November 1998
Contents |
|
|
|
3... 2... 1... Lift-off! Stellenbosch University's Satellite Programme is
set to make history early next year with the first ever launch into orbit
of a space asset constructed on African soil. If everything goes according
to plan, SUNSAT-1 will be launched from the Vandenberg Airforce Base,
California, on the 8th (now 15th) of January 1999. Designed and built
almost in its entirety by postgraduate students from the University of
Stellenbosch, SUNSAT-1 heralds South Africa's entry into the Space Age.
| |
During 1991, academic staff at the Electrical and Electronic Engineering Department of the University of Stellenbosch identified the need to afford their M.Eng. students the opportunity to increase their involvement in practical design activities. A long-term project, similar to those at the Robotics Laboratory at Stanford University and the Spacecraft Engineering Research Unit at the University of Surrey, was envisaged to produce industry-ready students. Thanks to the initiative of Professors Arnold Schoonwinkel, Garth Milne, and Jan du Plessis, and later, Mr Sias Mostert, it was decided to embark on the ambitious project of designing and building a fully functional micro satellite. Thus, the SUNSAT Programme was born in 1992 with the following expressed goals:
| |
Who is funding the launch?Who is funding the launch? |
|
Launching a satellite is a very expensive exercise, and SUNSAT's launch could have been unaffordable to a startup project. Fortunately, international collaboration has provided SUNSAT with a free launch opportunity, following the signing of a Memorandum of Understanding in May 1996 between NASA and South Africa. Under this agreement, the Geodynamics Group of NASA's "Mission to Planet Earth" arranged for NASA to provide the launch of SUNSAT as an auxiliary payload on their ARGOS mission. In exchange, SUNSAT will carry an experimental global positioning system (GPS) receiver and a set of satellite laser retro-reflectors (SLR) supplied by NASA's Jet Propulsion Laboratory. These will enable NASA to make position measurements with millimetre accuracy from their various laser ranging stations on the earth. The data gathered in this manner will be used to monitor geodynamic parameters and to update gravitational models of the earth. | |
SUNSAT will be launched from the Vandenberg Airforce Base in California,
on board a Boeing Delta II launch vehicle. The main payload of the mission
will be the 2½ ton US Airforce ARGOS satellite. SUNSAT and another micro
satellite from Denmark, called Ørsted, will be the secondary payloads.
Currently, the launch date is scheduled for the 8th of January 1999.
A Delta II launch incident in January 1997 and technical delays with the
ARGOS satellite had caused earlier launch dates to be postponed.
| |
What will be the sequence of events leading up to the launch date? |
|
On the 19th of November 1998, Air Malaysia will transport SUNSAT to California at no cost to the university as a favour to SUNSAT for hosting an experiment by the Malaysian University of Kebangsaan. SUNSAT will then remain at NASA's facilities at the Vandenberg Airforce Base, where SUNSAT project members will install the satellite's Nickel-Cadmium flight batteries. These extremely sensitive batteries have spent the past several years in a frozen state, however, and will first have to be defrosted with care. The SUNSAT team will then install the satellite's solar panels, as well as the pyrotechnic cutters needed to deploy its gravity gradient boom. Following this, the SUNSAT team members will hand over the satellite's payload adapter assembly (PAA) to Boeing, who will mate it to a payload adapter fitting (PAF) of their own, which will later be bolted to the launch vehicle. The mated PAA and PAF unit is then returned to the SUNSAT team, who must screw and lock it into the satellite's underside. SUNSAT is scheduled to be secured on board the Delta II launch vehicle on the 11th or 14th of December 1998, approximately 4 weeks before lift-off. Once in space, SUNSAT and Ørsted will be separated from the launch rocket at the same time. A pair of pyrotechnic cutters will release both satellites and release them into their orbits, after which SUNSAT's magnetotorquers will stabilise it. SUNSAT's tip mass will then be released and its boom will deploy. | |
SUNSAT is a low earth orbit (LEO) micro satellite, weighing 64 kg, with dimensions of 45 cm x 45 cm x 60 cm. It will follow an elliptical polar orbit of between 620 km to 850 km above the earth's surface and will circle the globe approximately once every 100 minutes at a travelling speed of nearly 7.5 km/s, or 27000 km/h! | |
What is the main function that SUNSAT will perform in space? |
|
