[evlatests] Using S-band is Tough!

[Dan Mertely]

A useful tracking and information program for the Sirius/
XM constellations (and others) may be found at:

http://www.n2yo.com/whats-up/?c=33

Just select the satellite you wish to track and/or get
info on.

Notice that it will also make 5 day predictions, highlighting
those passes that are within "visible" range.  This is quite
useful for strong X-band and Ku band earth resource radar SAR
satellites such as TerraSAR-X:

http://www.n2yo.com/passes/?s=31698

and CryoSat-2:

http://www.n2yo.com/passes/?s=36508

Both of which could have a significant "impact" on EVLA/VLBA
observing (instantaneously as well as possibly permanently!)
should we happen to be so unfortunate as to have
a chance, beam-on-beam event.

-Mert


Rick Perley wrote:
>     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
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