DESIGN STRATEGY FOR RADIO FREQUENCY INTERFERENCE PROTECTION, DETECTION AND MITIGATION OF THE COPERNICUS IMAGING MICROWAVE RADIOMETER

P. G. Arpesi¹, M. Grilli¹, C. Sopranzi¹

¹OHB Italia,

The Copernicus programme is an evolution of the previous Global Monitoring for Environment and Security (GMES) Programme. The Copernicus Imaging Microwave Radiometer Mission, namely CIMR mission, will provide improved monitoring of the Polar Regions in terms of measurement bands, spatial resolution, temporal resolution and geophysical accuracy as well as European operational continuity of L-band and enhanced AMSR type measurement capabilities.

CIMR mission space segment consists in a microwave imaging multi-frequency (L, C, X, Ku and Ka) radiometer instrument with the capability to uniquely observe a wide range of floating sea ice and related ocean parameters, in particular sea ice concentration and sea surface temperature, and to serve operational systems in non-precipitating atmosphere conditions, day and night.

Radio frequency interference (RFI) is an increasingly serious problem for passive and active microwave sensing of the Earth, even though allocated bands for the onboard passive sensors are protected through the ITU regulations. In addition, to satisfy their measurement objectives, spaceborne passive sensors may need to operate in unprotected bands.

Therefore, the possibility of RFI occurrence has to be duly addressed to reduce the risk of damage or measurements contamination with impacts on the data quality. This problem is even more critical when observation frequency channels in the low microwave spectrum are used, as in CIMR, since these spectra are generally more crowded as largely reported by measurements from spaceborne radiometers operating in L-band such as SMOS and SMAP.

When dealing with RFI, there are basically two objectives. First and mandatory objective is to protect the sensitive receivers against damage from the most powerful RFI emitters, mainly due to ground-based radars. Second but not secondary objective is to detect and mitigate those in-band RFI signals with lower power levels that may contaminate the brightness temperature measurements.

This paper reports the design strategy against RFI adopted for CIMR instrument within the radio frequency (RF) receiver and processing chains, focusing on the design solutions implemented in the analogue receivers for preventing permanent degradation of the performance. The CIMR radiometric chain is composed of the antenna subsystem, the calibrator units, the receivers and the RF digital processors.

An extensive survey of possible on ground and in space RFI sources is performed to establish the RFI environment with the list of all potential emitting sources identified in terms of frequencies (in-band and out-of-band) and power levels at the receivers’ input. This classification of the RFI is the basis for designing the input protections.

Several trade-off analyses are carried out among different technical solutions (filters, limiters and robust technology devices) with respect to a number of performance parameters (losses with impact on radiometric sensitivity, size and mass, power consumption).

The protection against the RFI is finally sized for survival only (no damage, no permanent degradation) to minimise the insertion losses degradation and the noise figure increase as well. This leads to the optimum radiometric sensitivity at instrument level. The option with protection for full performance, that is full linear operation, would require heavier filtering structures at the input of the RF amplifiers providing for higher rejection of the interference power but with inherent higher losses and unacceptable effect on the sensitivity as well.

Consequently, when the RFI signals have the highest power levels, the calibrators and receivers’ amplifiers may get deeply compressed and saturated. This is important to be known since the recovery time to come back to the nominal performances (linear gain) will not be negligible. Moreover, the current consumption might also increase during the compression event, according to the compression level and the device biasing scheme.

As a conclusion, the baseline design for the RFI protections, for all bands regardless the use of limiter or filters, is such that the maximum RFI power level at the input of the amplifiers is lower than the specified absolute maximum rating of the relevant active devices.

As far as in-band interferences with power levels comparable to the natural earth emission are concerned, these RFI would provide for an unwanted increase of the measured brightness temperature which is adversely affecting the validity of the scientific measurements (contamination of the measured brightness temperature). Specific digital processing functions have been added in the design to provide for the required RFI detection and mitigation capabilities, with dedicated Radio Frequency Processing Unit (RFPU). This unit converts the analogue signals from the receivers, and then digitally processes the converted signals in time and frequency domains to detect the characteristics signatures of the RFI and remove the contaminated elements of the time-frequency matrix. The algorithms for detection are embedded within an FPGA design.

The on-board RFI processor provides for continuous interference detection and data mitigation in all channels, also presenting on demand the capability to download to ground the complete set of data of the time-frequency matrix to allow on-ground data quality analysis of the on-board RFI processing.