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!

High-resolution images

Here are some of the long-awaited images from the main on-board payload, a F#4 linescan imager (refractive design) with 400mm focal length, and resultant GSD of 6.25m from an orbit height of 505km.  See separate posting describing the system limitation as applicable to these images.

Buenos Aires (Argentina)

Date:  21 Dec 2009

Scene size:  60km x 52km

Colour bands:  Red, Red-Edge, NIR

Post-Processing:  SAC (CSIR)

Cropped from main image (100% zoom factor)

Nkandla (Kwazulu, Natal, RSA)

Date:  6 Dec 2009
Scene size:  40km x 52km
Colour bands:  RGB
Post-Processing:  SAC (CSIR)

East London (RSA)

Date:  3 Feb 2010
Scene size:  12.5km x 7.5km
Colour bands:  Red, Red-Edge, NIR
Post-Processing:  SAC (CSIR)

Sossusvlei (Namibia)

Date:  14 Feb 2010
Scene size:  16km x 10km
Colour bands:  Red, Red-Edge, NIR
Post-Processing:  quick view (WHSteyn)

Stellenbosch (RSA)

Date:  20 Feb 2010
Scene size:  6.5km x 7.1km
Colour bands:  Red, Red-Edge, NIR
Post-Processing:  quick view (WHSteyn)

Cape Town (RSA)

Date:  24 Feb 2010
Scene size:  17km x 19.6km
Colour bands:  Red, Red-Edge, NIR
Post-Processing:  quick view (WHSteyn)

Miami (Florida, USA)

Date:  16 Mar 2010
Scene size:  8.4km x 7.8km
Colour bands:  NIR
Post-Processing:  none

Dubai (UAE)

Date:  24 Mar 2010
Scene size:  8.5km x 7.7km
Colour bands:  NIR
Post-Processing:  none

Note shadow of Burj Khalifa

6 months in space today!

Time flies when you're having fun!  We are indeed grateful for this milestone and look forward to many more happy anniversaries.  Some mission stats to date:

• Number of orbits: 2,750
• Months in space: 6 (today!)
• Distance travelled: 0.74 AU
• Activities uploaded: 1900
• Images taken successfully: 93
• Firmware uploads:  25
• ADCS SW updates:  12

Striking image taken by Horizon Sensor

Star camera commissioning

The star camera is currently being commissioned.  Below is a picture taken by the camera with four stars indicated that were extracted by the on-board algorithm used to detect valid stars.

GPS commissioned

The graph below shows the telemetry from the on-board GPS over three orbits.  The bottom trace indicates a valid "lock"; the yellow trace the number of satellites tracked; and the top three traces the X,Y,Z position vectors.

In a perfectly circular orbit, the satellite velocity would be constant.  In a near-circular orbit (as is the case with Sumbandila), the velocity fluctuates slightly between a minimum at Apogee and a maximum at Perigee.  This effect is clearly visible in the measured velocity data from the GPS (see below).  It shows a difference of 30m/s between Apogee and Perigee.  Also note the very low level of "noise" on the measurement.

Orbit correction

The graph below shows the effect on the Semi-Major Axis (SMA) of the Sumbandila orbit after the successful propulsion events.

Propulsion system commissioned

Sumbandila has a small (2.5l) Butane propulsion system which we have "fired" twice over the last couple of days:  first for only 15s and the second time for 60s. 

The telemetry of the second event can be seen below. The top trace (red) is the pressure in the propulsion tank.  The second trace (purple) is the nozzle pressure.  The green and yellow traces represent the temperature of the pipe leading to the nozzle (thruster) and the nozzle temperature respectively.  Lastly the blue trace shows the pitch angle of the satellite, which has been maneuvered to 90° prior to the event in order to align the thruster with the satellite velocity vector.

 

The image above shows the propulsion system being integrated into the satellite (bottom compartment).

Commissioning timeline

There remains just under two months available to complete the outstanding commissioning activities.  Thereafter the plan is to transfer operations of the satellite to the Satellite Application Center (SAC) at Hartebeeshoek (Pretoria).

The following is a list of the main outstanding commissioning activities to complete:

• Star camera
• Propulsion system
• GPS
• Resistive magnetometer
• Experimental payloads
• Viewfinder functionality

The graph below shows the summarized timeline since launch:

Imager refocusing

Initial images taken by the main payload imager, indicated that a focus shift had occurred, probably due to a combination of launch vibrations and an elevated thermal environment (higher than expected temperatures).  The images furthermore indicated that this shift was towards the blue image band (away from red).

The imager is designed with a built-in mechanical refocusing mechanism, so we attempted the refocusing of the imager by making use of a live PAL video feed (9 Dec 2009).  This feed originates from the matrix sensor that is co-located on the focal plane with two line sensors.  The focusing was done on four steps as shown below with the snapshot images (cropped) from the matrix sensor.  These images have a resolution of 8.4m compared to the resolution of the main imager of 6.25m.  A marked improvement was visible on both the video feed and the snapshot images that were downloaded afterwards.  The focus was shifted by 300um towards the red band.

High-resolution images taken by the line sensors after the focus effort confirmed that the imager is now again in focus.