[time-nuts] Speaking of Costas loops (WAAS)
Jim Lux
jimlux at earthlink.net
Thu Jul 11 14:56:13 UTC 2013
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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