The primary purpose of SUNSAT will be to take low cost, high resolution photographs of South Africa. By analysing the spectral contents of these images, the type and density of vegetation on the ground can be determined. This information should be of benefit to both the forestry and agricultural industries. A high resolution camera, also referred to as a pushbroom imager, has been developed by the university in conjunction with the Council for Scientific and Industrial Research (CSIR) and will form the main payload of SUNSAT. It operates like a fax machine and can take 3-colour stereo images (in green, red and near-infrared) with a width of 3456 pixels and a resolution of 15 m per pixel at an altitude of 800 km. The types and quality of images that SUNSAT hopes to produce promises to be exceptional for such a low cost university micro satellite and should approach that of large and expensive commercial satellites like SPOT and LANDSAT. The camera design has, in fact, been so successful that Korea and South Africa have already signed a technology co-operation agreement which will allow Korea to use a similar camera on board their next KITSAT-3 satellite. | |
What other important secondary functions will SUNSAT perform? |
|
Apart from taking photographs, SUNSAT will also perform a number of secondary tasks. Firstly, SUNSAT will test a "store-and-forwarding" communications system that will provide a bulletin board service (BBS) to radio amateurs across the globe, allowing them to store and retrieve files and other information in a "mailbox" on board the satellite. The SUNSAT team decided to deliver this service as a token of their appreciation for the important work done by the Amateur Radio Movement in developing new radio skills and stimulating technology interchange across the globe. Secondly, SUNSAT will also play host to a number of school projects and outside experiments, which in turn will yield valuable data to the scientific community. For example, NASA's GPS and SLR systems will be used to monitor variations in the earth's gravity field, which is influenced by the flux of water on the earth's surface, the variations in the polar ice caps and regional variations in ground water. SUNSAT will also permit experiments in exchanging large data files between remote locations on earth which are not well served by other communications systems. | |
Other school projects and outside experiments on board SUNSAT |
|
SUNSAT hosts the following experiments as part of its aim to promote science and technology among school children and to establish strong international ties:
| |
SUNSAT is made up of a cubical satellite body and a circular tip mass, which is separated by a 2.2 m collapsible gravity gradient boom (see figure 1). The tip mass and the boom will help to stabilise the satellite as it orbits the earth. The boom features an ingenious fold-up mechanism made at the university, which will unfold after the tip mass is freed from the top plate of the satellite body by a pyrotechnic cutter. The body of the satellite is organised as a stack of eleven trays, resembling a concertina, which contains all the necessary electronics and equipment (see figure 2). The bottom tray, which will always face towards the earth, contains a high resolution imager, two rechargeable Nickel-Cadmium battery packs and the reaction wheels. Directly above the bottom tray comes the power tray. Its function is to control the recharging of the batteries by the four solar panels situated on the outside of the satellite. The third and fourth trays are the VHF and UHF communication trays which allow the satellite to send and receive data from the ground. The next tray contains the NASA GPS receiver, the microwave electronics and two school experiments. The sixth tray is the telemetry tray. Its purpose is to monitor the satellite's vital functions, such as the temperature at various points, enabling the ground crew to determine the health of the satellite. Next comes the telecommand tray, which is responsible for optimising the satellite's power consumption by switching various parts of the satellite on or off as they are needed. The telecommand tray also allows the SUNSAT ground control team to upload new software and to re-boot the on-board computers when needed. The eighth tray contains the satellite's on-board computer. This 80188EC micro-processor is the brain of the satellite and controls all the other systems. On top of this tray, is another on-board computer (80386EC) that will serve as a backup should the main computer fail. The next tray is the RAM tray. It contains 64 Mb of memory which is needed to store images taken of other world regions until SUNSAT passes over Stellenbosch and they can be downloaded.
