THE BRAZILIAN SCIENTIFIC MICRO SATELLITE – SACI-1
José Ângelo C. F. Neri
Ijar M. Fonseca
National Institute for Space Research - INPE
Av. Astronautas 1758, P.O. Box 515
12223-010 S.J. Campos, S.P., Brazil
ABSTRACT - SACI-1 is a small 60-kg scientific satellite being developed by INPE, S.P., Brazil. The philosophy of the project is to involve Brazilian universities and the Brazilian emerging space industries, under the coordination of INPE. The project should also be low cost, accomplished in a short period of time, and reliable. The mission has being implemented within the "smaller, cheaper, and better" philosophy. The team engaged in the SACI-1 design is small. However the team is integrated and fully responsible for the mission success. The challenge and motivation for the group is the innovation regarding the design of a multi mission bus, including the development of an on-board computer and the implementation of an autonomous attitude control. The accomplishment of the project is done with only the minimal necessary bureaucracy. The scientific experiments also motivate Brazilian scientists that develop their payloads in conjunction with partner from other countries. SACI-1 orbit will be polar and circular at a 750-km altitude. The spacecraft will be spin stabilized with geomagnetic control. SACI-1 will be launched as a secondary payload from China by Long March–4B launcher in October 1998. This paper presents an overview of each subsystem and also an overview of the scientific experiments.
The SACI-1 micro satellite was conceived by the National Institute for Space Research – INPE [Sobral, 94], based on the micro satellite advanced technology and on the lessons learned from the progress carried out by the Brazilian Complete Space Mission (MECB). SACI-1 is a 60-kg polar low Earth orbit satellite to be launched as a secondary payload of the China-Brazil Earth Resource Satellite (CBERS) scheduled for October 1998. The project is totally financed by the Projects and Studies Financier of the Brazilian Government (FINEP). The scientific mission feature of the spacecraft comprises four experiments developed by Brazilian scientist with the participation of partners from other countries.
The platform is multi mission and objectives
The main characteristics of the satellite are:
2.1 - Structure
The structure of the SACI-1 is modular. According to this approach the subsystems are designed as standard modules. The use of a standard layout makes the whole preliminary activity of design faster and easier. Another advantages are compactness, high reliability, and lower production costs.The structure is a 640 x 470 x 470 mm parallelepiped-like spacecraft in the configuration with the solar arrays closed. An exploded view of the satellite is shown in Fig. 1. The structure includes a main body (platform and payload), four deployable solar panels and the holddown and release mechanisms (HDR). Only three of the solar panels are covered by solar cells. One of them will be used only to counterbalance mass around the longitudinal axis of symmetry. The platform consists on a pack of 9 aluminum ribbed frames of different thickness stacked horizontally and connected by 12 stud bolts. The analysis of this structure was entirely done at INPE by using a Finite Element Model (FEM) for static, normal modes, frequency response, transient response and random vibration analysis. The FEM of each part was developed based on two and three-dimensional AutoCAD drawings and solids pre-processed on a PC and exported to the pre-processor of the MSC/NASTRAN. This procedure decreases the time spent on building the model on the computer [Souza, 96].
Fig. 1 - SACI-1 exploded view
2.2 - Thermal Control
The thermal control subsystem is characterized by the use of passive elements such as super-isolators, paintings and coating with thermal-optic properties. This subsystem has been developed entirely by INPE based on experience with previous projects. To accomplish a good thermal control solution the external walls of the modules were partially covered with a mosaic of white/black paint and multi-layer insulation blanket (MLI) over alodine 1000. The project was validated through simulations and tests of the thermal behavior of the satellite considering the orbital parameters and the internal heat dissipation [Olavo, 96].
2.3 - Communication Subsystem
The communication subsystem will implement the telecommand reception, the telemetry, and the payload data transmission to ground. A dual cold redundant transmitters will provide an ESA standard S-band downlink with a BPSK modulation at 500 kbps data transmission rate. A 2 watts RF output power from the cold redundant transmitters shall be sent to the ground by two antennae. The microstrip satellite antennae gains assure the data reception by a 3.0 m diameter ground antenna. The dual hot redundant receivers perform an ESA standard S-band uplink with a 19.2 kbps data transmission rate.
