FASat-Bravo (Fuerza Aerea Satellite - Bravo)

FASat-Bravo is the second Chilean experimental microsatellite in orbit, built under a technology transfer program between the Chilean Air Fore (FACh = Fuerza Aerea de Chile) and SSTL (Surrey Satellite Technology Ltd), Guildford, UK. The FASat program included the training of Chilenian engineers with aerospace experience (at SSTL) and to operate the Mission Control Station (ECM-Santiago) in Chile. The program's primary objective is that of acquiring the basic scientific and technological experience required to take more advanced steps.

Background: Prior to FASat-Bravo, FASat-Alfa was built at SSTL and launched on August 31, 1995 (as a secondary payload to SICH-1, a payload of Ukraine) on a Russian Cyclone-3 vehicle. Orbit of FASat-Alfa: Sun-synchronous near-circular orbit, perigee=651 km, apogee=682 km, inclination = 82.5º, period = 98.7 minutes. Unfortunately, the separation mechanism to release the microsatellite from SICH-1 failed to operate. This implied non- operation (or hibernation) for FASat-Alfa. Some time after the launch, FACh and SSTL have declared the spacecraft as lost. The SICH-1 S/C was able to perform its observation functions unharmed with the FASat S/C permanently attached to it. ¹⁾
The FASat program was initially proposed in 1993 by FACh to the government of Chile. In response to an invitation to tender, SSTL offered the most tempting project, including partnership construction of the probe, technological transfer and postgraduate training for FACh technicians, and was awarded the contract in 1994.

Spacecraft:

FASat uses the proven modular UoSAT bus design (MicroSat-70 platform); the S/C is stabilized by an Earth-pointing gravity boom and by a three-axis magneto-torquing system. First introduction of a yaw-axis reaction wheel into the micro bus to provide a nadir-pointing non-spinning platform. The FASat structure consists of 11 module trays including the battery box. Eight of the module trays are used for the platform systems, three module trays for payloads.

S/C mass = 50 kg, power = 35W peak (25 W orbit-average), size = 60 cm x 36 cm x 36 cm. The FASat onboard data handling design introduced a CAN (Controlled Area Network) bus; each onboard instrument/subsystem (processing node) was able to access the RAM memory through CAN.

Figure 1: Illustration of FASat-Bravo (image credit: SSTL)

RF communications: The RF system consists of two redundant transmitters, three receivers and the associated modulators/demodulators. Up to 600 images of 300 kByte each can be stored on the satellite's solid state data recorder. The downlink data rates are between 9.6 to 76.8 kbit/s to a single ground station in Chile. MSC-Santiago (Mission Control Station - Santiago at the Los Cerrillos Air Force Base) is in charge of controlling all aspects of satellite operations.

Launch: FASat-Bravo was launched successfully as a secondary payload [along with Thai- Paht-1 (TMSat of Thailand), TechSat/Gurwin-II (Israel), WESTPAC (Australia), and SAFIR-2 (OHB, Germany) - and RESURS-O1-4 as the primary payload of Russia/Ukraine] from the Baikonur Cosmodrome on a Zenit-2 launcher on July 10, 1998.

Orbit: Sun-synchronous near-circular orbit, mean altitude = 820 km, inclination = 98.1º, local equator crossing time at 21:37 hours.

Status of FASat-Bravo mission: FASat-Bravo was operational until mid 2001 (3 years of operational life).

Sensor complement: (OLME, EIS, DTE, GPS Receiver, EdEx)

The FASat-Bravo mission includes the same sensors/experiments as those of FASat-Alfa.

