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AO-40 Q&A

Several questions regarding AO-40 have been posed in the last couple of days.  The answers to these lie in the available telemetry and telemetry specs.

>Now the sat is back on alon/alat 0/0 and the warble is back, what has
>changed to cause this? since it is not radar it has to be something else...
>what systems have been turned back on that may be causing this?
>Any ideas?

The telemetry is all available on the amsat web site, courtesy of Paul Willmott.  This can be downloaded and played back through P3T.  You'll  quickly see what has been turned on/off and when.

>I cannot understand why i should have the freedom to overload and desense
>everybody in the passband using the L-band and i cannot do the same using
>the U-band because of LEILA and i cannot understand why LEILA is not
>operating  for the L-band or cannot be switched time to time between the
>U-band and the L-band in a more balanced operating democracy.
>Technical problems or what  ?
>73" de i8CVS Domenico

There are no technical problems, per se.  Everything related to Leila works as it should, as far as I know.  However, if you review the telemetry specs. you will see that U-band can be connected to either Leila-1 or Leila-2.  L1 can only be connected to Leila-1 and L2 can only be connected to Leila-2.  Therefore, we have several options.  We can run L1 and U through Leila-1 and disconnect L2, we can run L2 and U through Leila-2 and disconnect L1, or we can run both Leila's.  Because Leila's are not perfect and they often chase "ghosts" having both Leila's connected means twice as many false alarms on the passband, disrupting QSO's and draining transmitter power for the siren.  The command team would prefer not to pursue this option at the moment.

The huge AGC suppressions on U-band (I saw -23 dB last night!) mandate that Leila remain active on this band.  Otherwise, a few powerful signals will suppress the U-band AGC to the point that others cannot use the passband.  This is a major cause of the "fades" heard on the downlink from time to time.  The AGC on L-band shows little or no response to the uplink signals, so signals on L-band are not suppressing L-band receiver sensitivity.  They may ultimately affect the S2 ALC, but I see no evidence of that in the recent telemetry.  Nonetheless,  it is likely that as L-band use grows Leila will need to be added to this uplink from time to time.   To do so will require shutting down one of the L-band links and it will also mean that Leila will be spread "thinner" in the sense of having two uplinks to police.  This means that while Leila is notching out a strong L-band signal which is not suppressing the L-band AGC or affecting the S2 ALC, other U-band signals may be severely depressing the U-band AGC. 

Leila is not a cure all, and is no substitute for common sense and good operating practices.   Fortunately, the great majority of AO-40 operators are very good in this regard.  However, telemetry capture is still hampered by occasional individuals who insist on zero beating the beacon or repeatedly swishing through it.   Although S2 has 250 KHz of passband,  from time to time someone feels the need to tune directly on top of another QSO.  Leila cannot affect these occasional poor operating practices.   Leila does not treat all signals the same.  CW signals stronger than the beacon are not uncommon and will escape Leila's attention.   SSTV signals are relatively immune to Leila.  Even the tone of ones voice seems to affect Leila's "attention."  Leila's current sensitivity setting seems about optimal.  Turning the sensitivity up will result in too many false alarms, decreasing sensitivity would lead to even more AGC suppression.   Leila, like everything else in life,  is a compromise.

I have been asked how long we can remain at ALON/ALAT = 0/0.  We can hold this position until early March 2003, at which point we must begin slowly moving ALON toward 35+ degrees, running ahead of the sun.  This will allow some time for the sun to move up the ecliptic.  In mid- to late April, we can then raise ALAT to about +25 degs, drift past the sun for a couple of weeks, drop ALAT back to 0 at ALON ~325, then chase the sun back to 0/0, as before. 

 Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html 
  Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key      

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