Distributed SAR imaging from space exploits the distribution of the key system resources, normally concentrated in a single, large and complex satellite, among many small-sized and simpler sensors, thanks to the proper combination of the signals from each single node of the swarm. The simultaneous operation of the satellites represents a Multiple-Input-Multiple-Output SAR system (MIMO-SAR), for which several concepts have been presented in the scientific literature, [1]–[6].Indeed, the theoretical maturity of this technique is significant, however the practical feasibility has not yet been demonstrated. In 2020, the demonstrative Mission SATURN [7] was proposed and is currently being studied with the support of the Italian Space Agency (ASI) being part of ALCOR programme promoting the development of the next generation Italian CubeSats. The main target of the SATURN mission is to demonstrate the key technology "Cooperative MIMO Swarms of SAR MicroSats" for innovative, low cost and versatile Earth Observation applications.In this work, we present the architecture and the implementation of a miniaturized SAR payload, called MiniSAR, suitable for operation in a MIMO SAR system. The paper reports the design solutions, the architecture and the qualification of the SAR Electronics by means of functional, performance and environmental testing in thermal-vacuum chambers and vibration facilities.

Pits are depressions in the ground that occur due to the collapse of the surface layer. The characterization from orbit of their internal structure using optical images is challenging due to uncontrolled illumination geometry. In this paper, we propose a methodology for the characterization of pits’ interiors by exploiting Synthetic Aperture Radar (SAR) images. The methodology analyzes the amplitude and range of the radar echoes originating from the pit’s interior for determining its geometric characteristics through data inversion. The experimental results demonstrate that a set of bright reflections in the radar image can be attributed to the response of pits’ vertical walls and floor. By applying the proposed methodology and interpreting the radar reflections, we are able to derive a geometric characterization (e.g., depth) of a given pit. The retrieved geometric parameters from SAR data of a terrestrial pit denoted asWell of Barhoutalign well with the ground truth. The findings of this study have implications for both Earth observation and planetary exploration.

The comprehension of pit craters on Venus’ surface is still limited both in terms of genetic process and geometric characteristics. Pit craters are valuable features for planetary scientists and geologists as they offer a window into a planetary body’s history, geological processes, and environmental conditions. In this context, morphometry is a valuable tool for unraveling the geological history of these structures and for comparative investigations with analogous features occurring on other celestial bodies. In this study, we present a methodology utilizing orbital synthetic aperture radar (SAR) data to estimate the characteristics of Venusian pits. Our findings demonstrate that it is feasible to estimate the depth of a given pit crater by analyzing reflections from the pit’s wall facing the radar illumination direction. This method offers an additional estimate of the pit’s depth, complementing literature techniques based on radar shadows, which may not always be available depending on the acquisition geometry. Depending on the configuration of the pit’s interior structure, the depth estimation derived from the pit wall’s reflections is potentially less prone to geometric distortions than the one obtained from the radar shadow. The morphometric analysis of Venusian pits, based on the radar-derived parameters from Magellan SAR data, reveals that these pits exhibit very large collapse volumes and remarkable morphological similarities with lunar pit craters. This discovery has implications for comparative planetology, shedding light on potential shared mechanisms and processes governing the formation and evolution of such features.

Circular sinkholes (i.e., pits) are depressions in the ground caused by the collapse of the surface layer. These type of features are commonly found on Earth and on other celestial bodies such as the Moon and Mars. Sinkholes may provide access to an underground cave system but it is very difficult to understand their accessibility and internal shape from optical images. In this paper, we propose a methodology for characterizing circular sinkholes by exploiting X-Band Very High Resolution (VHR) Spaceborne Synthetic Aperture Radar (SAR) images. The analysis of our experimental results show that a set of bright reflections in the radar image can be attributed to the response of the circular pit vertical walls and interior. Accordingly, it is possible to characterize the internal morphology of the pit. The results of this work have implications for planetary exploration as the proposed methodology can be applied for characterizing planetary pits.

