Radio-frequency interference mitigating hyperspectral L-band radiometer

Abstract

Abstract. Radio-frequency interference (RFI) can significantly contaminate the measured radiometric signal of current spaceborne L-band passive microwave radiometers. These spaceborne radiometers operate within the protected passive remote sensing and radio-astronomy frequency allocation of 1400-1427 MHz but nonetheless are still subjected to frequent RFI intrusions. We present a unique surfacebased and airborne hyperspectral 385 channel, dual polarization, L-band Fourier transform, RFI-detecting radiometer designed with a frequency range from 1400 through ≈ 1550 MHz. The extended frequency range was intended to increase the likelihood of detecting adjacent RFI-free channels to increase the signal, and therefore the thermal resolution, of the radiometer instrument. The external instrument calibration uses three targets (sky, ambient, and warm), and validation from independent stability measurements shows a mean absolute error (MAE) of 1.0 K for ambient and warm targets and 1.5 K for sky. A simple but effective RFI removal method which exploits the large number of frequency channels is also described. This method separates the desired thermal emission from RFI intrusions and was evaluated with synthetic microwave spectra generated using a Monte Carlo approach and validated with surface-based and airborne experimental measurements.

Modelling the L-Band Snow-Covered Surface Emission in a Winter Canadian Prairie Environment

“…The radiometer antenna is a 19-element air loaded conformal muffin tin design that has a 30 • half-power (−3 dB) beamwidth. A method was developed for separating out the thermal spectrum from RFI-contaminated channels to get unique RFI-free T B from the measured spectrum [28]. Only the protected radio-astronomy frequency spectrum of 1400-1427 MHz was used to calculate the T B .…”

Spatial Variability of L-Band Brightness Temperature during Freeze/Thaw Events over a Prairie Environment

“…For the present campaign, the radiometer system was mounted on a mobile manual forklift in order to be moved between plots, with the ability to adjust the height above ground to a standard acquisition height of 2.75 m above the surface at each measurement plot. The integration time of the radiometer was set to acquire 1 measurement every ≈3.9 s for all 385 channels at each polarization [22].…”

“…L-Band measurements at the seven monitoring plots were acquired by a surface-based, hyperspectral, 385 channel, dual polarization, L-Band Fourier transform, radio frequency interference (RFI) detecting radiometer (Radiometrics Corporation © , Boulder, CO, USA) with a frequency range from 1400 to ≈1550 MHz (see [22] for a complete description of the instrument). For the present campaign, the radiometer system was mounted on a mobile manual forklift in order to be moved between plots, with the ability to adjust the height above ground to a standard acquisition height of 2.75 m above the surface at each measurement plot.…”

“…Toose et al [22] developed a method to remove RFI contamination. To find an RFI-free T B value representative of the observed spectrum, a 3rd order polynomial of sort-rank versus T B is calculated, and the 2nd derivative of the slope of this cubic polynomial is derived.…”

“…During the campaign (see also [13]), 16 three-point calibrations were conducted to calibrate the radiometer including the measurements of sky at zenith (estimated at ≈5 K: [12,23]), an ambient black body target and a heated warm black body target (≈340 K). Independent sky and calibration target measurements (91 in total, including 21 taken within the KSMN), show that the mean absolute error of the calibration accuracy (MAE) was between 1.0 and 1.5 K over the course of the entire campaign [22].…”