Over the past couple of decades passive microwave radiometry observations have shown an increase in man-made RF interference corrupting measured radiances and thus impacting geophysical retrievals. These man-made sources also known as Radio Frequency Interference (RFI) have impacted various geophysical retrievals such as soil-moisture, sea-surface salinity, wind-speed and wind-direction, atmospheric water-vapor, precipitation retrievals etc. The spectrum environment for passive microwave measurements is expected to become worse over the coming decades due to an increasing demand from the commercial industry.
The CubeRRT (CubeSat Radiometer Radio Frequency Interference Technology Validation) mission was selected under NASA’s In-space Validation of Earth Science Technologies (InVEST) program to demonstrate on-board, real-time RFI processing.
The CubeRRT payload has three critical pieces of technology, a wideband antenna unit, a radiometer front-end (RFE) unit, and a radiometer digital back-end (RDB) that performs the on-board detection and filtering of RFI. The main objective of the CubeRRT mission is to demonstrate the RFI mitigation technology on a flight-ready hardware in space, increasing the technology readiness level (TRL) from 6 to 7.
Prof. Joel Johnson from the Ohio State University leads the CubeRRT mission. The algorithm validating back end technology is built at Jet Propulsion Laboratory, California Institute of Technology and the radiometer front end is built at NASA Goddard Space Flight Center.
The CubeRRT antenna subsystem consists of three circularly polarized tapered helical antennas. The antennas are being designed, developed and tested at The Ohio State University. The series of antenna are necessary to provide sufficient gain over a wide range of frequencies from 6 to 40 GHz. The current design provides a gain of 12 dBi at 6 GHz and 21 dBi at 40 GHz.
The CubeRRT radiometer front-end (RFE) is designed to sweep from 6 to 40 GHz with a 1 GHz bandwidth being injected into the RDB. The radiometer is a single tunable superheterodyne receiver. At the front-end of the radiometer the RFE has a four-position switch to choose between the three helical antennas as well as a reference load for calibration. The RFE contains a coupled wideband noise-source to for full internal calibration of the radiometer. The RFE achieves frequency tuning via a phased-locked oscillator (PLO) and sub-harmonic image rejection (IR) mixer. The design allows flexibility between choosing upper and lower side-bands to completely cover the 6 to 40 GHz regime. The architecture sacrificed radiometer performance to meet within the size, weight and power requirements of the 6U system.
The CubeRRT digital back-end (RDB) is designed to digitize a 1 GHz bandwidth signal and perform advanced digital signal processing algorithms on an on-board FPGA for RFI mitigation. The RDB ADC is capable of ingesting the IF signal produced by the RFE from 1-2 GHz aliased region. The FPGA proceeds to produce a 128 frequency spectra of the incoming signal using a front-end polyphase filter-bank. The output of the 128 channel spectra then undergoes gain adjustment to account for the non-uniform pass-band shape of the RFE signal. The higher order statistical moments of the data per channel are calculated (2nd and 4th moment) as a pre-cursor to RFI detection and mitigation. The second moment is uncalibrated power of the signal. The first RFI detection algorithm applied is a simple threshold detection algorithm across the spectra to detect frequency outliers. The second RFI detection algorithm is more advanced and uses the fourth and second moments to calculate kurtosis of the signal as a test of normality. Any signal that deviates from normality is flagged as being corrupted by RFI. The flags of the two algorithms are combined and the power in each frequency bin is summed to produce mitigated and unmitigated accumulated power. CubeRRT will downlink all 128 channels to verify performance of mitigated and unmitigated uncalibrated power outputs. Most of the thresholds, coefficients, gain-adjustment values within the RDB are updatable from the user perspective.
The CubeRRT bus is being developed by Blue-Canyon Technologies. CubeRRT is expected to operate with a duty-cycle of 30%. CubeRRT will mostly be operational over landmasses where the occurrence of RFI is expected to be significantly higher. CubeRRT is designed for 12 months of commissioning and operations.