[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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