[evlatests] Another S-band power setup failure

[Todd Hunter]

Hi Rick,
Attached is an az/el plot of the 3 Sirius satellites for the VLA site.
Todd

On 1/16/14 12:59 PM, Rick Perley wrote:
>      Dan, et al.:
>
>      Not quite.  The HEOs rise in the SE, head to just north of the
> zenith where they execute a little loop, then head off to the SW.  This
> is a result of their highly elliptical, inclined, geosynchronous orbit.
>      This path does mean that there are direction safe from them -- to
> the NW, NE, and (perhaps) to the south at an elevation of, say, 70
> degrees.
>
>      Rick
>
> Dan Mertely wrote:
>> 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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>>>      
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