[evlatests] Using S-band is Tough!

[Rick Perley]

    Todd:

    Thanks for this.  
    We learned about the interesting Sirius orbits about 3 years ago, 
when Bob Hayward and I did the first S-band antenna tests.  Naively 
thinking that Sirius was geostationary (and conveniently ignoring the 
instructions that came with my Sirius receiver that told me to point the 
antenna a bit to the east), we did sky dips to characterize the 
spillover, etc.  At elevation 50 degrees, we noted the bandpass totally 
collapsed due to some whopping great signal.  With some help from Vivek, 
we later discovered that we had hit one of those three satellites 'dead 
on'!   Fortunately, even the VLA's forward gain is not enough to cause 
damage to the amplifiers...  The Sirius orbit is highly elliptical, with 
the apogee high above Nebraska, and the perigee more or less where 
Tierra del Fuego is.  Indeed, the three satellites are always above the 
same hemisphere!   It took me a while to figure out why this is.  :-)

    Regarding 'zero dec' for geostationary satellites -- I meant to type 
'zero dec or so', where the 'or so' was something like 10 degrees.  But 
actually, it's quite a bit more.  Vivek has just posted on the wall the 
az-el plot for a whole set of geostationary satellites.  Some of them 
are not very stationary, having been allowed to wander about in 
elevation by 15 or degrees or more above and below the 'belt'.   Most of 
the geostationary satellite are better behaved than this, however. 

    We've got to figure out how best to handle this tough observing 
neighborhood.  It's worse than L-band by quite a factor (due mostly to 
the very high powers).

    Rick

Todd R. Hunter wrote:
> On Fri, 1 Jul 2011, Frazer Owen wrote:
>
>> Rick Perley wrote:
>>>     Jim:
>>>
>>>     We'll need filters in two IFs (like, A and C), for at least a 
>>> couple
>>> of antennas.  I don't think we need as many as four (we're not checking
>>> closure here).
>>>
>>>     Anybody else with an opinion?
>>>
>>>
>>   If I understand correctly XM/Sirius affects all declinations, so it
>> makes sense to block it out.
>
> The Sirius satellites do have a very interesting elliptical orbit that 
> allows them to maintain significantly higher elevations than the 
> geostationary band as viewed from North America for over 8 hours. This 
> probably helps reception in inner cities.  There are 3 of them equally 
> spaced to provide 24 hour coverage (there may be more today, this was 
> 3 years ago). I could post plots if people are interested.
>
>> However, the downlinks only affect 0 dec so
>
> I cannot resist offering a small correction here.  Although 
> geostationary satellites orbit above the equator, they do not appear 
> at 0.0 dec unless you observe them from the equator. From the EVLA 
> latitude, they appear at about -5.5 deg dec. This shift is due to 
> their relatively small distance in units of Earth radii. I remember 
> calculating this effect back in high school when our earth science 
> class was donated a dish antenna so that we could receive GOES images 
> in addition to the polar orbiters we tracked with crossed yagis.
>
> Of course, older satellites whose orbits are no longer well-controlled 
> can still be geosynchronous (not stationary) but can have significant 
> diural variations in declination.
>
> Todd