SunSpace was established in 2000 as a spin-off from Stellenbosch University following the successful launch of SUNSAT by NASA with a Delta-II launch vehicle. During the past decade SunSpace has developed and delivered numerous satellite components, subsystems and two flight model satellite, now operational in space. This blog documents the mission of Sumbandila (funded by the DST), launched on a Soyuz-2.1b on 17 September 2009. SunSpace places a high premium on a strong partnership with the South African Government for supplying the ongoing satellite needs of our government.

30 March 2010

System limitations

Although a hugely successful mission thus far, the past six months have unfortunately not been without incident.  We suffered a permanent loss of the Z-axis reaction wheel early on during commissioning, where after the ADCS algorithms have been adapted so as to still allow for controlled imaging with the remaining two wheels - Yaw-control was then slightly impeded and the off-Nadir Roll angle limited to ±20°.
Due to a power system anomaly, we have sadly lost access to one of the two CCD control boards.  Each control board interfaces to one three-colour CCD.  The board we lost contains the Green, Xantrophyl and Blue spectral bands, and the remaining board the Red, Red-Edge and Near-Infrared bands.
We have recently also lost the Y-axis reaction wheel (most likely also due to a power distribution failure), this meant the loss of three-axis control, but amazingly we can still take high-resolution images as before:  the ADCS algorithms have been updated very innovatively whereby the satellite now “flies” in a derived Y-Thompson mode (the satellite tumbles “head-over-heals”).  The spin rate in the Pitch axis is now 0.959°/s which results in an Forward Motion Compensation (FMC) rate of 4.  This allows the imager to scan a target from back-to-front from the point where the satellite is pitched 15° forward over the target, to the point where it points 15° backwards.  During the period that the image is scanned, the Roll angle is controlled by the remaining X-axis wheel, where the off-Nadir Roll angle is now limited to ±5°.  The magtneto-torquers are used to phase the Pitch-rate such that the sub satellite point overlaps with the desired target at the center of the image.
We have obtained very good imaging results in this new control mode (see Miami and Dubai images in a separate post).  The same controller is also used during data download sessions, but with a Pitch rate of 0.6°/s, and a Roll angle close to zero.  During three download sessions last week on the same day, we managed to capture 6.3GB of image data via the S-band transmitter link, running at 72 Mbit/s!

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Current Keplerian Elements

Last Update: 1 Febr 2010
Element set: 404

1 35870U 09049F   10032.48635300  .00000433  00000-0  21444-4 0  4040
2 35870 097.3525 084.8560 0011505 166.1189 300.7196 15.22856092 20818