| Abstract: |
Microwave radiometers measure weak thermal emission from the Earth, which is broadband in nature. Radio frequency interference (RFI) is man-made, originates from active transmitters, and are typically narrow band, directional and continuous or intermittent. The Soil Moisture Ocean Salinity (SMOS) [1], Aquarius [2] and more recently the Soil Moisture Active Passive (SMAP) missions have provided evidence of RFI globally at L-band [3], despite protection at this frequency. The Global Precipitation Measurement (GPM) Microwave Imager has also seen RFI caused by ocean reflections from direct broadcast and communication satellites in the 18.7 GHz allocated band [4]. This work focuses on the use of a complex signal kurtosis algorithm as well as the real signal kurtosis algorithm (similar to that used on SMAP) to detect direct broadcast satellite (DBS) signals at 18.7 GHz.
The complex kurtosis algorithm is a test for Gaussianity similar to that of the real kurtosis algorithm. A wideband RFI algorithm testing environment was developed using the Reconfigurable Open Architecture Computing Hardware System (ROACH). The complex kurtosis detector, which was modeled and simulated, showed improved performance over the real kurtosis detector under certain conditions. Both algorithms were then implemented in hardware at 200 MHz using the ROACH.
In collaboration with the Jet Propulsion Laboratory and Purdue University, a reflectometry experiment was conducted in August 2017 at the Harvest oil platform, located about 10 km off the coast of central California [5]. Data was collected for RFI detection and altimetry using direct and ocean reflected DBS transmissions in Ka and Ku-band, from a commercial geostationary satellite. An RF front end, which included two direct TV dishes, and a digital back end (using the ROACH) were designed to obtain raw captures at 400 MHz bandwidth at 18.4 to 18.8 GHz and also at 12.3 to 12.7 GHz for altimetry and for testing alternative RFI detection algorithms. The system also operated in non-raw capture mode to collect 200 MHz of bandwidth at 18.7 GHz for testing the real and complex kurtosis detection algorithms. The presentation will concentrate on the results of this experiment.
Preliminary results indicate that both kurtosis algorithms can detect the direct signal originating from the DBS transmissions. The algorithms show much less sensitivity to the ocean reflected signal likely due to time dispersion. Plots of the real and complex kurtosis versus interference to noise ratio will also be presented.
REFERENCES
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[2] D. M. Le Vine, P. De Matthaeis, C S. Ruf and D. D. Chen, “Aquarius RFI Detection and Mitigation Algorithm: Assessment and Examples”, IEEE Trans. Geosci. Remote Sens., vol. 52, no. 8, pp. 4574–4584, August 2014.
[3] P. N. Mohammed, M. Aksoy, J. R. Piepmeier, J. T Johnson and A. Bringer, “SMAP L-Band Microwave Radiometer: RFI Mitigation Prelanch Analysis and First Year On-Orbit Observations”, IEEE Trans. Geosci. Remote Sens., vol. 54, no. 10, pp. 6035–6047, October 2016.
[4] D. W. Draper and D. A. Newell, “An assessment of radio frequency interference using the GPM Microwave Imager,” in Proc. IEEE Geosci. Remove Sens. Symp., Milan, Italy, 2015, pp. 5170-5173, DOI: 10.1109/IGARSS.2015.7326998.
[5] R. Shah, S. C. Ho, J. Garrison, P. N. Mohammed, J. R. Piepmeier, A. Schoenwald, R. Pannu and B. Haines, “Coastal Altimetry Using Ku/Ka-band Signals of Opportunity: Results From a Recent Experiment at Platform Harvest,” Ocean Surface Topography Science Team Meeting, Miami, FL, October, 2017.
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