CESAR Mission—Cooperation Española-Argentina

Acra Astronautica Vol. 46, Nos. 2-6, pp. 121-126,200O 0 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0094-5765/00 $ - see front matter PII: SOO94-5765(99)00209-X

CESAR MISSION COOPERATION

ESPAROLA-ARGENTINA

Lucia Acedo, Alvaro Urech Institute National de Tknica Aeroespacial Carretera de Ajalvir, Km.4-28850 Torrejdn de Ardoz - Madrid, Espafia Daniel Caruso, Juan YelO’s Comisibn National de Actividades Espaciales Av. Paseo Co& 751- 1063 - Buenos Aires, Argentina ABSTRACT - This paper describes the CESAR Mission, an Earth Observation Satellite Mission developed in cooperation between INTA (Instituto National de Tecnica Aeroespacial) from Spain and CONAE (Comision National de Actividades Espaciales) from Argentina. The Mission, with a proposed launch date of the corresponding CESAR satellite circa 2002 / 2003, consists in the design, construction, launch and operation of a small satellite, less than 500 Kg, and the update of the existing Ground Segment Capabilities in Spain and Argentina to receive and process the CESAR generated data. The primary objectives will be: Cartography, Natural Resources and Geophysics, with a Satellite Payload composed of a Panchromatic Camera with 5 meters geometric resolution and 10 bits for cartography and topography studies: a radiometric resolution, Multispectral Camera with 6 bands, 34 meters geometric resolution and 10 bits radiometric resolution, for thematic studies; and Panchromatic High Sensibility Camera with 1Km geometric resolution and a Spectrometer, for geophysics studies.0 2000 Elsevier Science Ltd. All rights reserved 1. OBJECTIVE

2. INTRODUCTION

Remote sensing by satellite has proved to be one of the most effective ways to survey the surface of the Earth. Argentina and Spain, due to the matureness of the Remote Sensing communities in both countries, have decided to join their efforts and expertise in an international cooperative project called CESAR. This project would be a follow on of their national satellite programs, SAC and Minisat respectively. The “Instituto National de Tecnica Aeroespacial”, INTA, from Spain, and the “Comisibn National de Actividades Espaciales”, CONAE, from Argentina, are the responsible of the development of the program on a “fiftyfifty” basis.

The CESAR system is a multipurpose remote sensing system with the objective of fulfilling the demands of the user communities of both countries related to cartography, topography, land registry (basically rural land registry), crop evaluation, harvest predictions, knowledge of water quality, detection of underground water, evaluation of forest mass, oceanic and mineral resources, evaluation of ground and water contamination, as well as the evaluation of damage caused by fires and other natural disasters. The system will also provide data for geophysics applications, especially in the field of atmospheric composition and behavior, ozone destruction process, etc.

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To achieve the specified mission, the satellite will have the following instrumentation: l Panchromatic camera (PAN): in the visible range in the spectrum; it will be used for cartography and topography. This camera is being developed at INTA. l Multispectral camera (MUS): with six bands in the visible and near infrared range of the spectrum; it will be used for natural resources applications, especially for agriculture. This camera is being developed at CONAE and it is a follow on of the multispectral camera developed for the SAC-C satellite. l Spectrometer (ESP): to measure the concentration of the atmospheric gases involved in the ozone destruction process. This instrument is being developed at INTA, and it is inspired on the balloon-based system INTA has been flying for years. l High sensibility panchromatic camera (PAS): in the visible range of the spectrum (in a slightly wider band than the PAN), but with very high sensibility; it will be used to take images at nighttime of clouds and polar vortex. This camera is being developed at CONAE. To fulfill the mission requirements in terms of illumination and revisit times, a sun-synchronous orbit with an average height of 613 Km and a time of pass through the ascending node at 23:30 was selected. This orbit implies a repetition cycle of 47 days with revisit times that vary for the different instruments and latitudes. The mission life of the system will be 5 years and its expected launch date is late 2003. The pointing requirement for the

