SAC-C MISSION 2.0 CONAE INSTRUMENTS DESCRIPTION 2.1 MMRS

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1 SAC-C MISSION Carlos Alonso Carlos Hofmann SAC-C Program Manager SAC-C System Engineer CONAE (Comisión Nacional de Actividades Espaciales), Paseo Colón 751 (1063) Buenos Aires Argentina 1 Abstract. SAC-C is an international project led by CONAE (Comisión Nacional de Actividades Espaciales) of Argentina and NASA from the United States of America. Others countries like Italy, France, Brazil and Denmark also participate in the project. These countries participate in different ways, some of them developing instruments and others are more interested in SAC-C science and contributing to project development. SAC-C is the first Argentinean Earth Observation Satellite and it has been designed primary to fulfil the requirements of large countries with small population like Argentina. Its design is a good compromise between resolution and swath width that permits SAC-C to be an appropriate tool for global and high dynamic phenomena study. Large coverage, multispectral and panchromatic capabilities, medium ground resolution and short revisit time are its main characteristics. SAC-C has been launch on November 21 st 2000 from the Vandenberg Air Force Base, in California on a Delta II launcher. Currently SAC-C is performing nominal mission operations from mid-april The objectives of the SAC-C Mission, as well as particularities of the satellite design and programmatic considerations are presented in this paper. 1.0 Mission Objectives SAC-C Mission is basically an earth observation mission. There are nine instruments that will perform several studies. The MMRS (Multispectral Medium Resolution Scanner), provided by CONAE, Argentina, permits the study of desertification processes and their evolution in time (i.e. Patagonia, Argentina), to identify and predict agriculture production, to monitor flooded areas and to make studies in coastal and fluvial areas. The MMRS is associated with a High Resolution Camera (HRTC), provided also by CONAE, which permits improvement of the MMRS resolution on the areas where it is required. These two cameras are complemented with the HSTC, a high sensitivity camera for atmospherics research. The Whale Tracker, developed by CONAE and the Secretary of Environment of Argentina, will permit the study of the migration pattern and behavior of the Eubalanea Australis; the goal is to protect this specie from depredation. The same instrument will permit the collection of environmental data from platforms on ground. The instrument will be able to track whales from mid A GPS, provided by NASA JPL, is used to measure the long wavelength component of Earth s gravity field and to measure the refractivity of GPS signals as occulted by the Earth atmosphere and ionosphere. Others studies are being performed to observe reflection of GPS signals from the ocean surface for estimating capabilities for altimetric and ionospheric science. The MMP (Magnetic Mapping Payload), developed by the Danish Space Research Institute and NASA JPL, performs observatory quality measurements of the magnetic field. It consists of both vector and scalar magnetometers. The vector magnetometer is associated with a star imager to determine its pointing direction. ICARE (Influence of Space Radiation on Advance Components), provided by CNES from France, permits the improvement of risk estimation models for radiation effects on latest generations of integrated circuits technologies and to improve environment models for radiation responsible for degradations in electronic components. AADS and INES (Autonomous Attitude Determination From Stars and Italian Navigation Experiment), developed by the Italian Space Agency, constitute a technological payload that permits the testing of a fully autonomous system for attitude and orbit determination using a new generation star tracker and a GPS/GLONASS system receiver. 2.0 CONAE INSTRUMENTS DESCRIPTION 2.1 MMRS 1 CONAE Tel: / / Fax: ext calonso@conae.gov.ar /

