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S-band receiving requirements



I think I now understand why so many of us had unrealistic expectations about our S-band receiving systems.  It boils down to a series a cascaded assumptions that were incorrect.

After the V-transmitter failed, a lot of us cobbled together S-band receiving setups based on the available information which was mostly pre-launch examples described in the book P3G to P3D and many other articles.  For instance, P3G to P3D describes S-band downlink "Example 1" as follows: 60 cm dish + preamp with 1.5 dB NF to get 23 dB S/N. (this is the BETTER-performing of the two examples given)

The first mistake was not realizing that these examples assume a best-case situation of the S1 downlink and minimal squint.  The actual AO40 orbit is likely to require frequent use of the S2 downlink which has 8 dB less gain (because the low inclination orbit will require seasonal off-pointing of the antennas).

The second mistake occurred more recently.  The seductively strong S2 middle beacon fooled many of us into believing that our "pre-launch Example" receiving setups were satisfactory.  We didn't know that we actually need to copy signals 10 dB weaker than the Middle Beacon.  This was not widely known until AFTER the experimental UL/S2 transponder operation commenced.

Many of us discovered through trial and error that "pre-launch example" S-band receiving setups are woefully inadequate for reception of the existing spin-stabilized S2 transponder.  More uplink power will result in a readable downlink signal, but a higher authority has (correctly) decided for us that the solution must be to use receiving setups that are significantly better than the pre-launch examples.

It remains to be seen if "pre-launch example" receiving setups will be adequate part of the time with the final satellite configuration.  Squint near perigee will improve dramatically if 3-axis stabilization is successful.  There may be seasonal periods when the S1 antenna can be used for a portion of each orbit.  It is certain, however, that RELIABLE S-band reception requires a SIGNIFICANTLY BETTER receiving system than the "pre-launch examples".

In retrospect, the only thing that mystifies me is why today's "alligator critics" were SILENT when months of amsat-bb traffic was dominated by discussions about building helixes, modifying surplus converters, etc.  One or more of these silent experts should have spoken up to explain why those setups won't be good enough.  Then for the last few weeks the experts remained silent when several people reported modest success hearing the Middle Beacon.  Again, the experts should have pointed out that we need to hear much weaker signals than the Middle Beacon.  The "alligator critics" are vocal now, but they were strangely silent when their advice would have been most helpful.

73,
Wayne Estes W9AE

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