Radar Astronomy Research Articles

Saving the Planet with Radar Astronomy

Saving the Planet with Radar Astronomy

A Digital Backend with Pulse Detection for Radar Astronomy

In radar astronomy, the digital backend and data recording system process and store echo signals in real-time, facilitating the monitoring of near-earth objects such as space debris, satellites, and asteroids. In this paper, a radar astronomy digital backend (RADB), designed for radar astronomy observation and data recording, is proposed. With a sampling rate of 1.12 GSps, the RADB accommodates various analog intermediate frequency (IF) front-ends. In addition to continuously recording echo signals, the RADB has three pulse storage modes and employs an architecture that combines a two-stage decimation (TSD) unit and a decimated pulse detection (DPD) unit. The TSD unit reduces the sampling frequency based on the bandwidth of the signal, after shifting the IF signal to the baseband. Compared to a single-stage decimation, the proposed TSD structure effectively reduces FIR resource consumption without compromising performance. Meanwhile, the DPD unit identifies pulse echo signals and selectively enables the backend to store data only when pulses are detected. This process further reduces the burden on data transmission and storage. Furthermore, the matched filtering pulse detection method in the DPD unit enhances triggering performance, particularly under weak signal conditions. Preliminary performance evaluations in a laboratory demonstrate that the TSD unit reduces data volume by 56 times, while the DPD unit achieves a further reduction of 20 times. Concurrently, a Moon reflection experiment is also conducted at the Yunnan Kunming Electromagnetic Environment Observation and Research Station by using a 29 m antenna. Analysis and processing of stored data validate the effectiveness of the proposed design.

Integrated Ultra‐Wideband Dynamic Microwave Frequency Identification System in Lithium Niobate on Insulator

AbstractThe capability to identify the frequency of unknown microwave signals with an ultra‐wide measurement bandwidth is highly desirable in radar astronomy, satellite communication, and 6G networks. Compared to electronic solutions, the integrated photonic technology‐enabled dynamic instantaneous frequency measurement (DIFM) approach is attractive as it offers unique advantages, such as ultra‐wide frequency measurement bandwidth, high flexibility, and immunity to electromagnetic interference. However, so far the bandwidth of the reported DIFM systems based on integrated photonic technology is limited to below 30 GHz due to the finite bandwidth of electro‐optical modulators (EOMs), limiting their applications, particularly in the field of millimeter wave technology (30–300 GHz). Here, the first integrated dynamic microwave instantaneous frequency measurement system with a record‐breaking operation bandwidth (ranging from 5 to 65 GHz) and low root‐mean‐square (RMS) error (≈300 MHz) is presented on the lithium niobate on insulator (LNOI) integrated photonic platform. This demonstration paves the way for high‐performance millimeter wave photonic integrated devices using the LNOI platform.

Overview of High-Power and Wideband Radar Technology Development at MIT Lincoln Laboratory

This paper summarizes over 60 years of radar system development at MIT Lincoln Laboratory, from early research on satellite tracking and planetary radar to the present ability to perform the centimeter-resolution imaging of resident space objects and future plans to extend this capability to geosynchronous range.

The Fastest Rotators: Near-Earth Asteroids Observed with the Arecibo Planetary Radar System

Fast-rotating asteroids (FRAs) are considered to be small bodies having a rotation period (P) faster than the spin barrier of about 2.3 h, starting at diameters of less than 300 m. We selected the 20 fastest Arecibo radar-observed targets, with P<0.13 h (∼8 min). Some key measurements and calculations obtained from radar observations include: the Doppler bandwidth, the circular polarization ratio, radar astrometry, and (with enough signal-to-noise ratio) delay-Doppler images of the object. Rotation period data available from the Light Curve Database for the selected objects combined with the radar observations allow us to constrain the possible diameters and confirm the periods. Of the objects in this sample, the median absolute magnitude (H) is 24.9, and the median calculated diameter is 32 meters.The apparent rotation period indicated by the bandwidth can appear slower than the object’s true rotation, but not faster, providing an upper limit to the true rotation period. Asteroid cohesion required to prevent rotational disruption depends on rotation rate, density, and diameter; we performed calculations for the minimum values for cohesion via the Drucker–Prager (D–P) cohesion criterion. Most of these objects need a few to a few hundred pascals of cohesion; however four cases stand out: 2014 TV, 2015 RF36, 2015 GS2 and 2017 EK, needing a minimum cohesion on the order of a few kilopascals. These are comparable to very weak Earth rocks, and are larger than previously reported values for NEAs.

