SUNSAT is a 60kg, 45 by 45 by 62 cm micro satellite designed, built and tested by twenty-two Masters of Engineering students at the Electronic Systems Laboratory in the Department of Electrical and Electronic Engineering at Stellenbosch University.
Detailed design started in January 1992, led by Computer and Control System lecturers. Sunsat was originally designed for a sun-synchronous-type orbit on the Ariane 4 Helios mission, which is ideal for the main imaging payload. When launch costs became prohibitive, alternatives were sought.
NASA scientists have learnt much about the earth by detailed studies of the magnetic field and the gravitational field, and had arranged for the Danish Oersted microsatellite to be launched as a secondary payload on a USAF Delta II from Vandenberg Air Force Base on the P91-1 Argos mission in January 1996. NASA and Stellenbosch have now agreed to carry Sunsat into the same orbit instead of a counterweight for Oersted.
In exchange for the launch, Sunsat will carry a precision GPS receiver and a set of Laser retro-reflectors. These will enable NASA to study fine orbital perturbations for gravity field recovery, and for cross verification of GPS and NASA's laser tracking network. The orbit will be the same as Oersted, namely polar, 400 by 840 km. The equatorial crossing will initially be approximately 15:00, and drift an hour earlier every seventy days.
Sunsat is a complex microsatellite. Its developers expect they will not have time and manpower to utilize all its possibilities, and hope that other amateurs and universities will become interested in using it once it is fully commissioned. Since this is their first satellite, they reconise this may take many months to get right.
But first a look at Stellenbosch University:
Stellenbosch is the second oldest town in South Africa, not far from the Southern tip of the African Continent. It is about fifty kilometres East of Cape Town. Apart from the historical significance of the town, it is also known for the excellent wines that are made in the district and the excellent university situated in the town. Stellenbosch is popular amongst tourists and is a very attractive place.
The university has about 14000 students and is one of the oldest universities in the country and has an excellent academic record. The satellite is being built by students who have done all detail design and software, system level design was done by lecturers, some of whom studied at the Universities of Surrey, Stanford and MIT. It is therefore not strange that the satellite is very similar to the famous UoSATs built at Surrey. The amateur radio call area is ZS1.
The Amateur Radio communications payload comprises a packet radio service, a 2m band parrot speech transponder, and a Mode S transponder.
The use of an imaging system necessitates attitude stabilization. Coarse attitude stabilization will be by a gravity gradient boom and by magnetorqueing and is improved by small reaction wheels during imaging. Continuous attitude sensing is by magnetometers. Sun sensors, visible wavelength horizon sensors, and a star sensor provide 1 mrad accuracy when imaging from the sun-synchronous orbit. The average power of 25W enables images of South Africa and elsewhere to be taken on a daily basis for real time downlinking.
Availability of excellent linear Silicon CCD sensors able to operate in the visual and near-IR band led to a 3-color sensor system with bands similar to SPOT 4 and LANDSAT 6. These permit biomass production monitoring, which is of continuing interest in our water-short country. A linear CCD sensor with 3456 pixels of 10.7 microns spacing was chosen. The optical assembly is mounted in a tube which can be rotated forward or rearward for stereo images.
Communications payload:
The communications payload provides duplicated synthesized transmitters and receivers for the 2 m and 70 cm Amateur Radio bands and nearby frequencies.
A 1296 MHz receiver can operate as a fast uplink, or be coupled to the S-band downlink transmitter to provide a straight-through transponder.
Image downlinking:
The high resolution data will be transmitted in real time via the S-Band downlink to reception stations at Stellenbosch and Johannesburg. Small-area images stored in the RAM disk can be down-linked at much lower rates. For example, a 40 kbyte image covering a 4 km x 4 km area can be downloaded at 9600 baud in about 100s. The Sunsat team plans to be able to supply such images on request to amateurs once the satellite is fully operational.
Data collection and transfer:
At 800 km altitude, the 5 deg elevation footprint has a diameter of 5080 km, which spans 45 deg in longitude. Radio range varies from 800 km to 2800 km compared to the geostationary range of 36000 km.
Data communication with 10 Watt or lower powered transmitters and dipole antennas is practical, permitting data interchange with low cost terrestrial transceivers. Since large quantities of data can be stored in the satellite, global data transfer is possible. AX.25 data protocols will be used to ensure error-free operation.
