation waveform, can be related to the rms of L-band limited surface slopes. For wind-generated waves a connection can be established between the rms of surface slopes and the local wind. This relationship holds only when the local wind is the primary source of sea roughness in the vicinity of the reflection point, and the contribution from incoming swell can be neglected. During the last decade a number of airborne experiments have been performed to prove the feasibility of GNSS scatterometric technique to measure ocean surface winds. With new flying platforms and new GNSS signals becoming available there is a necessity to investigate this technique further. This technique might be attractive when considering high altitude/long endurance (HALE) Unmanned Aircraft Systems (UAS) because of the small size, small weight, and low energy consumption of GPS receivers. Use of high-altitude (~ 20 km) UAS platforms is especially beneficial providing swaths ~100 km wide. A version of software-defined GNSS bistatic radar capable to work with data volumes on the order of 1GB/minute for the GPS L1 civil signal was developed at Colorado University. This system was installed on the NOAA Gulfstream-IV jet aircraft and operated during flights in January, 2010 to test the system at high altitudes, ~13,000 m. The flight track ran across the Northern Pacific Ocean and the GPS reflected signal was recorded from all available satellites. Overall, 26 hours of reflection data were obtained during four flights. Wind speed and direction from dropsondes deployed from the same aircraft were available to assess the capability of this radar to monitor winds or rms of ocean waves. We report comparisons between GPS scatterometric wind retrievals and dropsondes measurements. The effect of swell on those retrievals is discussed. We analyze the effects of the platform high altitude on signal-to-noise ratio and on the sensitivity of reflected waveforms to rms of wave slopes. We also address the issue of Doppler filtering of the reflected waveforms due to a relatively high speed of the G-IV aircraft. For the next phase of the sensor development we expanded capabilities to all three GPS frequencies, including the wider band L5 signal (well suited for the ocean altimetry). This also required a development of the post processing algorithms for the collected data. During mid-late 2010 the new sensor was installed and flown on the NOAA P-3 aircraft. It was able to work with the GPS L1, L2C, and L5 civil bands as well as the signals from the test Galileo satellites. Due to the broadband, higher power, L5 transmission are expected to offer the most benefit for bistatic GNSS measurements, especially for the ocean altimetry. First scatterometric and altimetric results obtained with this GNSS bistatic sensor will be presented.