Numerical Evaluation of Planetary Radar Backscatter Models for Self-Affine Fractal Surfaces

Numerous analytical radar-scattering laws have been published through the past decades to interpret planetary radar observations, such as Hagfors’ law, which has been commonly used for the Moon, and the cosine law, which is commonly used in the shape modeling of asteroids. Many of the laws have not been numerically validated in terms of their interpretation and limitations. This paper evaluates radar-scattering laws for self-affine fractal surfaces using a numerical approach. Traditionally, the autocorrelation function and, more recently, the Hurst exponent, which describes the self-affinity, have been used to quantify the height correlation. Here, hundreds of three-dimensional synthetic surfaces parameterized using a root-mean-square (rms) height and a Hurst exponent were generated, and their backscattering coefficient functions were computed to evaluate their consistency with selected analytical models. The numerical results were also compared to empirical models for roughness and radar-scattering measurements of Hawaii lava flows and found consistent. The Gaussian law performed best at predicting the rms slope regardless of the Hurst exponent. Consistent with the literature, it was found to be the most reliable radar-scattering law for the inverse modeling of the rms slopes and the Fresnel reflection coefficient from the quasi-specular backscattering peak, when homogeneous statistical properties and a ray-optics approach can be assumed. The contribution of multiple scattering in the backscattered power increases as a function of rms slope up to about 20% of the backscattered power at normal incidence when the rms slope angle is 46°.

Planetary Radar—State-of-the-Art Review

Planetary radar observations have provided invaluable information on the solar system through both ground-based and space-based observations. In this overview article, we summarize how radar observations have contributed in planetary science, how the radar technology as a remote-sensing method for planetary exploration and the methods to interpret the radar data have advanced in the eight decades of increasing use, where the field stands in the early 2020s, and what are the future prospects of the ground-based facilities conducting planetary radar observations and the planned spacecraft missions equipped with radar instruments. The focus of the paper is on radar as a remote-sensing technique using radar instruments in spacecraft orbiting planetary objects and in Earth-based radio telescopes, whereas ground-penetrating radar systems on landers are mentioned only briefly. The key scientific developments are focused on the search for water ice in the subsurface of the Moon, which could be an invaluable in situ resource for crewed missions, dynamical and physical characterization of near-Earth asteroids, which is also crucial for effective planetary defense, and a better understanding of planetary geology.

Stochastic hyperbola fitting, probabilistic inversion, reverse-time migration and clustering: A novel interpretation toolbox for in-situ planetary radar

Ground-penetrating radar (GPR) is becoming a mainstream tool in planetary exploration, and one of the few in-situ planetary geophysical methods. There are currently three missions (Perseverance, Tianwen-1, Chang’E-4) with GPR-equipped rovers, and two future missions (Chang’E-7, ExoMars) that will include GPR in their scientific payload. The large number of GPR data, combined with the novel setup of the measurements, creates the need for new data processing and interpretation techniques to address the unique challenges of in-situ planetary radar. The current paper proposes an interpretation pipeline that starts with a novel stochastic hyperbola fitting that estimates the probability kernel density of the bulk permittivity at different depths. Subsequently, the bulk permittivity distribution is transformed via a novel probabilistic inversion to a 1-dimensional (1D) permittivity profile. The inverted 1D permittivity profile is then used as an input to a bespoke reverse-time migration (RTM) using the finite-difference time-domain (FDTD) method. RTM using FDTD does not assume a clinical homogeneous half-space; instead, it accounts for the expected layered structure of the investigated medium. Lastly, the migrated radargram is clustered in order to identify subsurface targets and distinguish them from the background medium. Each of the processing steps has never been reported in planetary radar; and together act as a complete processing toolbox tuned for planetary science. The suggested interpretation pipeline is validated numerically in a 1D case study with a complex layered structure and multiple subsurface targets. The proposed processing scheme is then applied to the GPR data from the Chang’E-4 mission at the Von Kármán crater, revealing a previously unseen layered structure and a complex distribution of rocks/boulders.

Spatiotemporal Search for Pulsed Radiation in the Photon Flux Detection Mode with Prolonged Observation Time of the Decomposition Element

The development of science and technologies of microwave frequencies (UHF) has contributed to the fact that the use of ultrawideband (UWB) pulsed signals has emerged as a separate scientific and technical area of radiophysics, requiring a deeper study of the methods of formation, emission, reception and processing of such signals. Aperture radiators are mainly used to transmit UWB signals. This type of emitters is a variety of modifications of mirror antennas, horn antennas of various shapes and profiles, open waveguides, planar and conformal antenna arrays, as well as lens antennas. The classical theory of analysis of radiation and propagation of electromagnetic signals is well known, but its application to the synthesis of UWB emitters in the time domain often becomes too cumbersome and incorrect. Progress in the fields of radar, communications, and radio astronomy in the 1960s stimulated the creation of aperture theory based on physical optics approaches. This theory makes it possible to analyze and calculate the characteristics of mirror radiators and antenna arrays with sufficient accuracy for practical application. It is often possible to use the aperture theory to analyze some horn emitters that have a large radiation surface relative to the spatial dimensions of pulsed signals.