2 The MMRS is a multispectral camera with 5 spectral bands: B1: nm Blue greenish B2: nm Green B3: nm Red B4: nm NIR B5: nm SWIR The MMRS has two operational modes, a highresolution mode of 175 meters and a low-resolution mode of 350 meters. For the first case the MMRS can operate in real time at a bit rate of Mbit/sec and also can store data in its own memory with an image dimension of 360km x 8000km, depending on the data compression ratio. For the low-resolution mode the MMRS transmits in real time at a bit rate of 0.943Mbit/sec. The low-resolution images are transmitted in real time. Small users, research institutes, Universities and schools use these images. The following are the main MMRS characteristics: Total Mass 22kg. Power Consumption 25Watts (peak) Spectral Bands 5 Down link bit rate 3.774/0.943 Mbit/sec Memory Capacity 96Mbytes On board compressor JPEG rev 08 Expected Compression Rate a) Real Time 4:1 b) Stored Mode worst/best 4:1/10:1 Ground Pixel size 175 meters Swath Width 360 km CCD s 4Xvnir Xswir 2100 Gain steps Programmable Integration Time Programmable Radiometric Resolution Corregistration among bands 8 bits +/- 2 pixels (life time) +/ pixel (one orbit) 2.2 HRTC The HRTC (High Resolution Technological Camera) is a panchromatic camera with a ground resolution of 35 meters. The spectral response is in the range of 400 to 900nm. The HRTC have two operational modes, stored data or real time transmission. It records the images in its own mass memory 96 Mbytes of capacity. The image size is 90km x 1150km. The HRTC main characteristics are: Total Mass 8.5 kg. Power Consumption 10.5 Watts Down link bit rate Mbit/sec. Ground pixel size 35 meters Swath width 90 km CCD Loral Fairchild CCD181 Nº of pixels 2592 Gain steps Programmable Integration time Programmable Radiometric Resolution 8 bits IFOV deg. FOV 7 deg. Operating Mode Stored Mass Memory 96 Mbytes 2.3 Data Collection System / Whale Tracker The Data Collection System / Whale Tracker experiment is a joint project between the Secretaría de Recursos Naturales y Desarrollo Sustentable y CONAE. The instrument has two modules: the Data Module and the Satellite Module. The Data Module, developed to be installed on a number of whales or as fixed platforms on ground, includes a GPS to determine position and absolute time and transducers to measure water temperature, pressure and environmental data. A transmitter and a data management unit complete the Data Modules system. All data received by the Satellite module is stored in the satellite Mass Memory and dumped every day during the nighttime passes. The Satellite Module on board the SAC-C satellite consists of a receiver to acquire data transmitted from the Data Modules and the necessary logic to handle data acquired. The instrument is currently used to collect data from Data Collection Platforms to monitor environment and meteorological data. 3.0 SATELLITE DESCRIPTION The SAC-C satellite has been designed for a minimum lifetime of 4 years. The maximum SAC-C injected mass was 475kg, including 12.5kg of hydrazine. SAC-C is a three axis stabilised spacecraft (earth pointed). Pointing accuracy is 0.2deg, pointing stability 0.003deg/sec and pointing knowledge accuracy 70arcsec (3σ) on the three axis. The orbit characteristics are:

3 Nominal altitude 707km Inclination 98.2deg Track error +/-10km EOL. Revisit Time 16 days The magnetic cleanliness achieved at 8 meter is better than 1nT (at the scalar magnetometer distance), required to have an appropriate magnetic cleanliness valid for the mission. The main satellite subsystems are: Structures and Thermal Control Command & Data Handling Attitude Control & Orbit Control Power Communications (RF) 3.1 STRUCTURE AND THERMAL S/S Structure Main structure is manufactured in aluminium. Antennas booms are manufactured in carbon fibre reinforced plastic and the batteries mounting frames in titanium. The structure subsystem has been design to carry 475kg. The subsystem mass is 65kg. SAC-C envelope in launch configuration was 1.85x1.68x2.4meters. The stiffness requirements were: First lateral natural frequency higher than 20Hz. First axial natural frequency higher than 35Hz SAC-C first natural frequencies were: Lateral 39.3Hz Axial: 41.7Hz Thermal Control The thermal control design implementation is based on MLI, Radiators and Heaters. The spacecraft bus is divided into three areas for their thermal control: the main room, the batteries environment, and the propulsion system. The main room uses radiators and an active system with heaters. The philosophy design is based on a power budget of 0 Watts. Main room thermal control is designed to maintain the temperature inside the required ranges using only the power excess and the equipment dissipation. The MMRS and the HRTC cameras have their own thermal control system. The operative and non-operative temperature ranges are: Main Room Op:-10/+40 Non-op:-20/+50 Batteries Op:-10/+5 Non-op: N.A. Propulsion System Op:+10/+40 Non-op: N.A. MMRS Op:-7/+7 Non-op:-20/+50 HRTC Op:-7/+40 Non-op:-20/ COMMAND & DATA HANDLING S/S The Command & Data Handling subsystem main features are, to receive, decode and execute commands in real time or time tagged, to read telemetry data, to supervise subsystems behaviour and to store telemetry and instruments data on the Mass Memory Unit. It consists of three units, the command and data handling unit, the payloads communication hub unit and the Mass Memory Unit. (see Fig-3) The command and data-handling unit includes two redundant units in the same box, in cold configuration. It has the capability for automatically switching through a dedicated watchdog type redundant hardware. Ground switching is also implemented through hard-commands. All internal communications are through a MIL STD 1553B bus. The Communications Hub unit was included to manage the payloads that required the use of the RS 422 interface. It consists of two redundant units in cold backup configuration; the interface with C&DH is through the MIL STD 1553B and with the payloads through the RS 422 at 9600 baud. The Mass memory Unit consists of two redundant units of 96Mbytes each. They could be in cold backup configuration or in use simultaneously, increasing the memory capability up to 192Mbytes. Housekeeping and scientific instrument data, excluding the MMRS, HSTC and HRTC cameras, is stored in the mass memory unit The mass memory unit includes also the function for down linking information from scientific instruments, cameras (real time and storage data) and Housekeeping. The MMRS, HSTC and HRTC cameras have access directly to the mass memory down linking electronics. It also codes the information to be transmitted using convolutional coding. 3.3 COMMUNICATIONS S/S The RF subsystem has been implemented to have the capability to: Receive commands (real time or time tagged) Transmit telemetry (real time and housekeeping) Transmit science data (recorded and in real time) The main components of the RF subsystem are: Two S Band command receivers (4Kbps up-link) in hot backup configuration, cross-strapped to both C&DH units. Two S Band transmitters (1Kbps downlink) of 1/0.1 Watts (switchable by command), for real time telemetry.

