SMAP Radiometer RFI Operations and Reporting

The Soil Moisture Active Passive (SMAP) radiometer operates within the 1400-1427 MHz frequency band protected for passive use only for the Earth Exploration Satellite Services (EESS); however, unauthorized in-band transmitters as well as out-of-band emissions from transmitters operating adjacent to this allocated spectrum have been observed in SMAP measurements throughout the mission lifetime. The SMOS and Aquarius radiometers, launched before SMAP, provided evidence of global radio frequency interference (RFI). Nonetheless, SMAP is the first spaceborne radiometer to fly a dedicated subsystem to enable detection and filtering of RFI. The on-board hardware detects signal in both time and frequency which are used in the RFI detection and filtering algorithm in ground software (Piepmeier et. al, 2014 and Mohammed et. al 2016).

The RFI challenge is further addressed by reporting interfering sources through official spectrum management channels using ITU interference reports (ITU-RRS.2106-0), to reduce interference and improve future detection. This includes work to identify and characterize the RFI sources. Sources are identified using an algorithm which operates on geolocated brightness temperature footprints in the SMAP L1B_TB product (Soldo et. al 2018). Persistence statistics, spectral information and intensity of interference are also derived from low-level radiometer data. Formal reports including these data are prepared and forwarded to the NASA spectrum office, which in turn submits them to the Satellite Interference Reporting and Resolution System (SIRRS), which then forwards to the country of interest. SMAP RFI operations are coordinated with the SMOS RFI team as both instruments see similarly high level RFI. The international effort of reporting aims to alert administrations of interference with the goal that the reported sources will be investigated and turned off. Reporting by SMOS and SMAP has resulted in a general decrease in global RFI at L-band (Bringer et al., 2021, Oliva et. al, 2012). However, a recent rise of RFI in Europe can be attributed to geo-political conflict.

NASA began reporting interference sources in SIRRS starting in March 2020. As of August 2023, 56 reports have been filed with 34 administrations. Thus far, the NASA spectrum office has received responses to reports from Brazil, Spain, Canada, the UK as well as the FCC within the United States. (Reports of interference in the United States are sent directly to the FCC.) Sources have been identified and turned off in Canada, the UK, and the United States due to the reporting efforts by SMAP. Investigators are encouraged to report details of the identified sources whenever possible. A surveillance camera and a CCTV camera were found and turned off in Canada. The five sources identified and seized by the UK include CCTV cameras, faulty DECT phones and a faulty indoor TV amplifier. The FCC identified two wireless cameras, a security system, two TV preamplifiers and a radiating TV antenna preamplifier. The presentation will include examples of the temporal and spectral signatures of the identified interfering sources in SMAP data. Knowledge of interferers can possibly help agents on the ground identify other similar sources and can also be used to optimize detection algorithms. The SMAP radiometer demonstrated the power of including RFI detection subsystems as part of the radiometer hardware, and such systems are now being incorporated in new passive remote sensing systems (e.g., ESA’s future CIMR mission).

References

2014. R. Piepmeier, J. T. Johnson, P. N. Mohammed, D. Bradley, C. Ruf, M. Aksoy, R. Garcia, D. Hudson, L. Miles, and M. Wong, “Radio-Frequency Interference Mitigation for the Soil Moisture Active Passive Microwave Radiometer,” IEEE Trans. Geosci. Remote Sens., vol. 42, no. 1, pp. 761-775, Jan. 2014.
2015. 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.
2016. Soldo et al., "Location of Radio-Frequency Interference Sources Using the SMAP L-Band Radiometer," in IEEE Transactions on Geoscience and Remote Sensing, vol. 56, no. 11, pp. 6854-6866, Nov. 2018, doi: 10.1109/TGRS.2018.2844127.
2017. Bringer et al., “Properties of the RFI Environment at 1400–1427 MHz as Observed by the Soil Moisture Active/Passive Mission Microwave Radiometer,” in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 14, pp. 7259-7267, 2021, doi: 10.1109/JSTARS.2021.3092996.
2018. Oliva et al., “SMOS radio frequency interference scenario: Status and actions taken to improve the RFI environment in the 1400–1427-MHz passive band,” IEEE Trans. Geosci. Remote Sens., vol. 50, no. 5, pp. 1427–1439, May 2012.