SUNSAT S-band downlink:
The 5 Watt EIRP S-band downlink will produce a 14.4 dB S/N ratio in a 40 MHz bandwidth at 2000 km slant range for a 4.5 m diameter 100K receiving station as planned for Stellenbosch. By adding an L-band receiver and appropriate switching, a transponder capable of 1 MByte/s with 2 m diameter ground stations can be implemented. Application of the system for Amateur Radio gateway service is possible.
Amateur Radio VHF and UHF payload:
The Amateur Radio payload definition was approved at the SA-AMSAT Spacecon 91 Conference. Store and forward digital packet radio will be provided, including 1200 baud AFSK for compatibility with terrestrial equipment common in SA. To provide sufficient uplink channels, one of the 2m band receivers has four IF sections displaced in 25 kHz steps, and connected to 1200 baud modems. Three 9600 baud modems compatible with the G3RUH standard are carried, and can be switched to various receivers and transmitters. Both 2 m up/down and 2 m up/70 cm down options will be included, together with full bulletin board facilities. The AMSAT Pacsat Standard Protocols can be supported.
The 2 m and 70 cm downlinks can be switched to 10 Watts output, producing a 0.5 uV signal (50 Ohm) at 435 MHz and 1.5 uV signal at 145 MHz with 0dBi receive antenna at full range. This power level will be used over critical areas to provide signal to noise ratios approaching 15dB for easy reception. At other times the power will be reduced.
A 2 m Parrot mode repeater is intended especially for Novice category users (under 16). Uplinked speech will be digitally stored and retransmitted on the same frequency. Novice school users will thus hear the re-transmission and know that they are getting through. The need to learn and apply operating protocols will definitely be experienced!
Antenna pattern studies:
The VHF and UHF radiation patterns must be smooth, with no holes, and with maximum gain at the horizon, which is 30 deg below local level. Circular polarization is required to avoid Faraday rotation induced nulls with linearly polarized ground antennas. SUNSAT's structure has a major effect on the VHF and UHF antenna patterns, requiring use of NEC2 for numerical evaluation of various antenna configurations. Sunsat's antenna design has had to be changed recently to be compatible with the Delta II launcher. Details will be published later.
Apart from the imaging and communications payloads just described, SUNSAT is a sophisticated satellite bus capable of fine attitude control and carrying alternative payloads. Since the attitude control system is of particular interest, and drives some other systems designs, it is described first in the following subsection. Thereafter the flight control, TT & C and power systems are detailed.
Attitude determination and control system:
The attitude determination and control specifications on SUNSAT are stringent for a microsatellite. The design goal is to be able to point the pushbroom imager boresight to within 1 km accuracy from an 800 km altitude, which amounts to a pitch and roll error of less than 1.2 mrad.
Five types of attitude sensors are used. A 3-axis magnetometer is used to measure the strength and direction of the geomagnetic field. This low power (100 mW) device can be operated continuously to provide attitude accuracy approaching 1 degree. Coarse attitude information is also derived to within 5 degrees from six cosine-law solar cells mounted on each facet of the satellite. Horizon sensos, a fine sun sensor and a star sensor serve as the accurate attitude measuring devices. The horizon sensors consist of two linear CCD and lens assemblies that obtain orthogonal measurements of the sunlit earth horizon. Pitch and roll attitude angles can be measured to an accuracy of 0.5 mrad. These sensors, based on a design by one of the SUNSAT team members, are currently flown on UoSAT-5. A fine sun sensor, consisting of a slit aperture perpendicular to a CCD array, will measure the sun azimuth to within 1 mrad. During imaging this sensor will face the sun and provide accurate yaw information to the attitude control system.
Finally, a 10 degree by 10 degree star image taken in the orbit normal direction is projected onto a matrix CCD sensor to provide back-up accurate 3-axis attitude information. V-6 magnitude stars will be detected to obtain at least two separated stars within the sensor's field of view. The roll and yaw angular resolution will be at least 1 mrad, while the pitch resolution will depend on the star separation.
Attitude control is achieved through a passive gravity gradient boom, combined with two redundant active actuation methods. Slow attitude motions and coarse pointing to within 1 degree is achieved through magnetorquers. These comprise air coils wound into channels in the solar panels on the sides of SUNSAT, and others in the top facet of the structure. These reliable, long life actuators are used for detumbling of the satellite, libration damping, nadir-axis spin control and momentum dumping of the reaction wheels. Accurate pointing and stabilization during imaging is provided by 4 servo-motor driven reaction wheels. The maximum torque per wheel is 3.5E-3 Nm, which provides a 180 degree slew around the nadir pointing axis within 90 seconds.
