South Africa's Sumbandila microsatellite has been in orbit for some seven weeks now.
During its first month in orbit, the spacecraft, whose name means "lead the way" in the Venda language, completed 457 orbits, amounting to a total distance of 0,1316 astronomical units (AU - an AU is the average distance from the Earth to the Sun, which is 149 597 870 km or 8,3 light minutes) or roughly 150 000 km.

It is in a low Earth orbit (Leo) at an altitude of 500 km. Its orbit is circular and Sun-synchronous, which means that it overflies South Africa at the same time each morning, around 0900.

SumbandilaSat, as it is known, has a mass of 81 kg and is intended to be both the prototype for a series of South African microsatellites (hence its name) in the coming years and a fully operational spacecraft in its own right. It was designed and built by specialist South African microsatellite company SunSpace & Information Systems (SunSpace), which is based in Stellenbosch in the Western Cape. It is the first member of SunSpace's second-generation "Small Satellite Technology Product Family", a family that will cover satellites with masses from 80 kg to about half a ton, and for missions such as high-performance Earth observation, Leo communications and space science.

SunSpace was spun-off from the University of Stellenbosch to exploit the expertise developed in the design, assembly, and operation of the university's own private-initiative satellite, Sunsat, which, in 1999, became the first South African satellite to reach orbit. Sunsat had a mass of 64 kg and carried a fairly small multi-spectral imager, with a resolution of 15 m (that is, one pixel equating to 15 m x 15 m on the ground) at an altitude of 600 km - the first of its kind on a small satellite in any country. Sunsat also carried an amateur radio payload and a small experiment supplied by the US National Aeronautics and Space Administration (Nasa) and, in return, was launched by the Americans free of charge.

The Sumbandila programme is funded by the Department of Science and Technology, which also acts as the custodian of the satellite. The primary mission of the spacecraft is Earth observation. To this end its main payload is a multispectral imager which operates across six spectral bands and has a ground sampling distance of 6,25 m - (one pixel equates to 6,25 m x 6,25 m). The microsatellite has a data storage capacity of 6 gigabyte to hold these images until they can be downloaded to the data reception ground station at the Satellite Applications Centre (SAC) of the Council for Scientific and Industrial Research at Hartebeesthoek, west of Pretoria. This is backed up by a redundant spare capacity of another 18 Gb. It also carries a lower resolution video camera.

In addition, SumbandilaSat is carrying four experimental payloads. Stellenbosch University has supplied an architectural radiation experiment for commercial off-the-shelf devices, acronymned to Arecots. The Nelson Mandela Metropolitan University has contributed a forced vibrating string experiment, and the University of Kwazulu-Natal a very low frequency radio experiment. The fourth experimental payload is SA Amsat, an amateur radio transponder a voice beacon and a "parrot" repreater, operating from the 2m band to the 70 cm band. In addition to these experiments, the Department of Communications also sponsored the inclusion of a store-and-forward communication payload, for which Stellenbosch University takes responsibility.

SumbandilaSat is equipped with two ultra high frequency (UHF) antennas for telemetry, tracking and control purposes, one S-band antenna to downlink imagery, and an amateur radio antenna. The microsatellite's control and payload systems and subsystems are connected to each other by two controller area networks, better known as Can buses - one Can bus is designated as primary, the other as secondary.

Flight dynamics

Keeping a satellite in orbit is not as straightforward as many assume. "SumbandilaSat's orbit may be circular, but the Earth isn't," highlights SunSpace CEO Bart Cilliers. The earth is, in fact, slightly flattened at the poles and slightly bulged at the equator. "This causes perturbations in our microsatellite's orbit over time. In addition, the solar wind also causes orbital perturbations." The solar wind is a stream of atomic particles, mainly protons and electrons, which pours outwards from the Sun's corona - it is the solar wind which causes the tails of comets to always point away from the sun. "The Sun-synchronous orbit was chosen so that these two forces could more-or-less cancel each other out."

But, of course, they do not exactly cancel each other out. Nor is this all. Although SumbandilaSat is 500 km up, there are still trace elements of the Earth's atmosphere there, and they do exert drag on the spacecraft, causing it to slowly lose altitude.

So the satellite cannot just be left to fly on its own. It must be continuously monitored and actively controlled, to keep it in its correct orbit. To this end, SumbandilaSat has three actuator systems, two of which form part of the spacecraft's attitude determination and control system (ADCS). These are magnetic torquer rods and reaction wheels, while the third system is a small propulsion system.

"The magnetic torquer rods use the Earth's magnetic field to exert moments of force on the microsatellite," explains Cilliers. "There are three reaction wheels, mounted perpendicular to one another, which are spun up or spun down as required, creating moments and counter-moments as needed, to manoeuvre the spacecraft." These are also used to adjust SumbandilaSat's attitude, to facilitate the taking of imagery.

"The thruster is for orbital maintenance. It uses very pure butane," he adds. "Probably once a month we'll release just a puff of the gas to restore the satellite's altitude. It is better to have regular and small adjustments. One has to be very accurate. The design life of the satellite, from the propulsion point of view, is five years. After five years we anticipate that we will have run out of butane." Thereafter, SumbandilaSat will lose altitude, slowly at first but increasingly rapidly as it falls ever lower until it burns up in the atmosphere.

Until then, however, the satellite faces a very active life.