system is better than 0.56” with a pointing stability during image acquisition better than 0.056”. Image location on-ground shall be better than 1 Km. The CESAR program has successfully completed its Phase A, Viability, and it is currently averaging Phase B, Preliminary Design. 3. THE CESAR SYSTEM The CESAR system configuration approved at the end of Phase A is evolving in the process of preliminary design. The system is composed of the following segments: l Space segment: The satellite, which is composed of the payload and the bus or service module. l Ground segment: It comprises the tracking stations as well as the mission and control centers. The Launcher market was analyzed, and several launchers were pre-selected as viable. Among them we can find European (Rocket, Cosmos), American (Athena, Taurus) and Asian (CZ-2D) launchers. The satellite envelope for the viable launchers is in the range from 1.7 m diameter (CZ-2D) up to 2.1 m diameter (Rocket). The satellite envelope for the preliminary design phase has been selected as 1.7 m, to be able to fit in all the pre-selected launchers and, in case of using a wider fairing launcher with higher launching mass capacity (like the Rocket), to be able to share the launch with other spacecraft in the microsatellite range; for instance one microsatellite Ariane 5 type (60x60~80 cm, less than 100 Kg) and two microsatellites of the Space Shuttle Gas Can type (34x34~40 cm, less than 68 Kg).

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Figure 1. CESAR satellite configuration 4. SPACE SEGMENT current satellite CESAR The configuration is that of a small satellite following a modular approach in its design. The modularity concept is believed to be the best approach in our case to tackle the parallel development of the system in Argentina and Spain. The power and mass increase with respect to a non-modular approach is believed to be well spent in exchange for cleaner, simpler and easier to control interfaces. The satellite is functionally and physically divided in two modules: the Payload module, which carries all the instrumentation and the mission specific equipment, and the service module or bus, which is in charge of supporting the Payload in terms of power, pointing, etc. In order to reach a viable configuration, the system was framed in the small satellite segment (mass less than 500 kg). The present configuration, which has evolved from the one proposed at the end of Phase A, is within that segment, with a launch mass of approximately 450 kg, taking into account that this figure is a mid-Phase B

figure, and that the design is still under evolution. The CESAR satellite will fly with a quasi-inertial sun pointing attitude, except during the image acquisition periods that it will fly Nadir pointing. The satellite actual flight configuration with the solar panels deployed is shown in Figure 1. The Payload of the CESAR satellite is composed of the cameras mentioned above and other mission specific 1 shows the Table equipment. characteristics of the cameras. The payload also carries two other instruments: l Spectrometer: It will provide vertical spectra of the atmosphere using the technique of direct sun occultation in two selectable ranges (330-460 nm and This will provide 470-600 nm. numerical data of gas concentration

[CM,[NW, WW,

etc.

Data Acquisition System (SAD). It will collect via an UHF receiver on board, the environmental data provided by on ground Environmental Platforms. The data collected by this instrument will support the MUS operation. l

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1 Revisit time @ Nadir Sensibility

day

mW/cm2srpm

1 47 0.03

1 2 10”

1 6i7

1

0.01

6i7 0.01

6~7

1 6/7

0.03

0.02

1

(

6/7 0.02

)

6/7

I

0.003

Table 1. Payload characteristics The payload also includes other mission specific equipment as: l Payload data processing and storage subsystem (PAD): It will collect the data provided by the instruments, and after some processing it will store them in an on board solid state memory with 64 Gbits capacity, from where they will be dumped to ground through the TM1 subsystem. l Image telemetry subsystem (TMI): It will dump payload data to ground using an X Band link with 150 Mbps data rate. It will use QPSK modulation and a conformed beam antenna. The units of this subsystem, although it belongs functionally to the payload, will be located in the bus. The service module that supports this payload includes all the generic and support systems of the satellite. Using a monocoke (box) structure, it will have the following general subsystems: l On Board Data Handling (OBDH): This on board computer, initially based on an ERC-32 processor, is in charge of collecting housekeeping telemetry and executing telecommands, as well as maintaining the system integrity during non contact periods. The mission control software, as well as the ADOCS software, will be run in this computer. The baseline is to use Ada as programming language, and to use a SVF to validate the software. The on

