On the ARISSat-1 pass just ended here (23 degrees max elevation), results were about the same as yesterday. I heard my CW downlink for about a minute, peaking 559. Several other CW signals were in there, but I couldn't get their calls due to the deep fades.
Bruce appears to be right that the transponder's performance depends largely on the spacecraft's orientation.
73 Ray W2RS
No joy on the 12:00Z pass (48 deg max elev) here... could hear the CW beacon, BPSK, and FM transmissions with deep fades, but otherwise quite well. No other stations heard in the transponder passband, nor could I hear myself on the downlink, from 5 to 75 watts uplink using omnis (M2 eggbeater w/preamp on the downlink, homebrew turnstile on the uplink). Next pass here not until 04:30Z.
It doesn't appear that the satellite has any active or passive attitude control system. I have heard that the spin should eventually settle around the same axis as the antenna, but is there any reasonable hope that that would happen during the satellite's operational lifetime?
George, KA3HSW
----- Original Message ----
From: "[email protected]" [email protected] To: [email protected] Sent: Sat, August 6, 2011 8:48:21 AM Subject: [amsat-bb] Transponder
On the ARISSat-1 pass just ended here (23 degrees max elevation), results were about the same as yesterday. I heard my CW downlink for about a minute, peaking 559. Several other CW signals were in there, but I couldn't get
their calls due to the deep fades.
Bruce appears to be right that the transponder's performance depends largely on the spacecraft's orientation.
73 Ray W2RS
On 8/6/11 8:23 AM, George Henry wrote:
It doesn't appear that the satellite has any active or passive attitude control system.
Correct.
I have heard that the spin should eventually settle around the same axis as the antenna, but is there any reasonable hope that that would happen during the satellite's operational lifetime?
Good question. Theory says it will *eventually* happen, but how fast depends on the mechanisms to dissipate mechanical energy in the spacecraft. Because the frame and the modules inside are so rigid, I think the flexible 2m antenna will probably do most of the work.
One way to follow this is to measure the satellite's spin period very accurately over time. Assuming no external torque (e.g., atmospheric drag, which will eventually become significant) then the spacecraft must conserve its original angular momentum. But as it dissipates excess energy and converts to a spin around the preferred axis (i.e., shifts its angular momentum to the body axis with the greatest moment of inertia), the spin rate will decrease.
E.g., if the moment of inertia around the axis of initial spin is, say, 1, and the moment of inertia around the preferred (antenna) axis is 3, then after it settles down the spin rate will be 1/3 of its original value.
So if we can follow the spin rate over time we can predict how long it'll take to reach a stable spin around the antenna axis. That does require knowledge of the moments and products of inertia, which I assume were measured before launch.
73, Phil
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