> The lack of attitude control forces us to use simple omnidirectional > antennas, which in turn keeps us on the crowded and narrow VHF/UHF > bands. Worse, there's really no such thing as an "omnidirectional > antenna" so our links are plagued by frequent deep fades of unlimited > (or at least unknown) duration. Fading has driven every one of my > modulation/coding designs for AMSAT telemetry links -- at the expense of > making them much less power-efficient. > > And "power efficient" means smaller ground antennas, and that means a > cheaper and more accessible ground station for the average ham. And THAT > means a much larger potential AMSAT membership. >
> With attitude control, our satellite could use directional antennas on > the microwave bands. Directional antennas on higher frequencies mean > much better link budgets. Yes, but with who? 95% of everyone in view is more than 45 degrees OUT of the main beam. Directional antennas have zero value on LEO birds that need to serve everyone in view at the same time. And if you only serve those in the main beam, then the duration is under 1 minute. > A lack of attitude control also plagues thermal design.... > I think he found the equilibrium temperature of a 1U cubesat > to be something like -30 or -40 C! I cannot believe that. The equilibrium of a nominally black (solar panels on all sides) spacecraft is something like about 0 to 30 C (32F to 90F) a very benign operational range. The only time you DO have thermal issues is when you DO have attitude control and have things that are not equally over time seeing the sun and dark sky. PCSAT is now 13 years in orbit and the above range is what it sees. And that range is over a 2 month period. THe orbit-by-orbit temperature changes are less than +/- 10 degrees C. The extremes are due to the "seasons" of the orbit. When it is seeing eclipses it averages to about 10C and when it is in full sun for weeks at a time, it gets up to 30 C (90F) I do agree that attitude control is nice to have, but my point is that it only makes the thermal problem much worse and that "gain" is of no value for a LEO where it must see everyone in a footprint at the same time. (Remmebr you cannot have wide beamwidth and gain at the same time). Gain for HEO's of course is another matter! (think AO-10, AO-13 and AO-40) Bob, WB4APR _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