2.4 - On-board Computer (OBC)
The OBC hardware was based on the classic architecture of Transputers and is totally redundant [Castro, 91]. The OBC block diagram is shown in Fig. 2. The hardware project was developed by INPE with the participation of the University of Ceará. The software project is based on modular and simple programs in order to increase the OBC reliability. The programming languages are OCCAM II and C previously validated in space missions. The subsystem has three processing units, based on a T805 transputer, and three I/O interfaces connected through a high-speed serial link (10 Mbps). The interfaces are fully redundant and are connected to two processing units. The OBC shall be able to execute all on-board tasks in the case of one processing unit failure. One single processor can operate in a degraded mode and perform all essential tasks in case of fail of two processors. Each processing unit has 8 Mbytes of extended memory and shall be used to store data gathered from the experiments. The OBC tasks comprise:
Fig. 2: On-board Computer Subsystem Block Diagram.
2.5 - The Attitude Control Subsystem (ACS)
The SACI-1 ACS [Fonseca, 96] combines the passive spin stabilization technique with geomagnetic control. The ACS will perform:
The ACS comprises four torque coils, two solar sensors and a three-axis magnetometer. The torque coils are redundant with respect to the spin axis (two coils) and the spin plane (two coils) to prevent failures. The coil interaction with the Earth magnetic field generates the necessary torque to execute all active control operations. The spin axis coil will perform the Sun acquisition (transient phase) and the attitude control (during the satellite normal mode operation). The other two torque coils will be mounted parallel to the spin axis plane at 90o from each other. Their functions are: the spin-up of the satellite up to its nominal spin rate of 6 rpm and the execution of the spin rate control. The satellite rates of inertia are in accordance with the major axis rule for spin stabilization. To assure such configuration some experiment sensors were attached at the tip of the solar panels. The spacecraft attitude with respect to the Sun shall be maintained within 10 degrees from its nominal value. To ensure this goal one weekly attitude correction is being planned. The satellite spin rate and attitude control are carried out automatically by the OBC. The spin rate requirement is 5£ 6 rpm. The control hardware has been manufactured by INPE with the participation of the University of São Paulo (USP) and Institute for Technological Development of São Paulo, IPT. The magnetometer and associated electronic has been purchased from especially qualified companies.
2. 6 - Power Supply Subsystem (PSS)
The spacecraft attitude shall keep a solar aspect angle about 0 degree to assure maximum power generation. During eclipse periods, the spacecraft is powered by two packages of 22 NiCd cylindrical battery cells with a nominal capacity of 4.5 Ah for each package. A maximum DOD of 1/5 of the battery capacity will enhance satellite lifetime. The battery is charged by two redundant peak-power trackers. The unregulated bus voltage is directly connected to the battery packages and to the DC/DC converter input without employing a discharge controller. The DC/DC converters will power locally each spacecraft subsystem in order to obtain a good performance related to the in-rush current and to reduce the noise interference from the power line.
The Power Supply Subsystem consists of two cold redundant battery chargers, two batteries, the solar panels, and the power conditioning and the power distribution boards [Neri, 96]. Fig. 3 shows the SACI-1 PSS. The subsystem has been partially projected and developed by INPE. The solar panels and their associated cells as well as the deployment mechanism have been purchased abroad. The technology adopted brought innovation in comparison with the conventional space projects and was based in power supplies with little power dissipation used in micro satellites. The batteries used were made at INPE by selecting NiCd commercial cells acquired from space project traditional vendors and screened through a specific program of tests.