4 Two dedicated S Band 5Watts transmitter for MMRS low-resolution images in real time (0.9435Mbps) and Mass Memory data transmission (1.887Mbps). Two X Band 3 Watts transmitters for MMRS High Resolution real time and stored images and HRTC stored images. The bit rate is Mbps for all cases. The main components of the Orbit Control Subsystem are: 8 thrusters in dual configurations 1 hydrazine tank (12.5kg.) 2 latch valves 1 filter fill and drain valves Redundant driver electronics 3.4 ATTITUDE AND ORBIT CONTROL S/S SAC-C is a three axis stabilised spacecraft (earth pointed). It will use dedicated sensors and actuator for the attitude control. Only in emergency cases would it be possible to use the propulsion system (orbit control s/s) for that task. The main components of the Attitude Control Subsystem are: Sensors: Star Tracker Coarse solar sensor Two Scanwheels Two Tri-axial Magnetometers GPS for orbit and attitude determination Actuators Three redundant torque rods. Two Momentum wheels Thrusters (in case of emergencies) Magnetic Mapping Payload Boom (8 meter boom) The Attitude Control Electronics (ACE) includes two redundant units in cold backup configuration. Pointing accuracy is 0.2deg, pointing stability 0.003deg/sec and pointing knowledge accuracy 70arcsec (3σ) on the three axis. The Orbit Control Subsystem main functions are orbit injection error correction manoeuvres and orbit maintenance. The system consist of four dual thrusters pointing in the velocity vector negative direction (there is a small tilt to provide torque around the three satellite axis). The satellite centre of mass is inside of the rectangle defined by the four thrusters. The Orbit Control subsystem works in closed loop with the Attitude Control Electronics. In this way, using an off modulation concept, it is possible to maintain the satellite attitude within the 5 degree from the nominal attitude. The Orbit Control subsystem operates, like baseline, using one of the two branches of four thruster. In case of failure or malfunction of the operative branch the system selects the other branch. In case of need both branches could be operated together. 3.5 POWER S/S The Power subsystem main functions are: to provide power distribution to the spacecraft bus and payloads, to measure load s current, to provide batteries thermal control and charge control, to limit bus voltage, to generate telemetry and to transmit it to C&DH through the MIL STD 1553B communication bus. The main components of the power subsystem are: Solar Panels 2 Nickel Hydrogen batteries (12Am/h) two Power Control Electronics in hot backup configuration The solar panels consist of two fixed wings with four panels each with different inclination with respect to the sun in order to have a power generation without excessive peaks of power. They are attached to the satellite body by one hinge per wing when deployed. (See Fig.1) The average power generated during sunlight is of 460 Watts. The orbital power consumption is Watts. 4.0 GROUND SEGMENT The SAC-C Ground Segment is located at the CONAE Center in Córdoba Argentina. A dedicated S Band antenna is used for T&TC and Mass Memory data down link. Other dedicated X Band antennae are used to receive high-resolution MMRS real time and storage images and HRTC images. 5.0 PROGRAMMATIC CHARACTERISTICS SAC-C is a co-operative program between the Comisión Nacional de Actividades Espaciales (CONAE) from Argentina and the National Aeronautics and Space Administration (NASA) from the United States. By mean of this agreement CONAE provides the spacecraft platform and three scientific instruments and NASA provides launch services and two scientific instruments. CONAE signed agreements with other space agencies to provide additional instruments valid for the SAC-C Mission.

5 In this way Italy provided two instruments and contributed with the program by supplying the solar panels design and a GPS tensor that is used by the bus for navigation and timing determination. Danish Space Research Institute provided part of the MMP payload and the 8 meter boom, CNES from France provided the ICARE instrument and INPE/LIT from Brazil provided its testing facilities for SAC-C structure and thermal subsystems qualification and system level test. For SAC-C development CONAE selected as prime contractor the company INVAP S.E. from Bariloche Argentina. SAC-C has been launched on November 21 st, 2001 from the Vandenberg Base, California (USA) with a Delta 7320 in a dual launch with the EO1 satellite from the NASA Goddard Space Flight Center.

6 Fig 1 Orbit Configuration Fig 2 Block Diagram SW2 SW1 TAM X,Y,Z CSS POWER ACS C&DH MM PROPULSION SYSTEM x2 x2 x2 SYSTEM SOL PAN BATT1&2 PDU 1553B CHU GPS LAGRANGE SAC-C ITALIAN STAR TRACKER S-BAND1 TXRX S-BAND1 TX RS422 S-BAND2 TXRX S-BAND2 TX MMP JPLGPS STAR TRACKER GPS TENSOR COMRAD VM UHF TXRX WHALE TRACKER X-BAND1 TX ST T SHM X-BAND2 TX HRTC MMRS UHF RX

7 SAC-C at Vandenberg

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