[time-nuts] Speaking of Costas loops (WAAS)

Bob Camp lists at rtty.us
Thu Jul 11 16:25:54 UTC 2013


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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Thursday, July 11, 2013 10:56 AM
To: time-nuts at febo.com
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.
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