[amsat-bb] Re: AO-7 calibration
deyoung_james at yahoo.com
Tue Nov 19 17:22:45 PST 2013
Bob et al.
A paper, by the undersigned--of course, on the variability of the uplink/downlink frequency for AO-7 appeared in the AMSAT Journal Vol. 30, No. 3, May/June 2007 p.23 - 26. The bottom line is the LO in the spacecraft is being affected by spacecraft temperature. The spacecraft temperature is related to the amount of time the spacecraft is in shadow/sunlight. This variability probably has always been there since the day it was launched and was deemed acceptable back in 1974. The frequency variation appears in both the transponder and the 2m beacon. Up to 6-KHz of variation was observed between full sun and maximum shadow time thru the transponder while the 2m beacon variation was around 2-KHz. The fact that both varied with spacecraft temperature shows clearly it is an AO-7 contribution.
A second paper concerning the solar array power actually used and analyzed the good high-speed RTTY telemetry that was transmitted by AO-7 during the March 2009 time frame, see AMSAT Journal Vol. 33, No. 1, January/February 2010, p. 4 - 15. Now that the solar array is the only power supply it is possible that voltage and current fluctuations do affect the AO-7 LO but are likely dominant around the period of entering/leaving sunlight or when transponder loads are high. When I operated AO-7 I remember many passes where the CW ops were pumping the passband which possibly caused some frequency variations, along with the FM'ing, etc. but are impossible measure or figure out.
To repeat, the dominate factor in the slow frequency variation over time is spacecraft temperature.
I would recommend getting the two papers from somewhere to get the full details rather than depending on the highly-condensed versions above.
Best regards, Jim, N8OQ (formerly KG4QWC)
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