Hi Having seen the number of signals being piggybacked on some transponders (> 100,000) it's safe to say that those transponders were not running saturated.
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jim Lux Sent: Thursday, July 11, 2013 10:56 AM To: [email protected] Subject: Re: [time-nuts] Speaking of Costas loops (WAAS) On 7/11/13 3:36 AM, Bob Camp wrote: > Hi > > The pseudo random spreading / looks like noise / buried signal thing is the most common way people piggyback low level signals on a bent pipe. > > Assuming that the bent pipe isn't running saturated, which I'm not sure is a valid assumption. Running TWTAs with enough backoff to be linear(ish) consumes a lot more power. I think that most of the transponders on commercial comsats are running linear (or linearized) at least for C and Ku band type applications. However, I wouldn't be so sure for more specialized applications. Consider the S-band Sirius/XM system, they basically designed the satellites for that service, and it could be run saturated, carrying a single high rate data stream that the single channel ground receiver in the car looks at. In fact, a bit of wikipedia research shows that each of the two Sirius satellite broadcasts only one carrier with 4 MHz bandwidth (different frequencies for different satellites). The receiver does both, to get diversity. XM uses 6 frequencies, in a similar scheme. i did find a block diagram of the Sirius payload using google in a book by Elbert (p 267), and while they use a huge pile of TWTAs all combined to radiate about a kilowatt, it does look like they're running two carriers through them (2322.1 and 2330.4 MHz) so they must be running at least somewhat linear. Sirius is S band, but there are also L-band DARS services in other parts of the world. I recall seeing some of the TWTAs for these things in a display case at the tube mfr (Thales, these days) in Ulm, and they are huge beasts. (I'm used to seeing the little helix X, Ku or Ka-band tubes we use for deep space comm or earth observing radar. A dual 300 Watt L-band cavity coupled TWTA is physically quite large.) This doesn't really answer the question about what the payload for WAAS/EGNOS looks like, though. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