Lava tubes are terrestrial tunnel-like natural subsurface caves. Mounting evidence suggests their presence on the Moon and Mars. Planetary radar sounders are nadir-looking instruments operating in the high-frequency (HF)/very-HF (VHF) part of the spectrum with subsurface penetration capabilities. Recently, several studies either proposed future mission concepts for lava tubes’ detection or attempted to locate them on the Moon and Mars with the available radar-sounding data. Lava tubes are typically modeled as quasi-cylindrical structures but their actual geometry and their influence on the radar backscattering in near-nadir regime have never been investigated in the literature. These are crucial information for understanding the feasibility of detecting lava tubes by current and future planetary radar sounding systems. Accordingly, in this article: 1) we assess whether lava tubes are self-affine fractal surfaces at horizontal scales relevant to radio and microwave scattering and 2) we evaluate the effect of lava tube topography on the radar backscattering response in the near-nadir regime. Our experimental results, which are inferred from 3-D terrestrial laser scanning (TLS) data of planetary lava tube analogs, show that lava tubes: 1) are self-affine fractals at horizontal scales relevant to radar sounding and 2) they are electromagnetically rough surfaces, especially in the VHF band. We provide quantitative values on the lava tube fractal parameters and radar roughness losses along with a discussion on both: 1) the implication of our results on current radar sounding systems’ ability to detect lava tubes and 2) the planning of future missions devoted to lava tube detection and characterization.

Caves are one of the last frontiers of human exploration on Earth. They are very relevant scientific targets as they host significant biodiversity and unique geologic formations. The presence of underground passages accessible for human or robotic exploration are revealed by localized collapse of the near-surface ceiling of a cave system (skylight). Remote sensing systems are a valuable tool for skylights detection as these features are often located on very remote and often inaccessible regions of the Earth. However, with the available remote sensing techniques and data analysis methodologies, it is very difficult to determine whether a skylight is providing access to a cave continuation or it represents only a closed depression with no extensions. In this article we propose a methodology, based on very high-resolution (VHR) orbital synthetic aperture radar (SAR) imaging systems, to estimate both caves geometric characteristics and accessibility information in the proximity of a skylight. To test our methodology, we acquired radar data over different Earth’s location by exploiting the Capella Space X-band microsatellite radar constellation. The experimental results show that our methodology effectively determines the caves geometric characteristics and accessibility under a variety of surface conditions. We also detected several unknown and unexplored large cave systems located near Volcan Wolf and Ecuador, Isla Isabela, Galapagos. The presented work has relevant implications for the field of geological studies, ecology, and space exploration research since optical imaging shows the evidence of potential cave systems accessible from skylights on other planetary bodies such as Mars.

Cave systems are one of the last frontiers of human exploration on both Earth and other celestial bodies. In this context, lava tubes are natural subsurface tunnels, which are not visible from the surface, that are ubiquitous on Earth as well as on the Moon and Mars. Skylights are one of the surface evidences of the presence of such conduits in the form of overhanging collapses of the cave ceiling, making these cavities partially observable and potentially accessible. In this paper, we propose a method for imaging and characterizing subsurface structures by spaceborne VHR SAR imaging. We performed several acquisitions over different Earth locations by exploiting Capella's X-band VHR SAR imaging radars in spotlight mode. The obtained results show that the proposed methodology is able to characterize the main geometric parameters of the first section of a lava tube (e.g. width, height) in the surroundings of a skylight and it provides an indication of the actual subsurface accessibility. The proposed methodology has several important implications for exploration and could be applied to different type of cavities other than lava tubes.

Spaceborne radar sounders are nadir-looking sensors devoted to subsurface investigations. The data interpretation of these sensors can be severely hindered by clutter originating from surface off-nadir reflections. Recently, the concept of distributed radar sounding has been proposed for synthesizing a narrow radar antenna beam with clutter suppression capability. The antenna beam is effectively synthesized by deploying an array of orbiting sensors in formation flight. In this paper, we assess the capability of distributed radar sounding to further discriminate clutter from subsurface returns by exploiting Direction of Arrival (DOA) estimation techniques. Accordingly, we propose an approach to design and evaluate the distributed radar sounder DOA estimation performance. The theory is complemented by radar simulations of several acquisitions over Greenland. The simulations confirm that clutter discrimination through DOA estimation is an effective approach for further improving the array capability in disam-biguating subsurface echoes from surface ones.

Several studies are in progress for proposing an Earth orbiting radar sounder (EORS) mission. Some of them consider as baseline system architecture a recently proposed distributed radar sounding array in formation flight with enhanced capabilities of clutter suppression. Besides clutter, the detectability of subsurface targets may also be affected by the propagation of the radar signal through Earth’s ionosphere. These effects include frequency-dependent phase dispersion and scintillations. In this letter, we present a subsurface detection performance assessment of an EORS with distributed architecture focusing on the ionospheric effects. The novel contributions of this work are: (i) simulation of the coherent radar response of a representative polar ice target (englacial layering) in the distributed radar sounding case; (ii) inclusion of spatially-dependent ionospheric scintillation effects on the distributed beam pattern; (iii) inclusion of phase dispersion effects for different values of total electron content (TEC), and (iv) analysis of the subsurface detection performance. Detectability analysis is performed after applying a state-of-the-art technique for compensating ionospheric phase-dispersion effects. The results show that the englacial layering is detectable by compensating the dispersion effects in the range between 1 and 21 TECU in the ionosphere. The layering is also detectable at higher values of TEC by improving the accuracy of TEC estimation. Moreover, even without compensation, the worst-case ionospheric phase scintillations of 25° produces a negligible effect on the detectability.

