D. Calabrese

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

COSMO-SkyMed: Image quality achievements

COSMO-SkyMed is the Italian Remote Sensing program which foresees the utilization of a constellation of four SAR Satellites in Low Earth Orbit, dedicated to the management, control and exploitation of Earth resources for civil and defense applications. The Program is completely funded by the Italian Government, mainly through the Italian Ministry of Research (MIUR) and the Ministry of Defense (I-AD). Thales Alenia Space Italia is responsible for the design, development and verification of the complete constellation. The complete constellation has been deployed in orbit (June 2007, December 2007, November 2008, and October 2010), the commissioning and the operative qualification (at satellite and constellation levels) with the achievement of full performance has been completed, too. From the very early days of the mission images were generated as nominal products by both the operational systems and the calibration&validation (CALVAL) facility. The data takes and level 1 products were analyzed to serve the main goal of the commissioning phase: the provision of calibrated and verified SAR products to the defense, civil institutional, commercial and scientific user community. The results of the statistical and individual product analysis added up in some instrument parameter adjustment and processing parameter optimization steps; these were performed in the a few ground segment processor releases of which the last took place at the conclusion of the operative qualification. From there on, all data takes and SAR products from spotlight (ES), stripmap (HIMAGE), alternating polarization (PP) and scansar (WR and HR) modes are considered as “operational” in terms of commanding, processing, product format and performance. This paper presents the different product types and the achieved product performance parameters. While product characterization and calibration measurements started early, the final SAR verification had to wait for the complete verification of all input products (specifically the orbit and attitude data) and the finalization of the overall SAR system performance and verification. Thus more than thousands “operational” products of all modes and variants have been used for final verification of basic characteristics like format, saturation degree, scene location and extent. A subset of specific acquisitions over calibration sites were processed to various product variants and analyzed with respect to point target responses and radiometry. These measurements confirmed the very excellent COSMO-SkyMed product performance in terms of: • focusing quality and resolution: since the overall SAR system performance and stability are outstanding, some margins in the spotlight azimuth beam steering sequence and processing have been invested in an improved IRF performance. This yields ratios to spatial resolution in azimuth and ground range, over an access region [20° ‥ 60°], better than 1m • sensitivity: the NEσ° is always better than the specified value (-22dB) for all the operation modes as well as the ambiguity to signal ratios • radiometry: the calibration effort, the pointing accuracy and the operational pattern correction and normalization result in products with a radiometric accuracy better than 1 dB.

Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): subsurface performances evaluation

According to the Mars Express mission, the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper pot-lions of the crust of Mars. Three secondary objectives are also defined subsurface geologic probing, surface characterization, and ionosphere sounding. In order to obtain the primary objectives the Radar Sounder design was based on the Ice/water interface and Dry/ice interface scenario: defining the material composition of the first layers and porosity and the pore filling materials. Concerning the surface, we have characterized the geometric structure in terms of a large-scale morphology, on which a small-scale geometric structure, due to rocks, is superimposed, taking into account also that recently the structure of the planets surface was described by means of fractals and in particular the new MARS surface models obtained by processing of the MOLA data. According to these models, this paper provides a description of the operational planning approach and expected performances of MARSIS.

MARSIS data inversion approach: Preliminary results

An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The constraints, due to the known geological history of the surface, the local temperature and the thermal condition of the observed zones and the results of other instruments on Mars Express and other missions to Mars, have to be considered to improve the validity of the utilized models and the obtained results that are given in parametric way.

MARSIS Data Inversion Approach

In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express. The inversion process allows the dielectric constant of the subsurface material to be estimated provided the dielectric constant of the surface is known. In addition, if impurity are present, it is possible to estimate the dielectric constant of any inclusions as well as the percentage amount of material in the inclusions relative to the host material provided knowledge of the host material up to the depth where the interface has been detected is available. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The data inversion has been performed at several frequencies in order to estimate the frequency dependent parameters affecting the behavior of the radar echoes. It is necessary that the surface and subsurface interfaces have the same roughness in order to estimate the Subsurface Fresnel reflectivity. As a preliminary approach, only flat surface have been selected. MOLA (Mars Orbiter Laser Altimeter) has already provided detailed data on the visible Martian surface and a simulator, with a facet model, has been utilized to use MOLA data in order to verify the correct selection of the frames that will be used for the data inversion (absence of clutter echoes).

