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W4DPH > SAT 22.12.02 04:50l 70 Lines 3712 Bytes #999 (0) @ AMSAT
BID : ANS-356.04
Read: DB0FHN GUEST
Subj: AO-40 Q&A
Path: DB0FHN<DB0ZWI<DB0HST<HB9OK<HA3PG<CT2GWY<CT3MY<WB0TAX<W4DPH
Sent: 021222/0323Z @:W4DPH.#TPA.FL.USA.NOAM #:37952 [CLW] FBB $:ANS-356.04
From: W4DPH@W4DPH.#TPA.FL.USA.NOAM
To : SAT@AMSAT
AMSAT News Service Bulletin 356.04 From AMSAT HQ
SILVER SPRING, MD. DECEMBER 22, 2002
To All RADIO AMATEURS
BID: $ANS-356.04
Stacey Mill, W4SM, recently took some time to answer a few questions
recently posted via the AMSAT-BB email reflector.
Q: Why is LEILA only switched into the U-band uplink when stations on
L-Band are much stronger? Is there a technical problem with LEILA?
A: 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 LEILAs. Because LEILAs are not perfect
and they often chase "ghosts" having both LEILAs 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 com!
promise.
Q: How long we can remain at ALON/ALAT = 0/0?
A: 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 deg, 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.
[ANS thanks Stacey, W4SM, for the above information]
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