Surface scattering model for dual-polarization planetary radars

This work presents a surface scattering model to interpret radar polarimetry derived from dual-polarization planetary radar systems, such as Arecibo Observatory and the Goldstone Solar System Radar. This model divides surface scattering contributions within a radar echo into quasi-specular and diffuse components, and further divides the quasi-specular component into single- and double-bounce scattering. In particular, the increase in the degree of linear polarization resulting from dihedral double-bounce scattering is emphasized, in addition to the increase in the circular polarization ratio. This mechanism is illustrated in radar observations of young craters on the Moon by the Mini-RF radar on board the Lunar Reconnaissance Orbiter. Arecibo radar observations of 8 near-Earth asteroids are compared to this model and the implications for the physical properties of these objects are discussed. This work provides new insight in interpreting radar polarimetry from dual-polarization planetary radar systems and is broadly applicable to all solar system targets.

Arecibo Planetary Radar Observations of Near-Earth Asteroids: 2017 December–2019 December

We successfully observed 191 near-Earth asteroids using the Arecibo Observatory’s S-band planetary radar system from 2017 December through 2019 December. We present radar cross sections for 167 asteroids; circular-polarization ratios for 112 asteroids based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for 37 selected asteroids using delay-Doppler radar images with a range resolution of 75 m or finer. Out of 33 asteroids with an estimated effective diameter of at least 200 m and sufficient image quality to give clues of the shape, at least 4 (∼12%) are binary asteroids, including 1 equal-mass binary asteroid, 2017 YE5, and at least 10 (∼30%) are contact-binary asteroids. For 5 out of 112 asteroids with reliable measurements in both circular polarizations, we measured circular-polarization ratios greater than 1.0, which could indicate that they are E-type asteroids, while the mean and the 1σ standard deviation were 0.37 ± 0.23. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for 11 S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich objects based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5.

Physical properties of near-Earth asteroid (2102) Tantalus from multiwavelength observations

ABSTRACT Between 2010 and 2017, we have collected new optical and radar observations of the potentially hazardous asteroid (2102) Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory, and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low-amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The convex light curve-inversion model, with rotational pole at λ = 210° ± 41° and β = −30° ± 35°, is more flattened than the two models reconstructed by including radar observations: with prograde (λ = 36° ± 23°, β = 30° ± 15°), and with retrograde rotation mode (λ = 180° ± 24°, β = −30 ± 16°). Using data from WISE, we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at the centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface covered in fine-grained regolith, consistent with radar albedo, and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces.

Radar Observations of Old Centaur Rocket from 1966

Abstract We report the results of radar observations of a near-Earth object discovered on 17 September 2020, with the Pan-STARRS 1 telescope at the Haleakala Observatory in Hawaii. Initially, this object was considered an asteroid and even received the standard provisional designation 2020 SO by the Minor Planet Center. However, its Earth-like orbit and low relative velocity suggested that the object may be of artificial origin, being the Centaur rocket booster from the Surveyor 2 mission that was launched to the Moon on 20 September 1966. In the period from November 2020 to March 2021, this object approached the Earth twice within one lunar distance of the Earth. Radar observations were conducted on 30 November in bistatic mode with the 70-m Goldstone Solar System Radar DSS-14 and 32-m radio telescope RT-32 at the Svetloe Observatory, while the object was in the visibility window of two antennas at about 200 thousand km from the Earth. The main goal of the study was to determine the physical properties of this object using radar astronomy to clarify its origin.

Altimetry Measurements From Planetary Radar Sounders and Application to SHARAD on Mars

Low-frequency radar sounders have the potential to generate altimetric profiles, but the feasibility of utilizing planetary radar sounding data as an alternative to laser altimetry has not been assessed using existing data to date. Therefore, we have developed, implemented, and evaluated an algorithm to process SHAllow RADar sounder (SHARAD) data on Mars (Experiment Data Records as available on the planetary data system) first into altimetry profiles and ultimately into digital terrain models (DTMs). The minimally processed data are pulse compressed, corrected for ionospheric distortion, zero-Doppler filtered, and incoherently summed. We then apply pulse re-tracking techniques adapted from terrestrial ocean altimetry to identify the surface return. From the surface return we compute the time-of-flight and hence the range from the spacecraft to the surface of the planet. The altimetry groundtracks are then co-registrated with Mars Orbiter Laser Altimeter (MOLA) to remove any biases resulting from residual ionospheric effects or timing issues. The altimetric profiles are finally used to create DTMs based on SHARAD data. While the SHARAD altimetry data have coarser inherent resolution than laser altimeters or imaging radars, we demonstrate that radar sounding data is still a viable source for satellite-based altimetry measurements. This is particularly important for future planetary missions not carrying laser altimeters but radar sounders, such as the upcoming Europa Clipper mission.