Research based on Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data detected unusual radar bright basal reflections located at about 1.5 km depth in a Mars region denoted as Ultimi Scopuli. These reflections were interpreted as a signature of subglacial liquid water even though this interpretation is still being debated in the literature. In this article, we propose a novel approach to the detection and imaging of candidate subglacial liquid water from radar sounding data. The approach combines the radar echo power traces with a suitable digital elevation model to provide a bidimensional representation of the surface. Even if the imaging method reconstructs a representation of the surface, we prove that it can be used to identify subsurface bright reflections in icy regions. Imaging is feasible even if the basal interface is not directly included in the processed data for image generation. To support this experimental evidence, we show that a relationship exists between the value of the reflected echo power originating from the englacial layers and the basal-to-surface-echo-power ratio. The observed relationship holds on both Ultimi Scopuli radar sounding data acquired on Mars and Lake Vostok data acquired on Earth. Our results show that the 2-D imaging provides an alternative way for locating candidate subglacial liquid water bodies on Mars over large areas also where the basal interface is not directly measured. The proposed approach complements previous research for further evaluation of the actual presence of liquid water on Mars.

Spaceborne radar sounders are High Frequency (HF) / Very High Frequency (VHF) nadir-looking sensors devoted to subsurface investigations. Their data interpretation can be severely hindered by off-nadir surface clutter. Recent literature showed that the clutter suppression capabilities of this class of systems can be greatly enhanced by deploying an array of orbiting sensors in formation flight synthesizing a narrow radar antenna beam. In this paper, we assess the capability of distributed radar sounding to discriminate clutter from subsurface returns by exploiting Direction of Arrival (DOA) estimation techniques. This is achieved by first outlining an approach for designing and evaluating the distributed radar sounder DOA estimation performance as function of the radar system parameters (e.g, inter-sensor distance) and external noise factors such as ionospheric scintillations. Then, the theory is complemented by radar simulations of several acquisitions over Greenland assuming a variety of subsurface geometries. The simulations confirm that clutter discrimination through DOA estimation is a viable approach to further improve the array capability in disambiguation of subsurface echoes from surface ones.

Lava tubes are buried channels that transport thermally insulated lava. Nowadays, lava tubes on the Moon are believed to be empty and thus indicated as potential habitats for humankind. In recent years, several studies investigated possible lava tube locations, considering the gravity anomaly distribution and surficial volcanic features. This article proposes a novel and unsupervised method to map candidate buried empty lava tubes in radar sounder data (radargrams) and extract their physical properties. The approach relies on a model that describes the geometrical and electromagnetic (EM) properties of lava tubes in radargrams. According to this model, reflections in radargrams are automatically detected and analyzed with a fuzzy system to identify those associated with lava tube boundaries and reject the others. The fuzzy rules consider the EM and geometrical properties of lava tubes, and thus, their appearance in radargrams. The proposed method can address the complex task of identifying candidate lava tubes on a large number of radargrams in an automatic, fast, and objective way. The final decision on candidate lava tubes should be taken in postprocessing by expert planetologists. The proposed method is tested on both a real and a simulated data set of radargrams acquired on the Moon by the Lunar Radar Sounder (LRS). Identified candidate lava tubes are processed to extract geometrical parameters, such as the depth and the thickness of the crust (roof).

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.

This paper presents the SaTellite RAdar sounder for earTh sUbsurface Sensing (STRATUS), which is a satellite mission for Earth Observation (EO) with an onboard instrument capable of probing the Earth's subsurface in polar and arid regions. STRATUS is based on an innovative distributed radar sounder (RS) with the unique capability to obtain continuous and large-scale subsurface measurements, with homogeneous and consistent quality in two of the least characterized and crucial frontiers of Earth: globally on the polar ice sheets, i.e., Greenland and Antarctica (primary objective), and regionally on the arid areas and deserts. STRATUS is a ground-breaking exploratory mission addressing crucial scientific questions. It provides new fundamental data that have not been acquired by any other past or present remote sensing mission on the Earth, with an expected high and genuine scientific return enabling the assessment of the climate change signature in the Earth subsurface.

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.

