Hirad Ghaemi

RFI Characterization and Mitigation for the SMAP Radar

The Soil Moisture Active-Passive (SMAP) mission will launch in late 2014 and will carry a combined L-band radiometer/radar instrument for the retrieval of global soil moisture and surface freeze-thaw state. Radio frequency interference (RFI) is a known challenge for Earth remote sensing in the L-band portion of the spectrum. This paper addresses efforts to characterize and mitigate RFI for the SMAP radar. A model for the RFI environment due to surface-based emitters is developed, and is shown to agree well with the observations of currently operating L-band radar systems. An analysis of the environment due to space-based emitters is also presented. Techniques to mitigate RFI in the radar band are described, and are shown to perform sufficiently well to meet the stringent SMAP measurement requirements. A companion paper addresses the different issues encountered with RFI in the radiometer band.

The DESDynI synthetic aperture radar array-fed reflector antenna

DESDynI is a mission being developed by NASA with radar and lidar instruments for Earth-orbit remote sensing. This paper focuses on the design of a large-aperture antenna for the radar instrument. The antenna comprises a deployable reflector antenna and an active switched array of patch elements fed by transmit / receive modules. The antenna and radar architecture facilitates a new mode of synthetic aperture radar imaging called ‘SweepSAR’. A system-level description of the antenna is provided, along with predictions of antenna performance.

Advances in digital calibration techniques enabling real-time beamforming SweepSAR architectures

Real-time digital beamforming, combined with lightweight, large aperture reflectors, enable SweepSAR architectures, which promise significant increases in instrument capability for solid earth and biomass remote sensing. These new instrument concepts require new methods for calibrating the multiple channels, which are combined on-board, in real-time. The benefit of this effort is that it enables a new class of lightweight radar architecture, Digital Beamforming with SweepSAR, providing significantly larger swath coverage than conventional SAR architectures for reduced mass and cost. This paper will review the on-going development of the digital calibration architecture for digital beamforming radar instrument, such as the proposed Earth Radar Missions DESDynI (Deformation, Ecosystem Structure, and Dynamics of Ice) instrument. This proposed instruments baseline design employs SweepSAR digital beamforming and requires digital calibration. We will review the overall concepts and status of the system architecture, algorithm development, and the digital calibration testbed currently being developed. We will present results from a preliminary hardware demonstration. We will also discuss the challenges and opportunities specific to this novel architecture.

Onboard digital beamforming: Algorithm and results

A one-dimensional digital beam-forming (DBF) algorithm that is suitable for onboard real-time implementation is described in this article. A couple of techniques for generating a set of DBF coefficients will be also proposed to improve radar system performance such as signal-to-noise ratio (SNR) and/or range ambiguity (as a part of multiplicative noise ratio, MNR), and range impulse response parameters including compression gain, range resolution, etc. Results from simulations demonstrate improvement obtained by these techniques at the output of digital beamforming process. Finally, via the proposed onboard DBF algorithm, a processed synthetic aperture radar (SAR) image generated from data of airborne SweepSAR experiment [1] is exhibited.

Assessment of the impacts of radio frequency interference on SMAP radar and radiometer measurements

The NASA Soil Moisture Active and Passive (SMAP) mission will measure soil moisture with a combination of L-band radar and radiometer measurements. We present an assessment of the expected impact of radio frequency interference (RFI) on SMAP performance, incorporating projections based on recent data collected by the Aquarius and SMOS missions. We discuss the impacts of RFI on the radar and radiometer separately given the differences in (1) RFI environment between the shared radar band and the protected radiometer band, (2) mitigation techniques available for the different measurements, and (3) existing data sources available that can inform predictions for SMAP.