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SUNSAT GOES SILENT



SUNSAT OSCAR 35 GOES SILENT

Sunsat Oscar 35 has gone silent. The last communication with South
Africa's first satellite was on Friday 19 January 2001 at 15:22 when the
ground control station at the University of Stellenbosch was engaged in
routine maintenance on the satellite.

The ground control station made this announcement yesterday after two
weeks of intense recovery action. All efforts to make contact with
SUNSAT have failed.

Recovery efforts will continue but the confidence level that any success
will be achieved is very low.
 
SUNSAT was the University of Stellenbosch's first satellite and has
contributed much to the development of satellite capability in South
Africa.  During its two years in orbit  the original goals were set:
Summary of achievements:

· to co-operate as OSCAR-35  with the amateur radio and amateur
satellite communities worldwide, contributing new standards in the
field.
· to demonstrate high resolution imaging not before considered possible
with a satellite this size and costs.
· to stimulate challenging research and technology development at
graduate student level
· to foster valued international ties in the science and engineering
community and
· to promote science, engineering and technology among the school
children of South Africa.


Sunsat was launched on  23 February 1999 and by 19 February 2001 had
completed 10 027 orbits. This translates to having travelled over 500
million km around the globe. During its operational life 51 high
resolution photo were s taken at places all over the globe,  in 3
spectral bands and 15 m pixel sizes on ground. Other statistics include:

· 937 command dairies uploaded in operating SUNSAT
· 241700 telecommands executed successfully
· 161.144 Mbytes of whole orbit data (WOD) downloaded
· 94868 GPS data points downloaded in support of JPL
· 3.144 Mbytes APRS digilogs leading to a new activity
· 1.656 Mbytes of Magnetometer data
· 888 Kbytes international school experimental data
· 7.052 Mbytes of data for the star camera experiment
· Several hours of PAL videotape data of Southern Africa


For more  back ground on SUNSAT visit : http://www.sunsat.ee.sun.ac.za 


RECOVERY ATTEMPTS
During the first year of SUNSAT's operations, the orbit provided ideal
lighting conditions for imaging, and eclipse periods during every orbit
to cool down the satellite. The various functions of the satellite were
commissioned and operated, such as the high resolution camera, the PAL
video camera, the amateur radio communications systems, the attitude
control system, the science experiments and the school experiments.

However, the non-ideal drifting orbit plane eventually exposed SUNSAT to
continuous sunlight.  This particular orbit was determined by the
requirements of the Danish Orsted satellite, together with which SUNSAT
was a secondary (and free) payload on the launch vehicle.  When the
satellite became exposed to full sun continuously, the SUNSAT operations
team started taking measures to alleviate serious problems of high
operating temperature and overcharging of the batteries, by continuously
re-orientating the satellite.   Amateur radio services were still
provided by SUNSAT during this phase and science and control systems
experiments were conducted.  However, imaging was discontinued due to
poor lighting conditions.   This non- eclipting situation continued for
5 months before the orbit plane again started to cause sun eclipses. 

When the eclipses restarted, it was clear that battery capacities were
low (fast voltage drop under load), as evidenced by some of the on-board
processors resetting during eclipses. The assumption of a typical NiCd
memory problem due to overcharging led to procedures that reconditioned
the battery cells via a number of forced fast  discharges.  This
exercise was successful to the extent that the resultant improvement of
battery capacity enabled SUNSAT to once again function throughout full
eclipses without processor resets.

Operations were back to normal when, after the communications pass of
Friday, 19 January 2001, radio contact ceased altogether.  It is
unlikely that battery failure
is the cause, since there was no evidence of gradual power failure from
the telemetry of the last few orbits of normal operations, and even
during direct sunlit passes,  SUNSAT is not reacting.  It is also
unlikely to be a solar cell malfunction, since this would also have been
evident from the solar cell current measurements and a lack of battery
charging over a number of orbits.

The functionality of the ground station was also verified through
communications with other amateur satellites, up- and downlinking via
both UHF and VHF frequency bands. 

All possible combinations of the RF-Modem-Telecommand- subsystems were
attempted unsuccessfully.  All other communication paths to and from
SUNSAT were thereafter investigated systematically, but without success.
This includes efforts to use telecommands directly on a command-
for-command basis (open loop) to place the subsystems in recognisable
states.



Probable causes for the failure:

>From the tests conducted, the possible failure scenarios in order of
likelihood, are:

(a) A possible failure cause would be simultaneous multi-point
failures.  However, the access experiments listed above tested all
possible single point failures
through previously used redundant routes.  The probability of a
simultaneous multi-point failure is consequently rather low.

(b) A battery cell or main power diode could have burst and caused
physical mechanical damage in the power supply system, since limited
communication would have been possible with either battery or solar
power.

(c) An external object in space could have caused major physical damage
to SUNSAT.


Hans ZS6AKV
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