Tuesday, October 7, 2014
Connecting Technologies - Connecting World
In 1994-1996 I worked in a small Canadian company Techwest Data Systems which developed a communication system via geostationary satellite for offshore applications. The system consisted of an antenna stabilized in 3 axis with gimbals, signal beacon receiver and the controller. Simple and reliable it utilized a rectangular scan pattern to search and lock the antenna on the satellite. It worked well in calm conditions. But in severe weather (when stable communication is especially important) it was loosing the signal and it was taking a very long time to re-acquire it. The solution for the problem was seen in using more powerful receiver which would make the system significantly more expensive. At that time I was responsible for developing control software for Startrack. I devised a non-orthodox algorithm which was implemented and tested - it resulted in the pointing accuracy and signal stability requirements of then customer ComSat to be exceeded more than twice. The system was successfully installed on a number of cruise ships in Caribbean and elsewhere.
Since then the company was bought by Data Marine Systems from Aberdeen, Scotland for their floating oil rigs in the North Sea. Later it found its way to China.
Recently I came across of the old publication which returned me back in time:
TECHNOLOGY Vessel Movement Influences Offshore Communications System Design.
Below is a direct quote from the publication:
"Startrack" (Techwest of Burnaby, B.C.) system could be modified economically to mount a C-band antenna.
An Andrew 3.6-m antenna was selected to enhance reliability during monsoon conditions and improve the "link budget." A radome protects the antenna dish from wind loading and prevents water fill because of the severe up angle that was required because of the almost overhead satellite.
A geostabilized platform has some interesting aspects. For one, in the link budget calculations, one may use zero pointing error. The stabilized platform homes on the satellite beacon, and constantly "nutates" (wobbles) around the signal to keep optimum signal strength. Thus, it does not matter how well the satellite crew keeps its "bird" on station. For this reason, one major market for stabilized platforms is for otherwise useless, unstable satellites.
On the other hand, the system is active, not passive pointing, and therefore must be operating correctly to receive a signal. The benefit to the satellite operator is that the offshore station cannot interfere with another satellite because it cannot point at it unless it has the same beacon.
Because of controversy about too many satellites in this same area, side lobes were of great concern. Asia Satellite required the running of a complete set of pattern tests on the antenna with the platform operating and fixed.
Another interesting engineering aspect of stabilized antenna platforms is that they are dynamic, and can be dynamically unstable. Because the system was modified with a much larger, heavier antenna than the original, Techwest used its motion simulator to run extensive tests.
It is quite impressive to see the floor rocking and rolling ... in a storm, and the big 3.6-m antenna solid as a rock on the signal.