Polarimetric Portraits

Abstract“Fully polarimetric” in its original radio science usage denotes a receiving instrument's ability to measure the four‐parameter Stokes vector characterizing the polarimetric properties of an electro‐magnetic (EM) field. In contrast, radar remote sensing specialists use “fully polarimetric” to apply exclusively to radars that evaluate the four complex scattering matrix elements. To disambiguate, polarimetric portrait is suggested, defined to be the Stokes vector of an unbiased EM field. For a passive system, this new name applies without qualification to the four Stokes parameters seen through a passive dual‐channel receiver. For an active system, the scene's illumination must have equal weighting between any two orthogonal polarizations to generate a polarimetrically unbiased reflected EM field. The suggested terminology is applicable to a variety of disciplines, including radiometry, alternative polarimetric radar architectures, radar astronomy, weather radar, and GPS/GNSS reflectometry. Polarimetric portraits obtained by dual‐polarized and quadrature‐polarized radars are shown to be equal.

Sic Transit Gloria Mundi [Microwave Surfing

Reports on Arecibo, a national research center supported by the National Science Foundation (NSF). The center focuses on such areas as radio astronomy, planetary radar and upper atmospheric sciences. The Arecibo Observatory’s principal astronomical research instrument is a 305-meter fixed spherical radio/radar telescope, one of the world’s largest radio telescope[s]. Its frequency capabilities range from 50 megahertz to 11 gigahertz. Transmitters include an S-band (2,380-megahertz) radar system for planetary studies and a 430-megahertz radar system for atmospheric science studies and a heating facility for ionospheric research.

Sic transit gloria mundi [Turnstile

Reports on the Arecibo Observatory and discusses the major research initiatives are part of its operations. Arecibo, a national research center supported by the National Science Foundation (NSF), focused on radio astronomy, planetary radar and upper atmospheric sciences…Arecibo Observatory’s principal astronomical research instrument is a 305-m fixed spherical radio/radar telescope, one of the world’s largest radio telescope[s]. Its frequency capabilities range from 50 MHz to 11 GHz. Transmitters include an S-band (2,380-MHz) radar system for planetary studies and a 430-MHz radar system for atmospheric science studies, and a heating facility for ionospheric research.

History of Antenna Technology at the Arecibo Observatory in Arecibo, Puerto Rico [Historically Speaking

The Arecibo Observatory first opened in 1963 and has been a marvel of engineering ever since. It has been a monumental instrument for scientific research in the fields of astronomy, planetary radar, ionospheric probing and high-frequency (HF) heating modification, and optical probing of the atmosphere.

The Next-Generation Ground-Based Planetary Radar

Whitepaper #434 submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032. Topics: planetary defense; other planetary bodies: cross-cutting/bodies with solid surfaces; ground- and space-based telescopes

Parametric Translational Compensation for ISAR Imaging Based on Cascaded Subaperture Integration With Application to Asteroid Imaging

Translational compensation is a crucial step in inverse synthetic aperture radar (ISAR) imaging. However, nonparametric compensation methods may collapse under the condition of a low signal-to-noise ratio (SNR), and high-order parametric compensation methods usually have a large computational load. In this article, two cascaded integration methods are proposed to solve the problems above based on the generalized Radon-Fourier transform (GRFT). The first method models the translational motion as polynomial and utilizes the proposed subaperture GRFT (SAGRFT), which is a fast implementation of the GRFT, to estimate the translational parameters. The SAGRFT divides the full aperture into several subapertures and realizes coherent integration by implementing moving target detection (MTD) within subapertures and the GRFT among subapertures. In addition, the distribution property of the blind speed side lobes (BSSLs) generated by the SAGRFT is analyzed. On this basis, the second method based on metaheuristic algorithms is proposed to further accelerate the parameter estimation process and solve the BSSLs problem. The proposed methods can not only play a role in stealth target imaging but also be utilized in near-Earth asteroid (NEA) imaging in radar astronomy. Finally, the numerical simulations of asteroid imaging and the experimental results of plane imaging are demonstrated to verify the performance advantages of the proposed methods.