Recent work has highlighted the simulated performance of passive synthetic aperture radar (SAR) using Jupiter’s radio emissions to probe the icy moons of Jupiter. Terrestrially, passive radar sounding using the Sun as a source for echo detection, ranging, imaging, and measuring ice thickness has also been recently demonstrated for the first time. With increasing advancements in passive radar sounders that use extended, incoherent radio-astronomical sources for echo detection, we revisit a potential limitation of the technique in terms of the sources’ spatial coherence properties. While previous work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening, there has been little work to date that has examined the spatial coherence constraints for passive sounding imposed by source size, wavelength, incidence angle, and altitude—all of which govern the potential performance of passive SAR focusing. Starting from antenna theory, the Van Cittert-Zernike (VCZ) theorem, and the coherence function for passive sounding, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes when using radio-astronomical sources; in particular, we analyze the scenarios for a spacecraft using the Sun and Jovian bursts as sources for passive sounding of the Earth, Mars, and Europa. While the results of our analysis and simulations show that the coherence requirements (in terms of both pulse broadening and spatial radius of coherence) are met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence at these greater altitudes creates an upper bound for orbital passive radar sounding. Our results therefore provide a richer understanding of the passive sounding technique, its viability, and a critical design constraint when planning future planetary and terrestrial passive sounding experiments.

We revisit a potential limitation for passive radar sounders that propose to use extended, incoherent radio-astronomical sources for echo detection and ranging. Recent work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening; we expand on this work by analyzing the spatial coherence limitations imposed by source size, wavelength, incidence angle, and altitude. Moreover, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes for a spacecraft using Jovian bursts as a source for passive sounding of the Moon and Europa. While the results of our analysis and simulations show that the coherence requirement is met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence creates an upper bound for an orbital passive radar sounder at Europa. Our results therefore provide a richer understanding of the passive sounding technique, its feasibility, and its potential limitations when designing future missions.

Global navigation satellite system (GNSS) signals are critically important for a wide range of commercial, military, and science applications. Recent studies have identified threats to the performance of GNSS from both intended and unintended sources of radio frequency interference (RFI). Understanding the distribution of the sources of RFI and the nature of the signals that they are emitting is critical to determine and mitigate their effects on the measurements made by GNSS receivers. Terrestrial RFI can be substantially detrimental to the received GNSS signals, affecting the interpretation of related science measurements. NASA’s Blackjack/TriG GNSS receivers are used for precise-orbit determination and radio occultation measurements, providing a data record spanning most of the earth’s surface for nearly 20 years. We have developed a highly sensitive detection algorithm that uses variations in the measured signal-to-noise ratio (SNR), on the order of 10–50 s, common to all satellites to identify times and locations subject to RFI. Initial work has focused primarily on the detection of the presence of RFI and using the receiver’s orbital solution to record the location of detection events. Our intermission analysis creates a unique record of global RFI with the potential for: 1) rigorous detection of the presence of interfering signals during science measurements; 2) geolocation of RFI sources; and 3) characterization of the nature of the transmitted signal to better identify intent. Preliminary analysis has shown that the presence of RFI is well correlated with regional conflicts and other geopolitical activities.

Passively sounding icy and rocky bodies in our solar system provides a way to observe the surface and subsurface of these objects without the need for costly transmitters. Jupiter’s decametric radiation provides a suitable source of radio frequency signals for sounding on geological scales of interest, but its spectral structure can introduce undesired artifacts. Recent studies have shown that Jovian bursts could be effectively applied as a source of passive sounding. However, it has been noted that the spectral properties of Jovian bursts, as measured with JUNO data, are not perfectly modeled as white noise, which can result in degraded sensitivity and vertical resolution for passive sounding. In response to this finding, we present conditioning processes that improve the echo detectability and sounding resolution for Jovian burst-like signals. More than 18 h of Jovian burst recordings are used to simulate conditioning of the natural spectral variation, demonstrating that high-quality corrections are possible with these processes for noise conditions in both the Jovian and Earth/Moon systems.

GNSS reflection measurements can be calibrated with data from SMAP to yield estimates of soil moisture with enhanced spatiotemporal resolution useful to certain hydro-logical/meteorological studies. Current approaches use simple models of the relation between the DDM (delay-Doppler map) and soil moisture and can fail in certain regions of the planet. Complex information contained in the complete 2D DDM could help in these areas, and can be extracted through the application of deep learning based techniques. Our work explores the data-driven approach of convolutional neural networks to determine complex relationships between the reflection measurement and surface parameters. We developed a neural network trained using CYGNSS DDMs and ancillary datasets aligned with SMAP soil moisture values; the results of which are analyzed and compared to existing global soil moisture products.