COSMO-SkyMed: Calibration & validation resources and activities

COSMO-SkyMed is the largest Italian investment in Space Systems for Earth Observation, commissioned and funded by Italian Space Agency (ASI) and Italian Ministry of Defense (MoD). It consists of a constellation of Low Earth Orbit mid-sized satellites, each carrying a multi-mode high resolution Synthetic Aperture Radar (SAR) instrument operating at X-band and a full featured Global Ground Segment to properly exploit space capabilities. In the framework of COSMO SkyMed mission, the CALVAL represents the system element that performs the calibration activities and the assessment of the image quality. In this paper main characteristics, driving requirements, technical choices and CALVAL first results are described.

Mars ionosphere data inversion by MARSIS surface and subsurface signals analysis

According to the Mars Express mission , the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Moreover three secondary objectives are defined for the MARSIS experiment: subsurface geologic probing, surface characterization, and ionosphere sounding.

New concepts and innovative solutions of the COSMO-SkyMed “Seconda Generazione” system

COSMO Second Generation (CSG) system has been conceived, according to the requirements stated by ASI and I-MoD, at the twofold need of ensuring operational continuity to the currently operating “first generation” COSMO-SkyMed (CSK) spacecraft constellation, while achieving a generational step ahead in terms of functionality and performance. The improved quality of the imaging service is among the foremost characteristics of CSG (see [5]), providing the End Users with new/enhanced capabilities in terms of higher number of images and increased image quality (i.e. larger swath, and finer resolution) with respect to COSMO-SkyMed (first generation) spacecrafts currently in operation, along with additional capabilities (e.g. full polarimetric SAR acquisition mode). The greater operative versatility in system resources management is one of the key aspects of the design approach. Some examples are the satellite agility both at platform and antenna level, used to increase the density of acquisitions in a fixed region, the capability to adapt operative profiles to system environment (e.g. sun illumination, downlink scenarios), service request planning process not only function of the priority but also able to maximize the system resources exploitation.

SHARAD design and operation

This paper describes the operating principles and the design of the Mars shallow radar sounder (SHARAD), an HF sounding radar devoted to the mapping of sub-surface features of Mars and currently operational on board the NASA/JPLs Mars reconnaissance orbiter spacecraft. Compared to its predecessor MARSIS, currently operating from ESAs Mars Express, SHARAD is characterised by an higher carrier frequency (20 MHz vs a max of 5 MHz for MARSIS) and a much wider signal bandwidth (10 MHz vs 1 MHz of MARSIS). This allows SHARAD to achieve a finer range resolution (15 metres unweighted in free space) at the expenses of ground penetration, which makes the instrument ideal to probe the shallow subsurface layers (to depths of hundreds of meters) which cannot be resolved from the surface by the much far- reaching MARSIS. SHARAD uses a 85 usec chirp signal with a PRF of 700 Hz and a peak power of 10 W, and radiates by means of a 10 meters fiber foldable tube (FFT) dipole antenna, with a wide-band matching network in charge of impedance matching with the transceiver. The most challenging requirement(especially considering the large fractional bandwidth of the system) is the level of the range sidelobes, which shall be below -55 dBc after the 6th lobe, to allow proper detection of the weak subsurface echoes in presence of strong surface returns. On this side, the design takes advantage from the wide download bandwidth made available by the MRO Spacecraft to keep the on-board processing to a minimum level (basically, only a programmable coherent presuming), and leave most of the processing (range compression and synthetic aperture) on ground. In this way it is easy to use Tx chirps and Rx transfer functions characterised on- ground as reference for range correlation, with the range sidelobes limited, basically, only by the stability of the RF hardware. The limited amount of on-board processing also helped in limiting the complexity of the instrument design and, therefore, its mass and power consumption. SHARAD uses a very simple architecture, with the transmit chirp generated directly on the RF frequency (using a digital chirp generator) before being amplified to the transmit level by a class C amplifier. The receiver is even more essential, providing direct amplification of the received signal (with programmable gain and band filtering) to an A-to-D converter operated in downsampling mode by digitising the signal at 26.6 MHz rate. In this way, the complete 10 MHz signal bandwidth can be represented unambiguously with only an acceptable amount of oversampling (30%) minimising the required hardware. Instrument control and processing tasks are performed by the same AD-21020 DSP (with the help of a couple of FPGAs). The presuming can be varied from 1 to 32 in powers of two steps, and the resolution of science data can be selected to be 4, 6 or 8 bits, to allow optimisation of the data rate vs the operating scenarios. The receive window position can either be controlled in open loop, using an a priori knowledge of S/C orbit and surface topography (which demonstrated to be a very robust approach) or in closed loop.

Discrete Stepped Strip (DI2S) for multi-swath acquisitions

The Discrete Stepped Strip (DI2S) technique introduces an innovative method to use a SAR in time sharing improving the system capability and flexibility. In this paper, the approach used by DI2S Multi-Swath technique will be described highlighting the main advantages in terms of performance and application.