SHARAD
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[edit] General
SHARAD (Mars SHAllow RADar sounder) is a subsurface sounding radar embarked on the Mars Reconnaisance Orbiter probe. It complements the MARSIS instrument on Mars Express, providing lower penetration capabilities (some hundred meters) but much finer resolution (15 metres - untapered - in free space).
SHARAD is developed under the rensponsibility of the Italian Space Agency (ASI, Agenzia Spaziale Italiana), and provided to JPL for use on board the Mars Reconnaissance Orbiter Spacecraft in the frame of a NASA/ASI agreement which foresees exploitation of the data by a joint Italian/US team. The INFOCOM dept. of the University of Rome "La Sapienza" is rensponsible for the whole mission, while Alcatel Alenia Space Italia (formerly Alenia Spazio) designed and build the instrument. SHARAD operations are managed by INFOCOM from the SHARAD Operation Centre (SHOC), located within the Alcatel Alenia Space facilities in the suburbs of Rome.
[edit] Science objectives
SHARAD is intended to map the first Km below the Mars surface, providing images of subsurface scattering layers with high vertical resolution (15 m), with the intent to locate water/ice/ deposits and to map the vertical structure of the upper subsurface layers.
[edit] Characteristics
SHARAD operates on a carrier frequency of 20 MHz, transmitting a "chirped" signal with a bandwidth of 10 MHz. Pulsewidth is 85 usec and the nominal Pulse Repetition Frequency is 700.28 Hz. Transmitted power is 10 W peak. The antenna is a 10 m dipole. A synthetic aperture is generated on-ground to reduce the unwanted surface returns from off-nadir scatterers at the same range of the subsurface echoes.
SHARAD is physically divided into two elements:
- the SEB (SHARAD electronic box), which contains all the electronics (instrument controller, transmitter, receiver and antenna impedance matching network), within a metal frame which acts as thermal radiator for the electronic modules inside (Mars Reconnaisance Orbiter is an open frame spacecraft, and SHARAD has an autonomous thermal control)
- the antenna, made by two fiber tubes, folded and stowed in a cradle (covered by thermal insulator to protect it from the heating induced by the aerobraking). Once released, the antenna extends into position thanks only to the elasctic property of the material. A metal wire running inside the non-conductive tubes represents the real radiating element of the antenna.
The instrument operates at fixed PRF (700.28 Hz) and the echo is received in rank 1 (i.e., after the second transmitted pulse). Two alternate (higher and lower) PRF are available to deal with the extended mission orbit range. An open-loop tracking system, based on a priori knowledge of the surface topography, is the nominal means to position the 135 usec receive window on the expected echo position (a closed-loop tracker is available as backup).
The instrument on-board signal processing is minimal, and consists in a coherent presumming of the received echoes (programmable between 1 and 32 in power of 2 steps) to reduce the generated data rate, with programmable number of bits (8, 6, 4).
The chirp signal is generated directly on the 20 MHz carrier by a digital chirp generator, and fed to the power amplifier, followed by a Transmit/Receive switch and the matching network. The receiver provides amplification, filtering and digital gain control directly at RF, and the digitised using a downsampling technique at a rate of 26.6 MHz. A single DSP processor provides both the control and processing function.
The instrument industrial team is composed as follows: - Instrument design, integration and test: Alcatel Alenia Space Italia (Rome plant) - DES (Digital electronics subsystem): Alcatel Alenia Space Italia (Milan plant - formerly Laben) - Chirp Generator, Receiver: Alcatel Alenia Space Italia (Rome/L'Aquila plants) - Transmitter, matching network: Galileo Avionica (Milan, Italy) - Antenna: Astro Aerospace (Carpinteria, CA, USA)
[edit] History
While the initial studies date back to 2001, full-scale development was released only in February, 2003. The Engineering Model (EM) of the instrument was delivered to Lockheed Martin Space Systems in Denver (rensponsible for the Spacecraft) on March, 2004, and integrated into the Orbiter Test Bed. The ProtoFlight Model (PFM) was delivered and integrated on the Mars Reconnaisance Orbiter Spacecraft in Denver in September 2004. Mars Reconnaisance Orbiter was launched from Cape Canaveral Air Force Station on August, 12, 2005, with an Atlas V-Centaur launch vehicle, and reached Mars orbit on March, 10, 2006. The aerobraking phase, needed to reach the operational orbit, lasted until August, 30, 2006. On September, 17, 2006, the SHARAD antenna was deployed, and the first in-flight test of the radar was successfully carried out on September, 19. SHARAD is operational since November, 2006.