board system clock will reside in the OBDH and it will be correlated with GPS. The electrical architecture will be based on a 1553B bus, with the OBDH as bus controller and the other units connected directly to the bus or using RTU’s; for the new design units the use of micro-RTU’s is being proposed. l Attitude Determination and Orbit Control Subsystem (ADOCS): In charge of the pointing and orbit control of satellite. the The pointing performance of the system during image acquisition will be better than 0.056”. To attain this performance the subsystem will use as sensors two star trackers (36 asec), coarse sun sensors, and magnetometers; as a backup for attitude determination, specially during propulsion maneuvers, it is proposed to use differential GPS. The actuators will be four reaction wheels and three torque rods; the propulsion subsystem could be used as attitude actuator in case of emergency. Orbit determination will be done by GPS, with an on-ground postfacto determination of 50 m. l Power: It will provide the energy necessary for the operation of the satellite by means of two solar panels (3.5 m2 providing -500 W), and a NiH2 battery (with at least 20 Ah capacity). It will also control power generation, battery recharge and power distribution through two specific boxes.

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Telemetry, Tracking and Command Subsystem (TTC): It will use S Band link to receive telecommands and transmit the housekeeping telemetry. The antennae configuration is under study, but currently it will use an S with ranging Band transponder two hemispheric capabilities and with RHCP to provide antennas omnidirectional coverage. l Propulsion: It will provide orbit injection correction capability, as well as orbit correction during the lifetime of the satellite. It will be a monopropellant system (Hydrazine). The system will use two redundant sets of four thrusters to be able to control attitude during the The initial calculations maneuvers. throw a need of less than 15 kg of propellant. The propulsion subsystem will be controlled by the ADOCS. l

5. GROUND SEGMENT The Ground segment is in charge of the CESAR system operation. Among its functions are collecting user requests, generating mission operation schedule, telecommand orbit determination, upload, receiving housekeeping and payload telemetry and the processing and distribution of the data. Argentina and Spain intend to use the existing capabilities in both countries to perform the functions assigned to the Ground Segment. This includes especially, the ground stations at Maspalomas (Spain) and Falda de1 Carmen (Argentina). Figure 2 shows the configuration of the Ground Segment.

ObwMl(lon

Images

.. r --...-. ....-..-..-........-.-.-........---.....--.. .---...

Figure 2. Ground Segment Architecture

reqU@

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Snail Satellites for Earth Observation

The basic building blocks of the ground segment are: l Control center: The control center is in charge of preparing the mission schedule having as inputs the observation schedules from both mission centers and the spacecraft maintenance activities. It will also prepare the telecommands as well as analyze the housekeeping telemetry to monitor the state of health of the satellite. Due to the dual nature of the CESAR system (Spain and Argentina), there will be two identical (as far as possible) control centers, but only one of them will operate alternatively (for periods of several months) as main control center; the other will be operating as backup or as alternative. l Mission center: There will be two mission centers, one in Argentina and one in Spain. These centers will collect the observational requests from the user communities of each country, generating a national observation schedule that will be sent to the main control center. The CESAR mission centers will be able to process the images up to five different levels with different levels of processing including the raw data divided in scenes, radiometric correction, standard systematic geometric correction, precision geometric correction with precise orbit data, and correction with ground control points. A single catalog of images will be kept.

Reception Stations. These stations will receive via an X Band link the payload data at a speed of up to 150 Mbps. The data will be transmitted to the respective mission center. Tracking to an X Band beacon in the spacecraft will be provided. The 15m station at Maspalomas and the 13m and the 7m stations at Falda de1 Carmen will be l

used. l TTC stations. These stations will use an S Band link to receive housekeeping telemetry and to uplink telecommands. The TTC stations in both countries will be controlled by the main control center that will use both of them indistinctively. The 5m and the 15m stations at Maspalomas and the 13m and the 3.6m stations at Falda de1 Carmen will be used.

6. CONCLUSION CESAR is a model example of profitably accomplishing good Earth observation and space science at low national cost by means of international cooperation. The complications arising from the international interfaces are more than made up for the ultimate results and reasonable costs. The process affords a positive synergism between the scientists and engineers from the participating countries, which forges pleasantly productive and lasting ties with colleagues from other lands and societies.