The SACI-1 mission experiments were selected by an Opportunity Announcement opened to all the Brazilian scientific community and foreigner partners under the Brazilian Academy of Sciences responsibility in 1994. The strategy for the SACI-1 project development aims for the fulfillment of scientific and technological benefits to the space sciences in Brazil. The scientific and technological objectives of the Opportunity Announcement incite
The creation of new space science centers through the participation of Brazilian Universities and research institutes in the satellite project stimulates the
Fig. 3: SACI-1 Power Supply Subsystem Block Diagram
4.1 - Study of Plasma Bubbles (PLASMEX)
The main objective of this experiment in the ionosphere is to investigate the generation, development and decay of the plasma bubbles, particularly in the Brazilian region. This investigation intends to elucidate the strong influence of the bubbles and associated plasma turbulence in several space application systems (remote sensing with radar, space geodesy, trans-ionosphere telecommunication etc). The discovery of such phenomenon in the Brazilian ionosphere was reported by measurements with rocket-borne photometers and ground-based ionosond. Several aspects of the phenomenon such as ionosphere environmental conditions and electrodynamics are still not well studied. More detailed knowledge of such aspects are fundamental to improve the predictability of the occurrence of the bubbles event. The plasma bubbles are region of plasma depletions aligned with the magnetic flux tubes extending over thousands of kilometers through the Earth’s magnetic field in both North and South hemispheres. Bubbles result of non-linear instability processes in the plasma such as the Rayleigh -Taylor mechanism. The effects caused by these irregularities are known as the equatorial "spread F".
The Sun synchronous orbit of SACI-1 is close to 22:00 local time meridian, which is the location where the bubble development reaches its peak. This fact offers excellent conditions for the bubbles observation. It is intended to perform several measurements such as density, temperature, and spectral distribution of the plasma irregularities. To accomplish this goal the following instruments will be used:
The Aeronomy Division of INPE, DAE/INPE, developed one version of the high frequency capacitance that was tested in several experiments carried out by the SONDA III rockets. The tests were implemented with the participation of the Brazilian Institute of Aeronautics and Space, IAE. A version of the Langmuir probe has also been developed by DAE/INPE and tested by using several SONDA III rocket flights. The electron temperature probe has been developed in cooperation with the Institute of Space and Aeronautical Science, ISAS, Japan.
4.2 - Airglow Photometer (FOTSAT)
This experiment has the objective of measuring the intensity of the terrestrial airglow emissions in global ranges of Oxygen OI 557.7nm, OI 630.0nm and OH(8,3) in global ranges. Special interests in this investigation are the equatorial ionosphere anomaly, the South-Atlantic anomaly, and the longitudinal and latitudinal variations of the dynamical process in the ionosphere.
The photometer system is composed of 4 sensors to measure 4 distinct wavelengths, i.e., 557.7nm, 630.0nm, 715.0nm and 724nm plus a frequencymeter and an interface for telemetry. The photometer was designed and manufactured by a technical team at INPE. This experiment has been conducted by INPE with the cooperation of the Federal University of Rio Grande do Norte, the Federal University of Paraíba, the ISAS and the National Institute of Polar Research both from Japan. The photometer is installed with its optical axis perpendicular to the SACI-1 spin axis. It will collect data across the orbit in the tangential direction of the Earth surface. As the SACI-1 orbit is polar and Sun synchronous the photometer will cover all latitudes and longitudes within 24 hours. That data will provide important support for the study of the global distribution of the airglow emission.
4.3 - Solar and Anomalous Cosmic Rays Observation in the Magnetosphere (ORCAS)
The main objective of the telescope ORCAS is to measure the Anomalous Cosmic Rays (ACR) fluxes, from C to Ne, "trapped" into the belt by using solid detectors and by identifying the time and the direction of particle arrivals. The ORCAS will allow studies of the space distribution and of the dynamics associated to the population of the trapped particles in conditions of minimum solar activity with intense fluxes. The SACI-1 will be launched during the period of this minimum solar activity. This fact will allow the ORCAS to measure fluxes, spectra, composition, and temporal and spatial variation of ions from He to Ne, and of protons and electrons with energy less than 100 MeV/amu.
