Louis McFadin wrote:

Unfortunately there is any way to put the antennas on the center of the spacecraft. There may be some spin modulation no matter what we do.

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

The designer of the code to demodulate class 1 uplinks in the SDX needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

Not quite true. For the phased arrays, if the required element phases are computed relative to the spin axis, then spin-induced Doppler is removed. You will get the spin Doppler only if you set the phase reference to be the center of the patch array.

One way this can be done is for some LO (common to all the elements of the phased array) needs to phase modulated with a rate equal to the spin rate and with a magnitude/phase calculated on the basis of the s/c-to-earth geometry. The only residual modulation to be compensated in the PLLs is then determined by the offset of the OBSERVER from the center of the earth.

Last nite I think I convinced Bob that the added S2 interferometric antennas (working against individual S2 array elements) will be able to determine the spin axis orientation (and rate) w.r.t. the line(s) connecting the s/c to dedicated terrestrial beacon(s) with accuracies ~1º or better.

73, Tom