PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "J. Ward, 2006-06-07, initial; S. Cull, 2007-05-18, revised; S. Slavney, 2007-05-21, revised" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "MRO" INSTRUMENT_ID = "SHARAD" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "SHALLOW RADAR" INSTRUMENT_TYPE = "RADAR" INSTRUMENT_DESC = " Instrument Overview =================== The SHAllow RADar instrument (SHARAD) is the sub-surface sounding radar provided by the Italian Space Agency (ASI) as one of six science instruments onboard NASA's 2005 Mars Reconnaissance Orbiter (MRO). SHARAD is a wideband radar sounder that operates on a 20 MHz central frequency with a 10 MHz bandwidth and transmits frequency- modulated radar pulses of about 85 microseconds in length. SHARAD is designed to create subsurface profiles with approximately 10 to 20 m vertical resolution (15 m in free space), 300 to 1000 m along-track resolution, and 1500 to 8000 m across-track resolution, depending on spacecraft altitude and terrain roughness. Radar penetration depends on the dielectric properties of subsurface materials, and is estimated from several hundred meters up to 1 km for the range of dielectric properties of expected Martian rocks. Information in this instrument description is taken from the SHARAD mission paper [SEUETAL2007]. See this paper for more details. Scientific Objectives ===================== The chief scientific objectives of SHARAD are: 1) to map dielectric interfaces in the Martian subsurface to depths up to one kilometer, 2) to interpret these interfaces in terms of the occurrence and distribution of expected materials, including rock, regolith, water, and ice, and 3) to determine the 3-D distribution and state of subsurface H2O. Subsystems ===================== SHARAD has three major subsystems: Antenna S/S, RF S/S, and Digital S/S. 1) The Antenna S/S is a dipole radiating element with a length about half the wavelength of the carrier frequency. 2) The RF S/S includes the transmitter, Tx/Rx switching net, and receiver down to the Analog-to-Digital Converter. 3) The Digital S/S includes the command and control functions, interfacing with the spacecraft bus, and the processing capabilities for digital synthesis of the radar pulse and generation of system timings. Detectors ===================== SHARAD's antenna is a 10-m dipole made of two 5-m foldable tubes, which, in stowed configuration, are kept in place by a system of cradles, and, when released, self-deploy because of their elastic properties. Electrically, the antenna is fed at the center, interfacing with the SEB by means of two wires (one for each dipole). The connected wires together form a balanced connection line. The line itself has no controlled impedance, and the load seen on the TFE side is therefore frequency dependent, requiring frequency compensation within the TFE. The antenna also includes its release mechanism (two solenoid controlled actuators to release the right and left dipoles). During deployment operations, heaters installed on the spacecraft panel will heat the antenna cradle hinges in order to keep the actuator mechanism within suitable temperature range. Electronics ===================== SHARAD's Electronics Box (SEB) includes all of the transceiver electronics and the signal processing and control functions. It is made of two separate electronic assemblies, mounted on a support structure that acts as radiator for thermal control and includes the heaters and temperature sensors. The two electronic assemblies are: 1) The Receiver and Digital Section (RDS) includes the Digital Electronic Section (DES), which includes the Digital Signal Processor (DSP) Module (Command and Control Board and Slave board), Service Module (Timing Board and DC/DC Converter board), and the Digital Chirp Generator (DCG) Module (DCG Board). The DES is responsible for most of SHARAD's functions, including: a) Command and control capability b) Formatting of science data and their transfer as telemetry to the spacecraft's solid state recorder (SSR) c) Generation of housekeeping telemetry and its transfer to the spacecraft's SSR d) Generation of the radar chirp signal e) Processing of the raw radar data f) Provision of a high-stability oscillator and generation of timekeeping and synchronization signals for all SHARAD units g) Power conditioning and distribution to the other SHARAD units The Digital Chirp Generator (DCG) synthesizes chirp signals in the DES using the Direct Digital Synthesis Technique: it generates discrete samples of a sine wave at different frequencies and reconstructs the desired waveform at the analog level. The DCG uses a Numerical Controlled Oscillator (NCO) to do this, which features: a) 32-bit frequency resolution (0.018 Hz @ 80 MHz clock) b) 0-32 MHz @ 80 MHz clock bandwidth c) 10-bit chirp signal generator d) Micron Processor Compatible input (16 bits Bus Address and 16 bits Bus Data) e) 520 mW @ 80 MHz power dissipation f) Automatic download of external look-up table PROM content g) All parameters previously configured (start frequency, frequency slope, phase compensation, pulse duration) The RDS also includes the Receiver (Rx), which amplifies, filters, and digitizes the received signal. 2) The Transmitter and Front End (TFE) amplifies the low-level chirp signals coming from the DES and couples them to the dipole antenna. The TFE also manages sharing the antenna between the transmitter and the receiver path. It also includes an internal DC/DC converter to supply all internal circuitry. Operational Modes ===================== Operational (or Measurement) Modes correspond to the different actions that the instrument may perform under the guidance of the Operation Sequence Table (OST). Each OST entry specifies details for the transition to, and the execution of, a given Operational Mode. Telemetry is always generated during Operational Modes and monitoring is active. In case of other error or anomalies during Operational Modes processing, an automatic transition is performed to Safe/Idle State. SHARAD can operate in one of four modes: 1) Subsurface Sounding Mode (SS) is the main measurement mode for SHARAD. In this Mode the instrument performs scientific measurements by transmitting radar pulses and collecting, processing and formatting received echoes. Pulse repetition interval and duration are variable depending on parameters specified in each OST entry. A variable Science Data rate is produced in this Mode depending on the specific processing parameters. 2) Receive Only (RO) Mode is used to perform passive measurements mainly during the on-orbit phase, but can also be used to check the performances of the instrument during the cruise phase (even with the antenna folded). No transmissions will be performed. A variable Science Data rate will be produced in this Mode depending on the specific processing parameters. 3) Calibration Mode is used to perform instrument calibrations during the on-orbit phase. 4) Test Mode is a debug mode used to generate a stream of science data simulating an instrument operational mode, to exercise all internal functions of the instrument. Data produced using these two Operational Modes have no scientific value and will not be used to produce EDR data products. Calibration =========== Before flight, the ground calibration checked the transmitter and receiver chain parameters, the reference oscillator parameters, the antenna radiation pattern, and the end-to-end parameters. During the Transition Orbit, SHARAD's antenna was both passively and actively calibrated. Both methods assumed that the spacecraft configuration does not affect the shape of the antenna pattern in the section of interest (+ 10 degrees pitch, + 20 degrees roll, with referenced to the nadir), but only its absolute amplitude. During Primary Science Orbit, SHARAD has undergone a number of calibrations. The best approach to calibration is to observe the same local and minimize scene dependence of the calibration. Calibration is complicated by the fact that it is difficult to fully characterize the antenna pattern due to the varying backscattering properties of the surface. Only a limited number of orbits can be devoted solely to this task during the science data collection phase. Operational Considerations ========================== SHARAD is able to operate at any time while the MRO spacecraft is orbiting Mars, regardless of solar illumination conditions. Constraints are a result of tradeoffs among the various instruments on board. The data volume for SHARAD is limited by the MRO allocation of 15% of its total data, which typically ranges from 40 to 90 Gb/day. SHARAD is fundamentally a nadir-pointing instrument, though it can still acquire good data up to 10 degrees off-nadir. SHARAD has considerable freedom in both its preprocessing parameters and its data production rate (300 kbps to >20 Mbps, depending on the pulse repetition frequency, presuming strategy, and number of bits per sample). " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "SEUETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END