This paper describes the science
payloads and experiments of the SUNSAT microsatellite which is
to be launched in 1997. A very modular satellite mechanical structure
has enabled the addition of addtional experiments at a late stage
of the program. The design and performance measurements of the
engineering and technology payloads (a 15 m resolution imager
and an accurate ADCS system) are reported. The science payloads
including an accurate GPS receiver, laser retroreflectors, a magnetometer
and school experiments is described, indicating that a very complex
microsatellite was developed as the first venture into space by
a South African university.
SUNSAT, an advanced microsatellite developed by graduate students at Stellenbosch University in South Africa, will be launched by NASA from Vandenberg Airforce Base on a Delta II rocket currently manifested for August 1997. The Flight Model hardware is nearly complete and is now being pre-flight tested.
This paper commences with a technical overview of the overall system, which supports the science and technology payloads and experiments. Laboratory measured performance data are provided for the 15 m resolution 3 colour CCD imager of SUNSAT, developed jointly by Stellenbosch University, the Council for Scientific Industrial Research (CSIR) in South Africa and the Korean Advanced Institute of Science and Technology (KAIST).
The performance of the Attitude Determination and Control System which is required to support the imager is described. An overview of the science payloads is followed by an investigation into the school science projects. The benefits and future of the microsatellite development laboratory at Stellenbosch University are also discussed.
The contribution of this paper
is to consider the SUNSAT micro satellite from a scientific and
engineering experimenters viewpoint. Previous papers have focused
on the satellite bus, application of the satellite services and
the communication payloads.
The SUNSAT programme set out in 1992 with three major goals, namely
The local development of a microsatellite with a number of novel subsystems has provided the technical challenge and excitement required to achieve these goals.
Now, 5 years after commencing the programme, more than 50 graduate engineering students at Master's and PhD. level have contributed to the endeavour through research and development work. Several apprentice technicians have also completed their practical training in the SUNSAT laboratory.
International collaboration has provided a launch opportunity for SUNSAT in 1997. NASA has agreed to sponsor the launch of SUNSAT as an auxiliary payload on their P-91 Delta mission. In exchange, the SUNSAT microsatellite has been modified to incorporate a NASA supplied GPS receiver, and a set of laser retro-reflectors. Under this agreement the SUNSAT microsatellite will supply NASA scientists with data from their instruments, which will enable them to study gravitational effects by means of orbit perturbations.
The appeal of space applications to school children is being used to involve them in the SUNSAT programme. Inside the structure of the microsatellite, room has been reserved for several projects designed and built in school science laboratories. Projects received so far from high school pupil teams include sensors that measure temperatures in space, structural acoustic noises and the effects of radiation on electronic devices.
The SUNSAT architects deliberately
decided that students should build as much of South Africa's first
satellite as possible. It was also decided in 1992 that SUNSAT
would not be a copy of any existing microsatellite. Attempts
have been made to achieve improved technical performance in selected
microsatellite subsystems, as will be described in the paper.
Engineering experiments
Science experiments
GPS receiver
Sunsat provides NASA with an opportunity to fly an advanced GPS receiver to perform experiments in atmospheric, ionospheric, and gravity mapping. The co-location of the GPS receiver and the satellite laser reflector will permit a check to the two reference frame systems for GPS and SLR geodesy efforts. Finally the Hermanus Obervatory magnetometer will provide a complementary measurement of the magnetic field to the Oersted geomagnetic satellite which will also be launched as a secondary mission aboard the Argus P-91 launch in August 1997.
Atmospheric mapping:
The TurboRogue receiver to
be flown aboard Sunsat is the first version of an advanced design
to be flown. The tracking system aboard the receiver is designed
to improve occultation measurements of the troposphere to measure
temperature and water vapor. Temperature and water vapor modify
the atmosphere's refractive index to GPS signals. As the GPS transmitting
satellites pass into occultation from the point of view of Sunsat,
the variation in phase of the signals can be measured in terms
of atmospheric temperature and water vapor. Temperature measurements
should have a resolution of approximately 1 degree centigrade.
If the GPS is run continuously in an occultation mode we expect
over 300 globally distributed measurements per day from Sunsat.
This compares to the 700 measurements per day of the global radiosonde
program. The Sunsat mission will be a very important step in developing
an inexpensive technique to monitor the Earth's atmosphere.
Ionosphere:
The TurboRogue GPS measures
arrival times of the GPS signals at two frequencies. The electron
content of the ionosphere can be measured in terms of the differential
arrival times of these two signals. Therefore the GPS receiver
can be used to continuously monitor the structure of the ionosphere.
This is important for several reasons, first for the Oersted and
Sunsat missions we hope to better understand the current systems
in the ionosphere which manifest themselves as diurnal and other
period variations in the magnetic field. Secondly, the ionosphere
is a source of noise in radar altimetry satellites which measure
the ocean structure and circulation. The combined measurements
of orbiting dual frequency GPS receivers such as the Sunsat receiver
and the ground GPS network gives us an unprecedented ability to
monitor ionospheric structure for the improved measurement of
ocean circulation. Finally, although not a factor to Code YSG,
a knowledge of ionospheric structure is important to forecasting
radio communications which is important to other organizations.
