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SCD (Satélite de Coleta de Dados) - Data Collection Program of Brazil

The SCD satellite program of INPE (Instituto Nacional de Pesquisas Espaciais) or “National Institute of Space Research” is to provide a spaceborne environmental data collecting capability for Brazil. This program is considered complimentary to the Argos DCS (Data Collection System) flown on the POES (Polar-orbiting Operational Environmental Satellite) series of NOAA, USA. Most data collected by the SCD spacecraft are related to meteorological and hydrological phenomena, gathered and transmitted by around 700 terrestrial data collecting platforms (DCP's) spread over the country (and some platforms installed in other South America countries).



 

SCD-1 (Satélite de Coleta de Dados-1)

SCD-1 is Brazil's first experimental Data Collection Satellite, a portion of MECB (Missao Espacial Completa Brasileira). The MECB program was approved by the Government of Brazil in 1979. INPE's responsibility in MECB was the space segment and associated ground segment of the program [six satellites were planned within this program: four for environmental data collection (SCD-1, SCD-2, SCD-2A and SCD-3) and two for remote sensing of the Earth (SSR1 and SSR2)]. The Brazilian Space Activities Program (MECB) has been reorganized in 2000. Now, the satellites are part of the Application Satellites Program. The DCP network in Brazil is equipped with sensors suitable for the fields of meteorology, hydrology, oceanography and atmospheric chemistry. 1) 2) 3)

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Figure 1: Line drawing of the SCD-1 spacecraft (image credit: INPE)

Spacecraft:

SCD-1 is a spin-stabilized spacecraft; mass = 115 kg; power = 90 W; design life = 1 year. The shape of SCD-1 is an octagonal prism, 100 cm in height, whose base fits into a 1 m diameter circle. The outer surface of the spacecraft is covered with solar cells.

ACS (Attitude Control Subsystem): The ACS is responsible to stabilize and control the spacecraft orientation with respect to the Sun. To perform these tasks, the ACS is provided with two digital one-axis sun sensors, one analog magnetometer, one spin axis air core magnetic coil, and a passive nutation damper. The stabilization is achieved by a rotation around its major principal axis imparted to the spacecraft by the launcher's last stage. Attitude determination and control are both performed on-ground, by using the telemetered sensor signals and commanding the appropriate coil polarity. 4)

RF communications: S-band TT&C communication is provided by a TM/TC subsystem for housekeeping telemetry transmission, telecommand and ranging (S/C mission control functions). On-board storage capability for TT&C data. A UHF uplink at 401.650 MHz and 401.620 MHz is used for data collection.

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Figure 2: The SCD-1 satellite being integrated in the INPE test facility (image credit: INPE)

Launch of SCD-1: Feb. 9, 1993 from NASA's Kennedy Space Center with a Pegasus launch vehicle of OSC (Orbital Sciences Corporation).

Orbit: Near-circular orbit; altitude = 750 km; inclination = 25º; period = 98.8 minutes.


 

DCS (Data Collection System):

The DCS onboard SCD-1 satellite is a real-time repeater of environmental data gathered on the ground by automatic data collection platforms (DCPs) providing random access service for DCPs. The real-time repeater concept implies that there is no onboard processing and storage capability but simply a relay function of data by the satellite.

Data reception on the ground is at the stations in Cuiabá and Alcântara, Brazil every time SCD-1 comes into view. This concept permits a data collection capability from all sites in Brazil and well beyond (coverage region of 3000 km radius). It is estimated that the system setup is capable of servicing up to 1000 DCPs randomly located in the coverage area with a probability of 85% of acquiring the data of any DCP at least once per day. The DCP data received by the Cuiabá station are processed by the MCC in Cachoeira Paulista and subsequently stored in a database for user access.

The random access method employed by SCD-1 for data collection is similar to and compatible with the Argos data collection system. This means:

• Access scheme is analogous to unslotted ALOHA, without a return channel

• Message format, data rates, and transmission frequencies are Argos compatible (message length of 32-256 bits, 400 bit/s uplink rate, 401.650 MHz and 401.620 MHz uplink frequencies).

• Brazilian DCPs in the ground segment can also be serviced by other satellites carrying the Argos DCS.

