[amsat-bb] Re: perfect groundtrack contest

[Howie DeFelice]

Hi Domenico,
Thanks for the link. Most of the commercial GEO satellites in orbit are bent pipe translators, similar to the amateur linear transponders. Only in very few satellites in recent years have used on board processing and switching. The Intelsat IRIS payload, the INMARSAT F4 satellites supporting the BGAN system, and the ViaSat-1 satellite are some of the recent satellites that use various forms of on board switching. Most VSAT networks have the hub/control on the ground. All remote to remote connection are "double hop". Only remote to hub are single hop. There are some systems that are full mesh and allow single hop remote to remote communications. 
Commercial bent pipe transponders are typically 36 to 72 MHz. wide and are very linear. Each satellite typically divides their available 500 MHz of C or Ku bandwidth into transponders. The frequency can be 100% reused by using polarization diversity. They usually have fixed gain on the transponder controlled by uplink attenuators and downlink  gain settings. It is up to the satellite operator to be sure that the total  power of all the transponder users does not saturate the transponder SSPA. In almost all cases, this is a manual operation and if a single uplink site is transmitting too much power that user needs to be contacted and the offending station adjusted, really not much different than ham radio :) VSAT network operators can usually control the transmit power of their remote sites from the hub station.  The current major GEO operators have an average of 80% capacity utilization. The going price for GEO capacity is in the range of $5,000 to $10,000 per Mhz. per month. 
-Howie AB2S

> From: domenico.i8cvs at tin.it
> To: howied231 at hotmail.com; w7lrd at comcast.net
> CC: amsat-bb at amsat.org
> Subject: Re: [amsat-bb] Re: perfect groundtrack contest
> Date: Sat, 16 Jun 2012 01:02:57 +0200
> 
> Hi Hovie,AB2S
> 
> You are correct but there is another reason that make very difficult to
> use a commercial satellites for amateur communications.
> 
> The Amateur Satellites and our ground stations are FDMA i.e Frequency
> Division Multiple Access in wich the satellite passband is shared at the
> same Time by all ground users depending on the frequency that they
> chouses for the uplink and downlink.
> 
> On the other side the Commercial Satellites are mostly TDMA i.e. Time
> Division Multiple Access in wich a master Ground Stations send to the
> satellite transponder the sincronization pulses to switch the onboard
> multiplexer at very high speed and specific Time between all client ground
> stations as explained in the following web page.
> 
> http://www.satsig.net/vsat-equipment/tdma-explanation.htm
> 
> In simple word the TDMA often called "switcher" operate as a citizen
> Telephone Central Station that is not on earth but on board of the
> satellite.
> 
> 73" de
> 
> i8CVS Domenico
> 
> ----- Original Message -----
> From: "Howie DeFelice" <howied231 at hotmail.com>
> To: <w7lrd at comcast.net>
> Cc: <amsat-bb at amsat.org>
> Sent: Friday, June 15, 2012 11:31 PM
> Subject: [amsat-bb] Re: perfect groundtrack contest
> >
> > Unfortunately the hardware on board isn't programmable. The shared
> > antenna's are connected to very frequency specific bandpass filters that
> > form the multiplexer, the frequency translators are fixed and if the HPA
> > is a TWTA it is also very frequency dependent. By the time the commercial
> > operators are done squeezing every last penny out of them, there isn't
> > much left over to use anyway :)
> >
> > - Howie AB2S
> >
> > Date: Fri, 15 Jun 2012 21:18:46 +0000
> > From: w7lrd at comcast.net
> > To: howied231 at hotmail.com
> > CC: amsat-bb at amsat.org; tomdoyle1948 at gmail.com
> > Subject: Re: [amsat-bb] perfect groundtrack contest
> >
> > I have also "wondered" if any of the older commercial surplus satellites
> > can be "reprogrammed"  for our use.  I realized that idea is no doubt an
> > over simplification.  Today's speculation can be tomorrows reality.
> > 73 Bob W7LRD
> >
> >
> >
> > From: "Howie DeFelice" <howied231 at hotmail.com>
> > To: amsat-bb at amsat.org, tomdoyle1948 at gmail.com
> > Sent: Thursday, June 14, 2012 12:48:06 PM
> > Subject: [amsat-bb] perfect groundtrack contest
> >
> >
> > I vote for Intelsat G-11 at 304.5 deg E. This is a Ku band only bird,
> > however, the semi-retired Intelsat 805, which is co-located with this sat
> > has a global C band beam with a downlink that extends into the amateur 9cm
> > satellite band (3400-3410 MHz). I often wondered if Intelsat would
> > consider "donating" a few Khz  to AMSAT. We could then tie a single,
> > commercially licensed uplink station via echolink and hear it on the 9cm
> > downlink. At least it would be a reason for people to use the 9cm band
> > before it's taken away for the latest mobile broadband gizmo.
> >
> > Howie AB2S
>