1.1. Experiment Overview
 Exploration from orbit and by rovers shows that Mars has undergone dramatic shifts in its climate and geologic regime over time. The most important of these is the transition from an epoch of abundant surface water after the end of heavy bombardment to a cold, dry environment. In this transition, much of the surface water was trapped as polar caps, circum-polar layered deposits, and extensive ground ice. The record of changing climate and geologic processes is preserved in the subsurface as layering associated with water-deposited sedimentary rocks, volcanic sequences, and the seasonal and long-term obliquity-driven movement of volatiles. Orbital remote sensing that probes to depths of hundreds of meters to kilometers offers the only practical means for studying such layering over large areas.
 Electromagnetic remote sensing typically probes to greater depths in a target medium as the illuminating wavelength increases (frequency decreases). To penetrate typical near-surface crustal materials and characterize layers at significant depth, the active signal must have a wavelength of several meters or more. Radio echo (radar) sounding is a technique that has been shown to reveal subsurface layers when used at the surface of the Earth (Ground Penetrating Radar), from aircraft, and from orbit around the Moon [Peeples et al., 1978]. Radar sounding at frequencies of tens to a few hundred MHz has been used to study the subsurface characteristics of the Earth's ice sheets, which can be many kilometers in thickness [e.g., Holt et al., 2006a].
 The SHARAD (SHAllow RADar) radar sounder provided by Agenzia Spaziale Italiana (ASI), now in orbit aboard the Mars Reconnaissance Orbiter (MRO) [Zurek and Smrekar, 2007], is designed to detect dielectric contrasts associated with geologic layering on vertical scales of 15 m or better and to probe to typically subkilometer depths. SHARAD will complement the spatially coarse, but deep sounding of the MARSIS instrument on Mars Express [Picardi et al., 2005] (see section 2.2). The SHARAD radar emits electromagnetic (EM) waves from its 10-m dipole antenna, and measures the reflections from both the Martian surface and subsurface. Waves that are transmitted into the subsurface may reflect from dielectric interfaces and return to the instrument at greater time delay than the surface echo. A two-dimensional picture (a “radargram”) of the surface and subsurface is built up in one direction by time delay and in an orthogonal direction by motion of the MRO spacecraft (S/C) along its orbit (see Figures 3, 4, and 8). The primary obstacle to the identification of subsurface echoes is the interference from off-nadir surface echoes (known as “surface clutter”) that arrive at the radar receiver with the same time delay as subsurface echoes. This can be mitigated by various methods discussed in section 8.
 SHARAD operates with a 20-MHz center frequency and a 10-MHz bandwidth, which translates to a vertical resolution of 15 m in free-space and 15/√ɛ m in a medium of relative permittivity ɛ. The transmitted signal is a 10 W, 85-μsec chirped (linear FM) pulse emitted from a 10-m-long dipole antenna that is used for both transmitting and receiving. Horizontal surface resolution depends on surface roughness characteristics, but for most Mars surfaces the cross-track footprint is 3–6 km and the along-track footprint, narrowed by synthetic aperture processing on the ground, is 0.3–1 km. The returned signal is recorded as a time series of complex-valued voltages. The pulse repetition frequency (PRF) over-samples the Doppler spectrum, allowing for coherent integration of pulses on board the spacecraft, while still allowing for Doppler focusing in ground data processing. Indeed, except for onboard presumming, all of the data processing will take place on the ground, including range and Doppler focusing of the chirp signals, and calibration of the processed data.
 Data will be processed at the SHARAD Operations Center (SHOC) of Alcatel Alenia Spazio (AAS) in Rome, Italy, under contract to ASI and under the guidance and control of the SHARAD science team. Data will be distributed to the community at large from the ASI Science Data Center (ASDC) in Frascati, Italy, and from the Geosciences Node of the Planetary Data System at Washington University in St. Louis, USA (WUSTL-PDS).
 The Italian portion of the SHARAD team was originally appointed by ASI, with Roberto Seu of Università di Roma “La Sapienza” as the Team leader. Subsequently, three American scientists were selected by NASA in response to an AO, with Roger Phillips of Washington University appointed Deputy Team Leader. US Participating Scientists likely will be added to the team. The full list of team members is given in Table 1, and includes Ali Safaeinili, the Instrument Scientist at JPL. The SHARAD team provided the instrument requirements to the prime contractor, AAS, and closely monitored the development of the hardware to ensure that the requirements were met.
|Roberto Seu||Team Leader/Radar Performance and Data Processing||Università di Roma “La Sapienza”|
|Roger Phillips||Deputy Team Leader/Mission Science Strategy and Interpretation||Washington University in St. Louis|
|Daniela Biccari||Mission Operations||Università di Roma “La Sapienza”|
|Roberto Orosei||Data Products||IASF, Istituto Nazionale di Astrofisica|
|Ali Safaeinili||Instrument Scientist||Jet Propulsion Laboratory|
|Arturo Masdea||Calibration and Commissioning||Università di Roma “La Sapienza”|
|Costanzo Federico||Data fusion and GIS||University of Perugia|
|Vittorio Formisano||Science Interpretation||Istituto di Fisica dello Spazio Interplanetario|
|Pierfrancesco Lombardo||Science Interpretation||Università di Roma “La Sapienza”|
|Lucia Marinangeli||Mission Science Targeting; E/PO||Università d'Annunzio|
|Giovanni Picardi||Surface Cutter Models; MARSIS||Università di Roma “La Sapienza”|
|Sebastiano B. Serpico||Science Interpretation||Università di Genova|
|Bruce Campbell||Clutter Detection and Mitigation||Center for Earth and Planetary Studies, Smithsonian Inst.|
|Jeffrey Plaut||MARSIS Correlation||Jet Propulsion Laboratory|
|Suzanne Smrekar||Science Interpretation||Jet Propulsion Laboratory|
 A review of the SHARAD radar experiment with an emphasis on its design is given by Seu et al. .
1.2. Scientific Objectives
 The primary objective of the SHARAD experiment is to map, in selected locales, dielectric interfaces to several hundred meters depth in the Martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials, including competent rock, soil, water and ice [Seu et al., 2004]. This is a seemingly cautious set of objectives, making no particular promises about the unique detection of any specific material (e.g., water). Nevertheless, the subsurface of Mars presents ample possibilities for dielectric contrasts. The dielectric constant depends on both rock porosity and rock composition, so boundaries between materials with differences in these properties are dielectric reflectors. This dielectric contrast could arise, for example, from sedimentary materials in contact with basaltic rock, from ice in contact with solid rock, or from ice-saturated porous rock in contact with ice-free porous rock. These contrasts alone lead to a large variety of subsurface targets for SHARAD to map. Examples include mapping of (1) the polar layered deposits, both their internal layers and contact with underlying bedrock, (2) the layering within sedimentary rock sequences, (3) buried impact craters in the northern lowlands, (4) buried channels, (5) volcanic stratigraphy, (6) shallow ice bodies, and (7) potential shallow water accumulations.