Lastly, the attitude determination and control system (ADCS) tray and top
plate close up the satellite. The ADCS system is crucial for SUNSAT to
determine its current orientation. Various sensors used by the ADCS system
are situated on the top plate. These include two horizon sensors, a sun
sensor, a star camera and a magnetometer.
| |
SUNSAT needs to be able to orientate itself with extreme precision in order to take high resolution images. This is accomplished by the highly advanced attitude determination and control system (ADCS) developed at the university. The system comprises various sensors that determine the satellite's current orientation by measuring the position of the earth's horizon, its magnetic field and the sun and star constellations. All these sensors were developed at the university and had to be thoroughly tested and calibrated. SUNSAT then uses this data to point itself in the right direction by using reaction wheels and electromagnetic coils, called magnetotorquers. | |
Has SUNSAT been tested under space conditions? |
|
SUNSAT has undergone various environmental tests to ensure that it will survive the severe stresses of the launch and the harsh conditions encountered in space. A number of electromagnetic interference (EMI) and radio frequency interference (RFI) tests were also conducted to verify that all the electronics would be able to function in the presence of different kinds of interference. The facilities at Houwteq (near Grabouw) were used to carry out most of these tests. A special vacuum chamber in the Electronic Systems Laboratory (ESL) at the university was used to simulate zero atmosphere conditions. This is an important test, since some electronic components can outgas and fail in vacuum conditions. The boom deployment and the satellite separation from a mock-up launch attachment were also tested at the ESL lab. NASA has verified all these stringent tests to ensure that SUNSAT won't jeopardise the launch. | |
No, SUNSAT would ideally have liked to be placed into a circular and sun-synchronous orbit for optimal imaging performance. Unfortunately, SUNSAT's orbit is dictated by the orbital requirements of the Danish satellite, Ø rsted, which receives precedence as it was the first secondary payload to be included in the mission. SUNSAT's orbit will, however, pass through its ideal orbit at certain times, allowing the performance of SUNSAT in the ideal orbit to be evaluated. | |
SUNSAT is equipped with the necessary transmitters and receivers that will allow it to exchange data on earth via UHF, VHF, S- and L-band links. For approximately ten minutes in four of SUNSAT's fourteen daily 100 minute orbits around the earth, the SUNSAT ground station will be in direct contact with SUNSAT via a 4.5 m diameter dish antenna at Stellenbosch University. While the UHF and VHF bands will be used mainly by amateur radio hams, image data will be downloaded using a separate S-band down-link at up to 50 Mbit/s. The L-band receiver will mostly be used to receive software uploads from the command station at 2 Mbit/s, and act as a backup receiver for telecommand of the spacecraft. | |
SUNSAT uses a conventional power system, comprising four high-efficiency solar panels that will be used to recharge the two rechargeable Nickel-Cadmium battery packs. This will produce an average power availability of around 30 W if in a fully illuminated orbit, but less than half of this during 12:00 local time orbits. | |
Except for the camera optics (CSIR) and the magnetometers (Hermanus Magnetic Observatory), SUNSAT was designed and built entirely by postgraduate students and technicians at the Electronic Systems Laboratory (ESL) at the Electrical and Electronic Engineering Department. Most manufacturing of components has also been done in-house by the university's Central Mechanical Services and Central Electronic Services (SEM). Somchem manufactured the carbon composite substrates used for the solar panels, while AMS assisted with the coating of circuit boards. The only foreign equipment on board the satellite is NASA's GPS receiver and a 300 g sub-miniature TV camera. | |