Geodesy and Gravity:
The GPS and SLR systems will
be used to accurately position SUNSAT to the centimeter level.
This will permit us monitor the long wavelength gravity field
and its variation in time. These measurements will be used to
monitor the flux of water of the Earth's surface such at the variations
in the polar ice caps and regional variations in ground water.
The accurate knowledge of the Sunsat orbit will also intercalibrate
the two geodetic systems, Satellite Laser Ranging and GPS. SUNSAT
will be using
Attitude determination
The accuracy requirements is determined by the pointing requirements for the imager. The image width is smaller than 50km for altitudes lower than 600km. The pointing accuracy and the requirement to take images on the earth at any point in time, is dependent on the accuracy with which the position and time of the satellite of the satellite over the earth can be determined.
The satellite has a speed of approximately 7.5 km/sec and thus time must be known on board the satellite with an accuracy of subseconcs and to determine the position better than 7.5km with an accurate orbital model. (SGP4 which uses NORAD elements)
The starimager is able to
measure the angular orientation of the satellite to resolutions
of beter than 0.01 degree. The earth facing satellite rotate
at 0.06 degree per second relative to the inertial space as it
orbits around the earth. If the time can only be measured to
an accuracy of one second, then the angular information (pitch
angle) around the orbital normal direction is degraded to 0.06
degree for the starimager.
Other engineering experiments on SUNSAT
One of the four body mounted solar panels are made with the MOVD process.
PAL TV camera
Parrot
Very high resolution imager experiment
The SUNSAT imager (also to be flown in Kitsat 3) has an entrance aperture of 10cm and a focal length of 560mm. Three TC104 linear CCD sensors are used, which have 3456 imaging pixels of a total of 3490 pixels per line. The pixel spacing is 10.7 micron.
(moon picture on transparency in colour?)
Measured results
The table below
There are 4 different school experiments on the SUNSAT satellite. Each school experiment module has to comply to the following requirements:
Each experiment will now be discussed:
Satellite sound and temperature experiment
This experiment was submitted by the George Campbell Technical High School in Durban. It consists of a temperature sensor and a sensitive microphone. Both of these sensors are bolted to the inside of the experiments tray. The purpose of the temperature sensor is simply to measure the temperature on the inside of the tray. It is calibrated for temperatures from -50 to +130 degrees Celsius. The microphone will pick up vibrations in the structure of the satellite. These include the shock when the piro bolt cutters release the boom and the rotation of the reaction wheels and the camera tube. The microphone circuitry includes an automatic gain control to increase its dynamic range.
The output of the microphone
can be placed on the audio bus for transmission on one of the
radios or digitizing by the on board computer. The temperature
sensor has a telemetry output that can be sampled by the 8031
control circuit.
Radiation damage to CMOS electronics experiment
This experiment was submitted
by the Rhenish Girls High School in Stellenbosch. Its purpose
is to investigate the effect of radiation on CMOS logic gates.
It contains various 4000 series CMOS gates with which various
circuits have been built. Additionally, there are two modules:
The main module which is inside the experiments tray and a small
'probe' module which is mounted on the outside of the tray (between
the solar panel and the tray). They contain the same circuits
and are separated like this to see if the aluminum of the tray
has any noticeable shielding effect. There is a two-inverter
oscillator and a few loose inverters of which the threshold voltages
are tested.
The output is a single telemetry line which is formed by an 8 to 1 multiplexer. The multiplexer is driven by a slow oscillator so that each of the 8 inputs is given a slot on the output.
Dust particle impact detection Experiment
This experiment was submitted by the Peninsula Technicon. Its purpose is to detect and measure the frequency of small meteorite impacts on the satellite. They have submitted two different boards, which each uses a different type of sensor. The sensors are mounted on the top plate of the satellite so that they are exposed to space. The one sensor is built around a piezzo-electric strip and the other by using a MOS-capacitor. This is a solid state capacitor manufactured on a piece of silicon. The capacitor is charged up to about 40V and when a particle penetrates the capacitor, it momentarily causes a short circuit and an associated voltage drop across the capacitor.
Both circuits give out a single telemetry signal. A counter in the circuit counts the number of impacts and then converts this to an analog value which is then measured by the 8031 again.
Material exposure experiment
The Malaysian experiment was submitted by the University of Kebangsaan in Malaysia. Its purpose is to measure the characteristics of a materials sample in the space environment. They are measuring the temperature and the conductivity of the sample. The material is a glassy carbon sample developed by the materials science department.
Telemetry lines are again
used to present the data.
Supporting science payloads
Acknowledgements
The whole SUNSAT team has
contributed to
References