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Live PAL colour TV images of the earth coming from Sunsat.

On 11 May, the first live TV images were received from Sunsat by the Overberg Test Range (OTB).

OTB, a division of Denel,  is two hour's drive from Stellenbosch. OTB have been helping us by tracking Sunsat with their 4 m and 10 m diameter S-band telemetry systems, and were the first to recieve TV from Sunsat. Thanks guys! OTB is a professional operation with quality equipment including diversity combiners etc, so provides a performance goal for our self-made ground station.

On 12 May our Stellenbosch S-band system was operating and received slightly noisy TV images. With the signal just at threshold, we could easily see that the signal strength was very stable - an observation made by all ground stations that have tracked Sunsat's S-band downlink.

On 18 May, we installed a better LNA and fixed some tuning errors, and received noise-free TV. Some of the video was shown on SABC TV 2&3 news at 19:00 and 20:00. It is really rewarding to see the images of the earth moving past under Sunsat. So many things have to be right for this to happen. Apart from the technicalities, the earth, sea, and clouds are beautiful to observe - particularly for Blue Planet junkies!

The Space Application Center of the CSIR at Hartbeesthoek also tracked Sunsat's 18 May pass to their horizon. But then they are practiced and are spoilt by having a 14 m diameter antenna!

Our S-band antenna has a single helical feed and preamp (for RHCP). We implemented a PC-based conical scan tracker to correct for any errors in orbital prediction or tracking. A detector at the 70MHz IF output of the satellite TV receiver is used as a signal strength indicator and feeds an A-D card in the PC.

We are presently scanning the whole dish, using the main AZ/EL servos, but are making a mechanism to displace the feed only. This will save the servos in future, and allow us to increase the conical scan frequency and tighten the tracking control loop.

We also still have to check the antenna's G/T using Y factor measurements with the sun as source. We can measure the TV receiver's 70 MHz IF power with a RF power meter and will use this for the Y-factor measurement. G/T was measured this way two years ago with a C-band LNB. We now need to do this with our S-band LNB. We also need to verify that the antenna is correctly focussed, but his may have to wait until Sunsat's TV reception is broken!

We now need more passes to fully evaluate the TV link performance, but expect to be able to receive good quality images at Stellenbosch for passes above about 30 degrees elevation - maybe better! If there is a need for extended reception ranges, we will rely on our friends at SAC with their big antenna!

The PAL camera on Sunsat is a small colour camera using the British/German PAL TV standard used in South Africa. Although not a 'professional' instrument, it delivers excellent quality images, and has an approximatyely 90 degree field of view. The image thus covers about 1500 km, showing how beautiful South Africa looks from space. Some images are available on http://sunsat.ee.sun.ac.za

At present we are not controlling the yaw angle of Sunsat when imaging, so the earth is seen passing past the satellite at arbitrary directions. Two of Sunsat's antennas are also visible on the picture, and obscure the earth a little.

The S-band downlink on Sunsat is only allowed to operate in range of South Africa, so if you are far beyond our borders, you won't be able to get the South African view of the world. We will have to refer you to a good travel agent!

© Electronic Systems Laboratory 1999
by Buchan Milne