[evlatests] Another S-band power setup failure

[Dan Mertely]

Remember that 3 of the Sirius satellites are *not* in geostationary
orbits, but rather Highly Elliptical Orbits (HEOs), otherwise known
as Tundra orbits.  I'm assuming that that means they can appear most
anywhere in the sky, not just the equator.  Sirius was assigned the
2320.0 - 2332.5 MHz section of the 2320.0 - 2345.0 MHz Satellite
Digital Audio Radio (SDAR) S-band allocation.  (XM gets the upper
12.5 MHz.)
-Mert


On 1/16/2014 9:23 AM, Rick Perley wrote:
>      I have found another instance where the power setup at S-band was
> grossly in error, resulting in serious compromising of the observation
> goals.
>
>      In this case, the source and calibrator are near dec = -10, so we
> are in a 'bad neighborhood', and trouble can be expected.  The B/D side
> set up more or less correctly, with the PSum values (for a central
> subband) ranging from 7 to 15 counts.  (The nominal level is ~ 14 --
> anything within 50% of that is considered o.k.).
>      But the A/C side was spectacularly in error, with Psum values
> varying from 0.1 to 4, with nearly all of them less than 1.  That's a
> factor of 15 or more too low!  Unsurprisingly, the data are seriously
> degraded, with the noise nearly doubled.
>
>      Almost certainly, the problem is that the antennas were pointed too
> close to one of the satellites which radiates strongly in subband 2
> (2180 to 2200 MHz is where the power lies), or subband 3 (2320 -- 2350
> MHz -- this is the Sirius/XM band) at the moment that 'set and remember'
> was doing its thing.  This is the second failure in the set/remember
> procedure that I've found in the 15 S-band databases that I've
> calibrated so far.  (Granted -- the first failure, which I reported on
> earlier this week, is bizarre, and likely has a different cause).
> Nevertheless, two out of 15 is too high, and I think we need to find a
> way to prevent these serious failures.
>
>      For S-band, I suggest two approaches:  (1) Avoidance:  The gain
> setup procedure should be done in a 'safe' direction.  Such directions
> exist (to the NW or NE), but this will often require a fairly long slew,
> hence lost observing time.  (2) Review:  When the system power (either
> PSum or an analog measure) is out of range by a significant value (I'd
> suggest a factor of two), an alert should be generated, and the
> procedure repeated.  (This of course brings up the question of how this
> should be done, and what is to be done if the next try also results in
> an unacceptable value).  Alternatively, default gain setting should be
> utilized.  I prefer this approach -- even stupid values wrong by a
> factor of two are better than what we are occasionally seeing now.
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