Calibration

It will be some months before the microsatellite is fully operational. "For the ADCS to be able to work, you first need to know where you are in space, and what your orientation is," points out Cilliers. "SumbandilaSat has a number of onboard sensors to do this - a magnetometer, a horizon sensor, a coarse Sun sensor, a fine Sun sensor, a star tracker, a global positioning system (GPS) and three laser ring gyroscopes. All are used for the three-axis stabilisation and control of the satellite in space. Each of these sensors needs to be calibrated, to ensure they provide accurate data."

Calibration requires that data from each and every one of these sensors be downlinked to mission control, over a period of several orbits. This data is then analysed and, if necessary, commands to adjust the sensor are uplinked to the microsatellite as it overflies South Africa. The sensors have to be calibrated individually and sequentially.

This calibration process - and the commissioning of the spacecraft as an operational unit - is the responsibility of SunSpace, as the designers and manufacturers. Consequently, at the moment, SumbandilaSat mission control is located at Stellenbosch University's Engineering Faculty.

Stellenbosch is, however, surrounded by mountains so SumbandilaSat must be at least 4˚above the horizon to be seen from, and be in communication with, mission control. Of the spacecraft's 457 orbits in its first month in space, only 107, or just 23%, met this criterion. And, on each such overflight, mission control only gets to communicate with the microsatellite for about nine minutes. The result was that mission control had a total contact time with SumbandilaSat of only 15 hours in that month.

This is one reason why calibration and commissioning is a slow process. "You really have to plan very carefully what you want to do when SumbandilaSat passes over," he says.

There are six people spending all or a significant proportion of their time working on the SumbandilaSat commissioning programme. Usually, two are in mission control for an overpass, while the others are analysing data from the spacecraft.

An early step was getting SumbandilaSat to point its imager and antennas straight down towards the Earth - what is called nadir pointing. (Straight down is the nadir point). This was achieved by early October. The magnetometer has been calibrated and mission control is currently in the process of calibrating the fine Sun sensor. The lower resolution video camera is now in operation. SunSpace hopes that the first high resolution image will be obtained within the next two months. The SA Amsat amateur radio payload has been activated, bringing in favourable responses from across the globe, and SumbandilaSat has been assigned the Orbital Satellite Carrying Amateur Radio (Oscar) number SO-67 (Sunsat was SO-35).

The process of calibration, commissioning, and training the personnel at the SAC who will actually operate SumbandilaSat, is expected to take about five months. "We are busy working down the calibration list. There have been no serious glitches so far," reports Cilliers. "But we can't forecast just how long each calibration will take."

Ground control to Sumbandila ...

Once fully operational, SumbandilaSat will be handed over to the SAC, which has established its own mission control at its complex at Hartebeesthoek. Unobstructed by mountains, this will be able to communicate with the spacecraft on more orbits and for a longer time on each over flight.

The SAC will use its existing 10 m and 12 m dish antennas to downlink imagery from SumbandilaSat. To uplink commands from mission control, the SAC has already set up a single rotating antenna mount, with several small yagi-type antennas on it. (Yagi antennas are often installed on the roofs of houses to receive terrestrial TV transmissions.) The SAC mission control itself comprises a set of high-specification but commercially-available desktop computers, connected in a network, with world-class, user-friendly and very flexible custom software, written by SunSpace.

"We're not used to operating our own satellite," points out SAC systems engineer Pieter Kotzé. "It is rather bold to declare this an operational mission as SumbandilaSat is really a prototype. So we're going slowly, carefully, to avoid major mistakes. We aim to develop experience, to be able to get to operate the second, third, and later satellites much more rapidly. We're taking baby steps now."

On the other hand, the SAC does have decades of experience in receiving and analysing data and imagery from satellites. "Our aim is to have images rolling off the system as fast as possible," he highlights. "The Earth observation side of SAC has issued studies as a preliminary to the establishment of a national imagery plan. We'll probably focus on providing a rapid response to the needs of our end users in government departments and agencies, as well as other institutions." The high-resolution images that will eventually be downloaded from SumbandilaSat on a regular basis will be available for use by all departments of national government.

SAC mission control will be headed by a Mission Specialist, responsible for the operational use of SumbandilaSat, and one or two Satellite Operators, who will actually transmit control orders to the spacecraft and monitor the reception of the data. The spacecraft and its control system have been designed by SunSpace to be as easy to use as possible. It is an automated system, meaning that the SAC controllers will merely have to tell SumbandilaSat what they want it to do - look to the left, look to the right, or whatever - and it will do it. They will not have to send detailed instructions, such as ordering a reaction wheel to spin up or down to a specified speed, for example.

"The orientation of the microsatellite can be moved by ± 30˚on either side of the nadir point," reports Cilliers. "Also, SumbandilaSat's video camera can be steered from mission control in real time by means of a joystick. Thus, if the spacecraft is overflying the country on a cloudy day, the video camera can be steered to shoot through breaks in the clouds. We don't know of any other civilian Earth observation satellite with this capability. No one knows what spy satellites can do!"

Regarding the experiments onboard SumbandilaSat, the data from them will be downlinked to SAC and then forwarded to the relevant university research teams, who will analyse the information.

The transfer of control from SunSpace to SAC will not end the company's involvement with the microsatellite, nor see the closure of the Stellenbosch mission control. "We, as the design authority, are responsible for the health of the satellite," explains Cilliers. "We will downlink telemetry every day, to check that all systems are working properly. We will ensure that the spacecraft is available and ready for use by SAC. This is the approach used everywhere in the world."