OLME (Ozone Layer Monitoring Experiment). The objective of the prime instrument is to monitor the distribution of ozone. The instrument consists of two nadir-pointing UV cameras, one operating with CCD detectors, the other with UV photodiodes. The objective is ozone layer monitoring by measuring the backscattered UV solar radiation. The measurements of OLME are in particular dedicated to overflights of the antarctic and sub-antarctic regions of Chile (correlations with ground-based observations of the UV radiometric network). OLME employes the well-known SBUV (Solar Backscatter Ultraviolet) principle to determine the total column concentration of ozone. ^{2) 3)}

OLME comprises two instrument packages: OUBI (Ozone Ultraviolet Backscatter Imager) - two dye-coated CCDs, capable of taking images of the backscattered UV light - and the OMAD (Ozone Mapping Detector), a four-channel UV-enhanced photodiode-based radiometer. The OUBI 380 nm camera images `albedo' UV light, while the 313 nm camera images the UV light scattered primarily from higher altitudes. The OMAD data are used in the analysis of the OUBI images, and in addition the OMAD 313 nm and 334 nm data are used to derive relative global maps of total ozone concentrations.

The OLME instrument was radiometrically and optically calibrated at SSTL and at NASA/GSFC between 1995 and 1998.

Table 1: Parameters of the OLME UV cameras

EIS (Earth Imaging System). EIS consists of two video CCD imaging cameras with two optical assemblies. The cameras are aimed in the same direction but offer a different FOV. The wide angle camera (WAC) image has a ground resolution of 2 km and a FOV of 1500 km x 1050 km (NIR 810-900 nm filter; 4.8 mm focal length). The objective is to provide snapshot imagery with good contrast between land, sea, clouds, and ice or snow. - The narrow angle camera (NAC) has a ground resolution of 200 m and a FOV of about 150 km x 100 km (red 610 - 690 nm filter, 50 mm focal length). The cameras feature a monochromatic design with optical filters chosen to contrast ground properties, the NAC highlights variations in soil moisture content and vegetation density. An image is taken in a snapshot or staring mode (as opposed to a mechanically scanned pushbroom array) and is read from each CCD by a Transputer Data Processing Experiment (TDPE) and, after processing, provided to the On- Board Computer (OBC) and recorded. Camera sensor (both cameras): EEV CCD04-06 image sensor + chip set; 578 x 576 array (interleaved fields); anti-blooming and electronic integration control. Operational Earth imaging is performed on a scheduled snapshot basis.

Table 2: Some parameters of the EIS camera system for FASat orbit

DTE (Data Transfer Experiment), S&F mode operation. The instrument consists of two redundant receivers and a redundant DSP (Digital Signal Processing) unit. The objective of DTE is to provide the space segment infrastructure (hardware, interfaces, firmware and software) to allow a variety of communication experiments of the following nature: a) uplink experiments, b) duplex/two-way experiments, c) data collection, d) data transfer (collection and distribution), e) downlink experiments, f) data distribution, g) technology demonstration, h) enhanced modulation/data rates/small terminals.

GPS Receiver (based on the Trimble TRANS-II 6-channel receiver). GPS data provides onboard position, velocity and time reference. The data is used by OBC to generate an orbital element set and to provide scheduling and synchronization to other onboard computers, and to allow ground stations equipped with a GPS receiver to experiment with applications for real-time DGPS. FASat (like its predecessor PoSat-1) is able to autonomously determine its orbit through the processing of GPS data into orbital elements (generation of Keplerian elements using a 32-bit transputer system).

EdEx (Educational Experiment). EdEx is intended to promote direct participation in space by Chilean schools to communicate with the FASat S/C. EdEx in turn uses DTE to generate telemetry signals that can be received by low-cost receivers and a PC. DSP chips with the DTE produce digitized voice data that may be received by simple receivers. EdEx takes place on a scheduled basis. The curricular benefit for the students arises from the following activities:

· S/C tracking (learning of some basic orbital parameters)

· Handling of satellite communications (telemetry reception)

· FASat satellite telemetry analysis (in particular those parameters related to electric and thermodynamic parameters).

2) C. I. Underwood, A. Valenzuela, M. Schoenherr , M. Arancibia, M. Fouquet , "Initial in-orbit results from a low-cost atmospheric ozone monitor operating on board the FASat-Bravo microsatellite," Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 361, No 1802, Jan. 15, 2003, pp. 71-76; URL: http://www.journals.royalsoc.ac.uk/media/7PED06P2AG7QULC93CBY/Contributions/3/G/P/7/3GP7EAL9W16YWGTQ.pdf

3) http://www.ph.surrey.ac.uk/satellites/main/ozone1_4.html