Flight control:
Flight control is provided by redundant heterogeneous computers of differing type. General flight management tasks such as scheduling, CCD imager control and communications management are performed by an Intel 386-SL processor, backed up by an Intel 80C188EC processor. Both have access to all peripherals, but the 386 is the preferred flight controller. A T800 transputer is dedicated to the fine attitude control system, but its tasks can be taken over by the 386 in case of failure. Three additional embedded 80C31 microcontrollers provide further support for telemetry, telecommand and attitude control subsystems. Each of the two main processors has its private static EDAC RAM, FLASH RAM and a small PROM with basic boot code. Separate rows of RAM can be isolated in case of permanent failure of a memory chip. A separate RAM disc of 64 M-byte, which is accessible by both processors, is provided for storage of imager data or large files for store-and-forward missions.
Telemetry and telecommand:
The telemetry data collection function and the data transmission functions are duplicated for redundancy. Telemetry data can be collected from acquisition modules in each subsystem of SUNSAT by either of the flight management computers. In case of flight computer failures, a backup discrete component telemetry system is also available to feed simply formatted data streams to 1200 baud modems, which can be switched to any of the transmitters. The telecommand system, being extremely critical, also has a backup system implementation with discrete logic components. Reliability against natural occurring errors and illegal commands have been prime design objectives.
Power system:
The power system is kept simple while providing for as many component failures as possible. Peak power consumption could reach 140 Watts so careful power management will be required to limit depth of discharge of the batteries to 20% to ensure a lifetime exceeding 5 years. Timeouts are included on all transmitters and the whole satellite is reset to a safe state if the battery enters deep discharge.
Satellite structure:
The structure supplies mechanical support during launch, and thermal and radiation protection in orbit. A guideline radiation requirement of 2 mm Aluminium between any electronic component and the exterior gives great stiffness to the layered box structure. The Payload Adapter Assembly (PAA) threads into the bottom plate. Mechanical design and verification of the structure, particularly the bottom tray and the PAA had been delayed until the launch vehicle was defined. McDonnell Douglas (MD) however, required a finite element model of Sunsat that had been verified by model survey testing by early 1995. Great interaction with MD and the orbital launch service team at NASA, and "star performance" by Sunsat's mechanical team saved the day. Modal survey tests in November 94 on a flight-like structural model showed that the finite element computer model of Sunsat require little modification to meet all requirements.
Composition of satellite:
The 11 trays are stacked on top of each other with the sensors on the top plate.
The bottom and top trays are milled from solid aluminium for structural stiffness. The bottom tray contains the batteries, power amplifier modules, and the battery charge regulator. The rest of the volume is used for the main payload which in SUNSAT 1 comprises the optical tube and the reaction wheels required to meet attitude control requirements of the imager.
The cabling between trays is divided into a slow bus and a fast bus on opposite sides of the satellite. Using subminiature `D' connectors, a maximum of 250 wires can be connected to a side of a tray. The slow bus carries signals that produce little radio frequency interference. The fast bus carries signals such as micro-processor busses, and will be screened with great care.
Four sides of the satellite are used for solar panels, the bottom (earth) side for antennas and launcher attachment ring, and the top side for attitude determination sensors.
Conclusions:
Sunsat was started at Stellenbosch to
a) Provide an exciting core project around which to expand graduate research activity in electronic systems.
b) Promote interaction between Stellenbosch and international groups.
c) Contribute to amateur radio internationally.
d) Stimulate interest in technology as a career for school children in South Africa.
The NASA sposored launch opportunity for January 1996 has significantly changed the status of the project. The SARL and SA Amsat will shortly start a programme to select a few small school-built experiments to be added to the GPS receiver tray in Sunsat.
This bulletin and future bulletins will soon be available by FTP. Details will be made known as soon as possible.
As radio amateurs in the Sunsat team, we of the Sunsat Bulletin Team look forward to utilizing Sunsat and it's launch to stimulate participation in our hobby.
Hans van de Groenendaal, ZS5AKV, AMSAT-SA
AMSAT@uctvms.UCT.AC.ZA
Member of SUNSAT ADVISORY BOARD (In charge of PR and Publications)
Return
to Future of Amateur Satellites