 

SCD-1 access scheme:

SCD-1 uses a “random time-division multiple access” scheme which is similar to “unslotted ALOHA.” However, SCD-1 and also the Argos-1 access scheme are simpler than ALOHA due to the lack of a return channel from the satellite to the DCP. This means in fact loss of data due to interference (interference occurs when the demand for service exceeds the system's capability; the result is loss of data from system `blockage'). However, the system works satisfactorily in spite of this disadvantage for the following reasons: 5)

• Interference is mitigated by Doppler shifts, which tends to spread the DCP transmissions

• The system operates at a low input rate (400 bit/s) so that a data packet has at least a probability of 80-90% of getting through without interference

• There is a lot of redundancy in the data of consecutive transmissions from the same DCP, so that successful reception of one out of two or even three transmissions is still satisfactory

• However, there is no catastrophic effect for subsequent communications if some data are lost.

DCPs in the ground segment: The various DCPs differ in the parameters measured according to the measurement objective and location. Thus, they are able to measure temperature, rain level, wind direction, solar radiation, carbon monoxide as well as many others, beyond its own location.

 

Mission status:

• On February 9, 2012, SCD-1 completed 19 years in space. During this time, the spacecraft completed 100274 orbits around the Earth and traveled about 4.5 billion km, which corresponds to 5910 round trips to the moon. The SCD-1 is the first satellite developed by INPE and it is still operating and relaying information to the weather forecasting and watersheds monitoring, among other applications. 6) 7)

• SCD-1 is operating nominally as of 2006, 13 years after launch - with a design life of 1 year. 8)

• In the time frame 2005-2006, there are ~ 700 DCPs in the ground segment

• As of 2001 there were 400 DCPs operational in Brazil; others were in the phase of installation or were being planned for the future (including those from other South American countries).

• After deployment, the SCD-1 was performing nominally.There were only 25 DCPs in the ground segment when the spacecraft was launched in 1993.



 

SCD-2 (Satélite de Coleta de Dados-2)

SCD-2 is a follow-on satellite with the same objectives as SCD-1, but with improved data collection performances, due to modifications in the antenna- and attitude control subsystems. The SCD-2 spacecraft has an aluminum honeycomb structure, the total mass = 117 kg, power = 110 W. The spacecraft design life is 2 years. The onboard data handling system was modernized. All functions are now performed by one single OBC (Onboard Computer). 9) 10)

SCD-2 is also a spin-stabilized spacecraft. However, the spin rate is now actively controlled (in the 32 - 36 rpm range) due to two magnetic torque coils. The new attitude system of SCD-2 permits to eliminate the reception antennas on the spacecraft bottom panel. There are only four UHF monopoles on the top panel.

RF communications: S-band TT&C communication is provided by a TM/TC subsystem for housekeeping telemetry transmission, telecommand and ranging (S/C mission control functions). On-board storage capability for TT&C data. A UHF uplink at 401.650 MHz and 401.620 MHz is used for data collection.

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Figure 3: Artist's view of the SCD-2 spacecraft in orbit (image credit: INPE)

 

Launch: The launch of SCD-2 took place Oct. 23, 1998 on a Pegasus-XL vehicle (with OSC's L-1011 Stargazer aircraft) from Cape Canaveral, FL. The launch site of SCD-2 was at 29.0º N and 78.3º W.

Orbit of SCD-2: perigee = 743 km, apogee = 768 km (average altitude of 750 km), inclination = 25.0º. The two S/C, SCD-1 and SCD-2, are phased in the same orbital plane.

The SCD-2 satellite carries also a data collection transponder, along with the same DCS payload as SCD-1.

 

Mission status:

• The SCD-2 spacecraft is operational in 2012 - in its 14th year on orbit.

The geographical locations of data collection platforms (DCP) in the Brazilian Environmental Data Collection System are obtained by processing Doppler shift measurements between satellites and DCP. When the signals travel from a DCP to a satellite crossing the terrestrial atmosphere, they are affected by the atmosphere layers, which generate a delay in the signal propagation, and cause errors in its final location coordinates computation. The signal propagation delay due to the atmospheric effects consists, essentially, of the ionospheric and tropospheric effects. - This work provides an assessment of ionospheric effects using IRI (International Reference Ionosphere) and IONEX (IONosphere map EXchange) models and tropospheric delay compensation using climatic data provided by National Climatic Data Center. Two selected DCPs were used in this study in conjunction with SCD-2 satellite during high and low solar activity periods. Results show that the ionospheric effects on transmission delays are significant (about hundreds of meters) in equatorial region and should be considered to reduce DCP location errors, mainly in high solar activity periods, while in those due to tropospheric effects the zenith errors are about threemeters. Therefore it is shown that the platform location errors can be reduced when the ionospheric and tropospheric effects are properly considered. 11)

• SCD-2 is performing nominally as of 2006.