Radar sounder surface imaging is a rather unexplored approach to the analysis of planetary bodies. While a radar sounder is an instrument specifically designed for subsurface investigations, a particular set of power measurements (denoted as off-nadir surface echoes) can be exploited together with an external digital elevation model to produce images of the investigated surface at meters wavelength. The use of the off-nadir data may also reveal the presence of previously undetected subsurface features. In this article, we present a method for producing surface roughness images by high-frequency (HF) radar sounder data. The study of surface roughness in the HF band is particularly useful for both geologic studies and landing-zone reconnaissance as it is evaluated at meters to hundreds of meters horizontal scale. The proposed method combines off-nadir data of the Shallow Radar Sounder (SHARAD) with the Mars Orbiter Laser Altimeter (MOLA) digital elevation model. The produced roughness images at 20 MHz (15-m wavelength) of the Martian surface provide higher coverage and resolution of the surface roughness characterization at a 10-100-m horizontal scale than previous SHARAD work. By comparing the experimental roughness image with the one obtained by radar backscattering simulations, it is possible to identify subsurface features. In our experiments, we were able to produce a bidimensional image of a previously undetected large buried crater (10 km ×12 km) located in the Nili Fossae. This finding opens up new possibilities in exploiting radar sounding data for better detecting shallow subsurface features.

This paper presents the Subsurface Radar Sounder (SRS) instrument onboard European Space Agency's (ESA) EnVision mission. EnVision is one of the three candidates selected for the Cosmic Vision 2015-2025 M5 medium-class missions. It is aimed at exploring the activity, the geologic history and the atmosphere of Venus. SRS is an orbital ground-penetrating radar with the unique science objectives of understanding the evolution of Venus' surface by searching for subsurface dielectric interfaces in the top hundreds of metres of the crust. In the paper, we describe the main science objectives of SRS, the performance evaluation under expected target conditions, the instrument design and the acquisition strategy that maximize the scientific returns.

Spaceborne radar sounders are nadir-looking sensors operating in the high frequency (HF) or very high frequency (VHF) bands with subsurface sensing capabilities. Due to technological limitations, this type of sensors often deploys omnidirectional antennas. This results in undesired artifacts such as off-nadir clutter which could hinder data interpretation. Very recent technological advancements open up the possibility of synthesizing very large antenna apertures in HF/VHF band by using small satellites array deployed in suitable orbital formation flying. Accordingly, in this study, we propose a novel concept of distributed radar sounder system. The proposed concept is complemented with a mathematical model for performance prediction which takes into account the uncertainty on the position of the sensors. Moreover, we discuss possible orbital solutions for the problem of the deployment of the distributed radar sounder system. The results show that a distributed radar sounder operating in small satellites formation flying is particularly appealing as it can: 1) substantially reduce the impact of surface clutter; 2) increase the across-track resolution; 3) increase the signal-to-noise ratio (SNR) (or, alternatively, decrease the overall required transmitted power with respect to a traditional single configuration radar sounder design); and 4) provide large flexibility in the data processing of the signals acquired by the different sensors.

Spaceborne radar sounders are nadir-looking sensors operating in HF/VHF bands. Their relatively low frequency allows the penetration of the transmitted signals into a given subsurface to infer its geoelectrical properties and composition by analyzing the radar echoes. These type of sensors are affected by unwanted artifacts such as off-nadir clutter mainly resulting from the constraint on the type of deployable antenna which is very often a dipole. Very recent technological advancements open up the possibility of synthesizing very large antenna apertures in HF/VHF band by using small satellites array deployed in suitable orbital formation flying. Accordingly, in this study we propose a novel concept of distributed radar sounder system. The proposed concept is complemented with a mathematical model to predict its performance. The results show that the distributed radar sounder operating in small satellites formation flying is particularly appealing as it can (i) drastically reduce the impact of surface clutter, (ii) increase the across-track resolution, and (iii) increase the signal to noise ratio (or, alternatively, decrease the overall required transmitted power with respect to a traditional radar sounder design).

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.

Lunar lava tubes are buried channels that contained thermally insulated lava during the volcanic period of the Moon. Nowadays, they are believed to be empty and thus, identified as potential habitats for humans. In recent years, numerous studies investigated the possible locations of these tubes by taking into account the distribution of gravity anomalies and the volcanic features of the surface. In this paper, we model lava tubes according to their electromagnetic behavior, and we propose a novel approach to locate lava tubes and estimate their physical properties. The method analyzes the subsurface reflections stored in radargrams to extract the desired features automatically. Then, these features and their relationships are processed by a fuzzy rule-based system to detect the presence or absence of lava tubes. The strategy was implemented and successfully tested on simulated radargrams with various surface properties and tunnel dimensions.

