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 present the results of a preliminary statistical analysis and classification of solar radio burst candidates detected by the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD). We first analyze the histograms of the MRO SHARAD burst candidates as a function of MRO-STEREO true anomaly difference and received peak power. We then show the results of performing logistic regression to classify the MRO SHARAD burst candidates. Our results highlight the need for additional burst data to further refine the classifier, additional parameters to determine if bursts are present, and potentially explore a different classification technique to assign burst candidates with improved accuracy. Analyzing SRBs detected by MRO/SHARAD (as a potential additional solar radio-observatory) would enhance our understanding of solar radio burst propagation physics and behavior. We conclude by discussing the potential application, and challenges, of using these bursts as a source for subsurface radio sounding for future terrestrial and Mars missions.

We seek to understand Jupiter’s radio emissions as a source for passive sounding by characterizing its signal properties for echo detection and ranging in terms of both temporal and frequency availability. As Jupiter is one of the loudest natural sources of radio emissions in our solar system outside of the Sun’s radio signals, this work explores the extent to which we can utilize these emissions for echo detection in terrestrial passive radar experiments. Our results show that while Jupiter is temporally available year-round, the source experiences elevation angle limitations at extreme latitudes. Our results also show the source’s minimum usable frequency can vary both diurnally and seasonally. We conclude with a discussion of three cases (ideal, conservative, and pessimistic) for the maximum terrestrial bandwidth based on the described constraints.

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.

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.

We present a passive radar sounding approach that performs direct signal suppression (DSS) with a single-channel receiver. The modified passive sounding signal processing algorithm leverages a blind deconvolution filter to estimate the direct path of the ambient noise source before using CLEAN processing for DSS. We then highlight the results of our single-channel DSS technique with both synthetic and experimental data, and we analyze its performance in terms of SNR improvement. We find that by combining an adaptive blind deconvolution approach with the CLEAN algorithm, we obtain an SNR improvement of over 10 dB for both synthetic and experimental data. The single-channel DSS technique is a further step towards low-resource, high SNR passive sounding with ambient radio noise, as it eliminates the requirement for long integration times and a two-channel DSS system.

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter's icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter's DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise.

Recent work has demonstrated a passive radio sounding approach that uses the Sun as a source for echo detection and ranging. As the Sun is a moving source with a position that is knowna priori, we evaluate this technique’s capabilities to measure the echo’s phase history, map topography, and perform synthetic aperture radar (SAR) focusing. Here, we present our approach to implementing passive SAR using a compact, temporally incoherent radio-astronomical source as a signal of opportunity. We first evaluate the passive system’s capabilities to obtain an echo from a rough surface by determining the critical signal-to-noise ratio (SNR) for reliably observing the Sun’s echo reflection with our passive instrument. We then demonstrate that our technique can detect the necessary changes in range, phase, and reflectivity of an echo from the Sun. We next present the experimental results of our passive radar testing using the Sun at Dante’s View, Death Valley, to highlight this technique’s ability to perform 2-D imaging. Finally, with synthetic data, we demonstrate that we can use time-domain backprojection to focus a planar white noise signal, perform passive SAR imaging, and improve the measurement’s SNR and azimuth resolution. The results of passive SAR focusing on white noise highlight the potential for the Sun and Jupiter’s radio emissions to perform surface and subsurface imaging for planetary and terrestrial observations.

Recent work has demonstrated a passive radio sounding approach using the Sun as a source for echo detection. We expand on our passive autocorrelation-based technique by demonstrating its potential to map topography as the Sun moves throughout the entire day. Here, we show with synthetic data and our experimental results of passive radar testing with the Sun at Dante's View, Death Valley, our approach to implementing two dimensional image formation with passive radar. We also determine the critical signal to noise ratio (SNR) required to reliably observe a Sun echo with our passive instrument. Finally, we show that our autocorrelation-based technique can obtain changes in range, reflectivity, and phase, which are measurements normally acquired with traditional active radar systems. Demonstrating that our technique can acquire these changes is the first step to developing passive SAR processing using the Sun.

Jupiter is one of the Solar System's most powerful source of radio waves in the MHz range. Very recent studies proposed that Jovian Decametric Radiation can be effectively exploited for probing the subsurface of Jupiter's icy moons Europa and Ganymede by using passive radio sounding. However, these studies results were based on the assumption that the Jovian noise is white and stationary. Therefore additional investigation into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding. In this paper, we investigate the actual spectral structure, temporal occurrence and general characteristics of the Jovian Decametric Radiation for understanding their impact on the radar sounding performance metrics. This is done by analyzing the recently available Jupiter's radio spectra acquired by JUNO Waves. The results show that the Jovian Decametric Radiation (i) is not perfectly white and some performance deterioration in the range response should be expected and (ii) has a varying temporal occurrence versus probing frequency.

While radio echo sounders are powerful tools used to constrain subglacial conditions, current ice-penetrating radar systems are too resource intensive for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low resource approach for observing the subsurface of ice sheets and glaciers. Although passive radar has been used for target tracking and military purposes, it has never been implemented for ice sounding. Nevertheless, recent work has proposed the passive radio sounding of Europa's icy shell using Jupiter's decametric radiation as a source for echo detection [1], [2]. Expanding on this idea, we evaluate and discuss the challenges of developing a passive radio sounder that uses the Sun for echo detection. Our prototype measures the Sun's direct and reflected path off the ocean to obtain the height of a cliff along the California coast. This serves as the first in-situ demonstration of an autocorrelation-based passive-sounder using a compact astronomical white noise source.

Ice sheet contributions to sea level rise present one of the greatest challenges that our society will face in the next century. However, models predicting sea level rise due to ice melt lack critical information regarding processes at the base of ice sheets. Although radio echo sounders are powerful tools that are currently used to constrain subglacial conditions, existing ice-penetrating radar systems are too resource-intensive in terms of cost, power, and logistics for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low-resource approach for observing ice sheets across a range of spatial and temporal scales. While passive radar has been used for target tracking and military purposes, it has never been used for the sounding of ice sheets. Some recent work has proposed using passive radio sounding of Europa's icy shell using Jupiter's decametric radiation. We expand on this idea by evaluating and discussing challenges and opportunities for developing a passive radio sounder using the Sun as an illuminator of opportunity for echo detection. Here, our prototype instrument sits on the side of a cliff and measures the Sun's direct and reflected path off the ocean surface. We then use an autocorrelation-based method to extract the amplitude and delay of the reflection. This serves as the first in situ demonstration of an autocorrelation-based passive sounder using a compact astronomical white noise signal.

The Jet Propulsion Laboratory (JPL) is developing the Radiometer Atmospheric CubeSat Experiment (RACE), which consists of a water vapor radiometer integrated on a 3 U CubeSat platform. RACE will measure 2 channels off the 183 GHz water vapor line, and will be used to validate new low noise amplifier technology and internal calibration methodology. RACE will advance the technology readiness level (TRL) of the 183 GHz receiver subsystem from TRL 4 to TRL 6 and a CubeSat 183 GHz radiometer system from TRL 4 to TRL 7.