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Figure 4: SCD-2 spacecraft integration with Pegasus (image credit: INPE)

Note: The launch of SCD-2 was preceded by SCD-2A, an identical spacecraft to SCD-2. This satellite was supposed to be launched from CLA (Alcantara Launch Center), Maranhao, Brazil, using the newly nationally developed VLS (Satellite Launch Vehicle) system, provided by the IAE/Maer. However, the launch of SCD-2A ended in a complete launch failure (an ignition problem in one engine of VLS caused the loss of the spacecraft). The launch attempt of SCD-2A took place on Nov 2, 1997.


 

Experiments on SCD-2:

• A new ERR (Reaction Wheel), a prototype subsystem developed by INPE, is flown for space qualification. All additional attitude control elements are similar to those of SCD-1.

• A solar cell experiment (SCE2) is flown as a subsystem. The objective is to use the data from the solar cell experiment, a self-calibration method, for the assessment of the albedo. SCE2 consists of an array of a glass cover, three solar cells, a kapton foil isolates the cells from the mechanical structure, and a PCB (Printed Circuit Board) that contains the components of the electrical amplifying signal circuit of the SCE2 (part of the satellite telemetry). 12) 13)

Note: The SCD-1 spacecraft features also a solar cell experiment, designated as SCE1.

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Figure 5: Schematic composition of the SCE2 experiment (image credit: INPE)


1) INPE brochure `SCD-1 Data Collection Satellite,' and fax information from Prof. P. M. Fagundes, Rio de Janeiro

2) “SCD-1 Satellite Description,” and “The Brazilian Data Collecting System,” papers provided by C. E. Santana of INPE, May/June 1992

3) O. Durào, “Data Collection Satellite Application in Precision Agriculture,” Proceedings of 34th COSPAR Scientific Assembly, The Second World Space Congress, Oct. 10.19, 2002, Houston, TX, USA

4) Valdemir Carrara, Ulisses Thadeu, Vieira Guedes, “Attitude Control Aspects for SCD1 and SCD2,” Journal of the Brazilian Society Mechnical Sciences (RBCM), Vol. 16, Special Issue, 1994, ISSN: 0100-7386, URL: http://www2.dem.inpe.br/val/publicacoes/carrara_attcontrol_rbcm_16_94.pdf

5) Information provided by C. E. Santana and by J. Kono of INPE

6) “First Brazilian satellite completes 19 years in orbit,” INPE, Feb. 8, 2012, URL: http://www.inpe.br/ingles/news/news.php?Cod_Noticia=282

7) William Reis Silva, Maria Cec¿lia F. P. S. Zanardi, Regina Elaine Santos Cabette, Jorge Kennety Silva Formiga, “Study of Stability of Rotational Motion of Spacecraft with Canonical Variables,” Mathematical Problems in Engineering, Volume 2012, Article ID 137672, 19 pages, doi:10.1155/2012/137672, URL: http://downloads.hindawi.com/journals/mpe/2012/137672.pdf

8) M. A. Chamon, “Scientific and Technological Satellites at INPE/Brazil,” Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B5.2.01

9) http://www.inpe.br/programas/mecb/ingl/satellite/scd2/scd2.htm

10) http://www.las.inpe.br/~veissid/eng18.html

11) Áurea Aparecida da Silva, Wilson Yamaguti, Hélio Koiti Kuga, Cláudia Celeste Celestino, “Assessment of the Ionospheric and Tropospheric Effects in Location Errors of Data Collection Platforms in Equatorial Region during High and Low Solar Activity Periods,” Mathematical Problems in Engineering, Volume 2012 (2012), Article ID 734280, 15 pages, doi:10.1155/2012/734280, URL: http://www.hindawi.com/journals/mpe/2012/734280/

12) N. Veissid, “New developments in using solar cells as remote sensors to gauge climate change,” Environmental Geosciences, Vol. 10 , No 2, 2003, pp.. 47-57

13) “Solar cell experiment of the SCD-2 Satellite,” URL: http://www.las.inpe.br/~veissid/eng20.html


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.