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

Jim Lux jimlux at earthlink.net
Sat Jul 6 15:43:35 UTC 2013


On 7/6/13 7:50 AM, Joseph Gwinn wrote:
>> Re: time-nuts Digest, Vol 108, Issue 29
>> On Fri, 05 Jul 2013 19:55:42 -0400, time-nuts-request at febo.com wrote:

> OK.  Given that the birds WAAS uses were built for communications
> purposes, not timing purposes, I'g guess that their frequency reference
> is a very good quartz unit. I suppose Rubidium is possible, but Cesium
> is very unlikely.
>

Except that apparently, the WAAS/EGNOS repeater payload is purpose 
designed, so it isn't necessarily the "same" bent pipe as is used for 
other purposes, although it could be: similar to other Mobile Satellite 
Service channels for instance.

I would think it very unlikely they are flying either Rb or Cs.  If they 
need high stability and precision, then they'd just recover the carrier 
from the uplink signal, because that could be as steady as you like it. 
  I would think it would be cheaper to do that than to put an atomic 
reference up.



> Bent-pipe channels do a frequency change to eliminate singing.  I
> imagine the datasheet for the rentable comm channels will give the
> frequency error and stability of the downlink signal.

The international allocations for up and down frequencies are separated 
by quite a bit (Earth to Space and Space to Earth, respectively).

For C band, up is around 6 GHz and down is around 4 GHz.  That makes 
building a filter to separate them pretty easy. So the WAAS signal goes 
up on 4 and comes down on 1.5.

What is really needed is a good description of the WAAS/EGNOS system, 
because it will give all those nice gory details.




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