In the interplanetary space, heavy nuclei of He and even Ne with energy below 50 MeV/amu, called anomalous cosmic Radiation (ACR), were discovered in the 1970’s by the space probes IMP and by Pioneeers 10 and 11. While the relation between the Carbon and Oxygen quantities (C/O) in the cosmic and solar intergalactic radiation (CGR, SCR) is less than 0.5 it is less than 0.1 in the ACR. The theoretical explanation for this fact is that the interstellar neural atoms are captured by the solar system in motion, then ionized and accelerated, and then transformed into ACR. The studies of the the ACR started with the Skylab III discoveries in 1973. Then a series of experiments on board of the Spacelab and Soviet satellites (in orbits about 500 Km) carrying recoverable plastic detectors have detected particle with energy and composition similar to the ACR near the Earth. The presence of these particles at such altitudes (despite the existence of the Earth magnetic field) is explainable only if they are ionized and "trapped" by the magnetosphere.
4.4 - Geomagnetic Experiments (PLASMEX)
The geomagnetic field controls the movement of charged particles in the space around the Earth and protects the planet from the direct incidence of the solar wind. Simultaneous measurements of the geomagnetic field on the Earth’s surface and in the near space plasma are essential to the study of geomagnetic phenomena. In the last 25 years electrical currents aligned with the geomagnetic field lines have been subject of important studies with satellite on-boarded magnetometers. The Polar Low Earth Orbit (LEO) satellites have performed remarkable research of the space plasma, by using fluxgate magnetometers. More than 170 geomagnetic experiments have been implemented in satellites, most of them dedicated to the investigation in auroral regions. These experiments represent about 67% of the scientific missions.
The main objective of the experiments is the investigation of the phenomenon related to the currents aligned with the trans-equatorial field and the plasma electrodynamics that involves the Earth, specially in the region of the South Atlantic Anomaly. To implement the experiment a triaxial high precision fluxgate is boarded on the SACI-1 to conduct the geomagnetic field measurements. In addition all instruments selected and related to the ionosphere studies will give important complementary information for the study of the geophysical effects.
5 - CONCLUSIONS
This paper has presented the main features of the SACI-1 satellite and associated scientific experiments. The spacecraft bus has the following subsystems: structure, thermal control, communications, attitude control, on-board computer and power supply.
Some of the developments implemented in the SACI-1 project will be employed in other Brazilian space projects. The experience on cost-effectiveness acquired during SACI-1 development has contributed to change the way people think about cost-risk in space designs in Brazil.
[Sobral, 94 ] Sobral, J. H. A. et al: "The Brazilian Scientific Satellite", Acta Astronautica, Vol. 32, No. 10, pp. 675-682, 1994.
[Souza, 96] Souza, P. N. et al: "Structural Design of the Brazilian Scientific Applications Satellite (SACI-1)", 10th Annual AIAA/USU Conference on Small Satellite, Sept 16-19, 1996.
[Olavo, 96] Olavo B. Oliveira et al: "Thermal Control of the First Brazilian Scientific Satellite", Small Satellites for Earth Observation, International Symposium of the International Academy of Astronautics (IAA), Berlin, November 4-8, 1996
[Castro, 91] Castro, H. S. A.: "Fault-tolerant Multitransputer System For Space Applications", Microprocessors and Microsystems, Vol. 15, No. 7, Sept , 1991.
[Fonseca, 96] Fonseca, I.M.; Souza, P.N., Neri, J.A.C.F. and Guedes, U.T.V.: "The Attitude Control Subsystem of the Brazilian Scientific Satellite - SACI-1", ICONE’96 Second International Conference on Non-Linear Dynamics, Chaos, Control and Their Applications in Engineering Sciences, São Pedro, SP, Brazil, August 05-08, 1996
[Neri, 96] Neri et al: "SACI-1 - A Cost-Effective Microsatellite Bus For Multimission Payloads", International Conference on Small Satellites: Mission and Technology, Madrid 9-13, 1996