It is difficult to put an exact price tag on the whole SUNSAT programme, since costing is intertwined with other university functions. The satellite has taken seven years to plan, fund and develop and approximately 25 staff-years and about a hundred and fifty student-years have gone into the project to date. Excluding the launch cost, the total investment is estimated at US $2 million (R11 million), half of which accounts for student support, while the rest has been spent on engineering materials and processing. Special care was also taken by the SUNSAT team to delay the acquisition of expensive components until they were absolutely indispensable. | |
There has been no commercial funding for the SUNSAT project, although industry has played a crucial role with the sponsoring of students and components and by making their facilities available at no cost. The Foundation for Research Development (FRD) matched some of this support through its THRIP programme and played an important role in helping to secure the NASA-sponsored launch opportunity. GRINTEK' sponsorship of a Chair in Satellite Systems has also been instrumental in the project's success. | |
SUNSAT will stay fully operational for as long as its power system can deliver power to the satellite's sub-systems. Unfortunately, the NiCad battery packs cannot recharge indefinitely and are expected to last four to five years. SUNSAT's orbit, whose perigee was recently increased to 620km, will cause it to re-enter after about 30 years in orbit. | |
Even when viewed against the ambitious goals formulated seven years ago, the SUNSAT project's results have surpassed its wildest expectations. Some of SUNSAT's main accomplishments include:
| |
It is hoped that the successful in-orbit operation of SUNSAT-1 will help to secure support for future SUNSAT projects. Plans are already on the table for SUNSAT-2, a micro satellite of similar design to its predecessor, but with notable performance upgrades. The experience gained from the development of the 3-colour imager on board SUNSAT has already led to ideas for the development of a new 9-colour multi spectral imager with a wider swath width for the next generation SUNSATs. Other upgrades which are in the pipeline include improved telecommand and telemetry systems, a new generation RAM disk capable of storing up to 1 Gigabyte of data, a new generation flexible satellite bus, and reduced volume, mass and power consumption. Hopefully, SUNSAT-2 will also be able to secure a circular and sun-synchronous orbit of about 800 km above the earth's surface to ensure optimal imaging performance. | |
When SUNSAT-1 leaves South Africa on the 19th of November, the SUNSAT team will be able to look back with pride at having achieved one of South Africa's latest technological feats. During the initial stages of the project, funding prospects often looked bleak, especially after many project evaluators expressed their doubt as to whether the project would ever be completed and even advised against funding support. But thanks to their never-say-die approach, the SUNSAT team for many years disregarded the fact that their project hovered on the brink of termination and simply stuck to their guns, until they accomplished their vision. And thanks to their tireless efforts, the world is now one step closer to having an inexpensive technique for monitoring the earth's atmosphere and obtaining high-quality images of the earth's surface. SUNSAT would like to thank the following companies and organisations for their support: Air Malaysia, Altech Alcatel, AMS, CSIR, Dimension Data, ECS, FNB Technology Innovations Group, FRD, Grinaker Electronics, Houwteq, Irdeto, Lombari Trust, MTN, NASA, Orbicom, Plessey Tellumat, Reutech, Reumech, Siemens, Somchem, Telkom, Dept. of Trade and Industry THRIP fund, and Vodacom. | |
For further information contact:
e-mail: milne@firga.sun.ac.za,
schoonwi@firga.sun.ac.za, or
mostert@firga.sun.ac.za
Telephone: * +27 (21) 808-4936
Fax: * +27 (21) 808-4981
website: http://sunsat.ee.sun.ac.za
(The site contains further information, as well as PAPERSAT, a paper model of SUNSAT that can be downloaded and assembled by children.)
Address:
SUNSAT Project Manager,
Department of Electrical and Electronic Engineering,
University of Stellenbosch,
Private Bag X1,
Matieland,
7602,
SOUTH AFRICA.
|
|
Figure 1 SUNSAT-1 in its operational configuration
|
Figure 2 SUNSAT General Structural Configuration (Exploded View)
compiled by Tim Müller
(Updated by G.W. Milne 12 Jnuary 1999)
