ORBITING AND GROUND RFI MITIGATION STRATEGY IN MICAP L-BAND INTERFEROMETRIC RADIOMETER

D. Han^1,2, H. Ma^1,2, C. Huo^1,2, H. Liu^1,2

¹Chinese Academy of Sciences, ²national space science center

1. Introduction

The Microwave Imager Combined Active and Passive (MICAP) is one of the primary payloads of Chinese Ocean Salinity Mission satellite[1], as shown in Fig.1. This satellite is launched in 14 Nov., 2024. MICAP has the capability of L/C/K multi-band passive and L-band active measurements, and can simultaneously retrieve sea surface temperature (SST), wind speed (WS), and sea surface salinity (SSS), which plays an important role in energy exchange between the ocean and atmosphere.The L/C/K band radiometer of MICAP are both one-dimensional Microwave Interferometric Radiometer (MIR). MICAP L-band MIR faces an ever increasing threat from man-made radio frequency interference (RFI) as the global wireless infrastructure has grown[2].

MICAP L-band MIR works at 1413±12.5MHz, which is located in the protected band. There is 4 Data Acquisition Units (DAU) corresponding 12 receivers with H- polarization and V-polarization. Each DAU includes 6 channels AD @120MSPS, which is much higher than Naquist sampling rate in order to improve out-of-band suppression. There is 1 Data Processing Unit (DPU) after DAU cards for digital signal correlation of all channels. As for the ground-based RFI processing, it will be addressed during the L1 level brightness temperature processing phase.

Fig.1 Chinese Ocean Salinity Mission satellite

2. Orbiting and Ground RFI Mitigation Strategy

An on-orbit RFI mitigation strategy is used in MICAP L-band MIR to mitigate the pollution of L0 data by RFI interference[3], and the enabling of on-orbit RFI detection is configurable. RFI detection logic is described in [4], as shown in Fig.2.

(a) Orbiting RFI detection (b) Orbiting RFI delete

Fig.2. Orbiting RFI detection and delete logic diagram

In Fig.2, RFI will be detected in multi domain:

• Frequency Domain: 1400~1426MHz band will be separated into 13 sub-bands, and signal processing will be implemented in every 2MHz sub-band.

• Time Domain: RFI data processing will be implemented in every 10ms.

• Polarization Domain: H/V/HV polarization will be calculated in each 2MHz&10ms cell .

• Probability Spectrum Domain: Kurtosis value is calculated in each H/V 2MHz&10ms cell .

• The 2MHz&10ms cell data will be deleted if anyone RFI detection(H-T-F /V-T-F /HV-T-F /H-K-T-F/ V-K-T-F) is true.

On ground RFI mitigation strategy is completed in three approaches

• RFI Threshold Detection : if BT > 350K, it will be identified as RFI signals.

• Frequency & Space domain Coherence Analysis RFI detection.

• Diff-Frequency brightness temperature imaging RFI detection.

Which as shown in Fig.3.

Fig.3. Ground RFI detection and delete logic diagram

3. Current Results and conclusion

onboard-ground RFI mitigation is employed in MICAP data processing. onboard-RFI mitigation focuses on the suppression of strong RFI signals to prevent the pollution of the L0 data. Ground-based RFI processing will employ more advanced signal processing techniques to achieve high-sensitivity RFI processing performance. Current RFI mitigation result is shown in Fig.4.

(a) No RFI mitigation (b) ONLY Orbiting RFI mitigation (c) Orbiting+Ground RFI mitigation

Fig.4. Current RFI mitigation result

In the future working:

• RFI processing performance should be evaluated quantitatively .

• Deep learning algorithms will be tried in the ground-based RFI processing.

• Multi-band joint RFI processing will be considered in MICAP.

References

1. Liu, H.; Zhu, D.; Niu, L.; Wu, L.; Wang, C.; Chen, X.; Zhao, X.; Zhang, C.; Zhang, X.; Yin, X.; et al. MICAP (Microwave imager combined active and passive): A new instrument for Chinese ocean salinity satellite. In Proceedings of the 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, Italy, 26–31 July 2015; pp. 184–187.

2. D. W. Draper, “Radio frequency environment for Earth-observing passive microwave imagers,” IEEE J. Sel. Earth Observe. Remote Sens., vol. 11, no. 6, pp. 1913–1922, Jun. 2018.

3. D. Han, T. Guo, X. Guo, L. Niu, H. Liu and J. Wu, “Radio Frequency Interference Mitigation in L-Band Radiometer of Microwave Imager Combined Active and Passive (MICAP),” IGARS, Malaysia, 2022, pp. 5274-5277, doi: 10.1109/IGARSS46834.2022.9884324.

4. D. Han et al., “Experimental Flight Verification of Radio Frequency Interference Mitigation Method in MICAP L-Band Radiometer,” IGARSS, Athens, Greece, 2024, pp. 9174-9177, doi: 10.1109/IGARSS53475.2024.10642212.