[evlatests] Another S-band power setup failure

[Todd Hunter]

Hi Mert,
    Their apparent path is a squashed figure-eight that dips below the 
horizon for only about 4 hours per day.  A few years ago, I plotted their 
az/el path as seen from Green Bank:

http://www.cv.nrao.edu/~thunter/sirius.pdf

At that time, all 3 had a similar path, just separated by about
8 hours (the red, blue and black dots in the plot).  Probably still the 
same.  I could plot the current paths from the VLA site if it seems 
worthwhile.

Todd

On Thu, 16 Jan 2014, 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.
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>