Reliable electromagnetic simulators are of prime importance for the design of radar sounder instruments and for supporting the subsequent interpretation of their data. In this paper we present a coherent simulator based on the facet method that can compute radar echoes from the subsurface of a target area with an arbitrary number of geological layers, thus going beyond the mere 1- or 2-layer descriptions usually modelled by coherent ray-tracing radar sounder simulators. The simulator has been validated using real radar data of lunar areas characterized by a multilayer nature collected by the Lunar Radar Sounder (LRS) instrument of JAXA's Kaguya probe. Results confirm the effectiveness of the proposed simulator.

Spaceborne low-frequency and wide bandwidth radar sounders are a promising technology to regularly investigate at global-scale Earth's icy and arid regions. However, Earth ionosphere distorts the radar signal impacting performance parameters, such as subsurface resolution, of the radar system. One of the most relevant distortions that a sounder signal in the lower part of the very high-frequency (VHF) band (e.g., 40-50 MHz) encounters is the distortion of the phase component that could become mission critical if not properly compensated. Low-frequency and high fractional bandwidth radar systems are particularly affected by this issue. Previous works on radar sounder ionosphere phase distortion compensation addressed the Martian ionosphere and used techniques based on the Taylor series expansion. In this letter, we focus on the Earth ionosphere and we exploit a recently proposed ionosphere compensation technique based on the Legendre orthogonal polynomials expansion, which proved to be more accurate than the compensation based on Taylor expansion. Simulations show that the method allows a nominal compensation of the phase distortions under realistic ionosphere scenarios expected during the acquisitions. Furthermore, it proved to be accurate and robust for total electron content conditions expected during nighttime for all the geomagnetic latitudes. The results confirm that the method can accurately compensate the distorting effects on the phase component of a spaceborne VHF radar sounder.

Reliable electromagnetic simulators are of prime importance for the design of radar sounder instruments and for supporting the subsequent analysis of their data. In this paper, we present a coherent, facet method-based simulator that can compute radar echoes from the subsurface of a target area with an arbitrary number of geological layers, thus going beyond the surface-only or the two-layer descriptions so far implemented in coherent ray-tracing radar sounder simulators. Propagation of fields throughout the subsurface is computed according to Snell's law following a ray-tracing approach. For each ray interacting with the surface, be it a direct reflection or a refracted ray coming from the subsurface, the phase contribution of each facet is calculated through the linear phase approximation, while the total field received at the antenna is computed using Huygen's principle. Validation simulations have been performed against the radar data of lunar and martian areas characterized by a multilayer nature and collected by the Lunar Radar Sounder instrument of JAXA's Kaguya lunar probe and the Shallow Radar instrument onboard NASA's Mars Reconnaissance Orbiter, respectively. Results confirm the effectiveness of the proposed simulator.

Radar sounders are unique instruments for subsurface investigation in both terrestrial and space applications. They are widely employed for monitoring changes to the polar ice sheets and for the study of planetary bodies (e.g., Mars). The analysis of the very large amount of data produced by such systems requires the development of automatic techniques for an objective, accurate, and fast extraction of relevant information from radargrams. In this paper, we propose a novel technique for the automatic detection of layer boundaries based on a local scale hidden Markov model (HMM), which models the radar response in the presence of a layer boundary, and the Viterbi algorithm (VA, which performs the inference step). The proposed technique is based on a divide and conquer strategy that executes the VA using the observation data and the HMM to infer the most likely layer boundary location within a small radargram portion. Finally, a detection strategy is defined to chain together the inferred local layer locations. Furthermore, a novel radargram enhancement and denoising technique tailored to support the detection step is presented. The effectiveness of the proposed technique has been confirmed by testing it on different radargrams acquired by shallow radar over the north pole of Mars. The results obtained point out the superiority of the proposed method in retrieving the position of each layer boundary (and thus of the related intensity and geometric properties) with respect to the state-of-the-art techniques.

In spaceborne radar sounder systems, the constraint on data storage imposes an upper bound to the radar receiving window length. Accordingly, an adaptive tracking procedure capable of shifting the receiving window in time according to the topography and spacecraft orbit variations is needed for maximizing the amount of signal acquired from the subsurface. Moreover, spaceborne sounder systems usually have constraints in terms of number of acquisitions and power consumption. Thus, tracking robustness and simplicity are mandatory requirements. In this paper, we present an adaptive range tracking technique derived from a combination of OCOG (Offset Center Of Gravity) leading edge estimation and a-p1filter. It gives significant improvements in the loss of lock detection. The performances of the overall technique are evaluated by different simulations of meaningful application scenarios which confirm its effectiveness.