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

[Joseph Gwinn]

> Re: time-nuts Digest, Vol 108, Issue 29
> On Fri, 05 Jul 2013 19:55:42 -0400, time-nuts-request at febo.com wrote:
>
> 
> Message: 6
> Date: Sat, 06 Jul 2013 00:27:33 +0200
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Speaking of Costas loops
> Message-ID: <51D74855.9090805 at rubidium.dyndns.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> On 07/05/2013 10:39 PM, Joseph Gwinn wrote:
>>> Re: time-nuts Digest, Vol 108, Issue 28
>>> Message: 2
>>> Date: Fri, 05 Jul 2013 09:18:39 -0700
>>> From: Jim Lux<jimlux at earthlink.net>
>>> To: time-nuts at febo.com
>>> Subject: Re: [time-nuts] Speaking of Costas loops
>>> Message-ID:<51D6F1DF.9090800 at earthlink.net>
>>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>> 
>>> On 7/5/13 8:44 AM, Bob Stewart wrote:
>>>> Wouldn't a Cs or Rb clock in orbit be slow due to relativistic
>>>> effects?  I'm pretty sure there is a relativistic correction to the
>>>> GPS clocks.
>>>> 
>>>> Bob - AE6RV
>>>> 
>>>> 
>>> 
>>> I believe that the original WAAS repurposed transponders intended for
>>> other L-band satellite signals (e.g. Sirius/XM/LightSquared).
>>> 
>>> As noted earlier in the discussion, the new satellites might have a
>>> specialized payload, which could have a purpose specific coherent
>>> transponder, rather than a linear translator.
>>> 
>>> If it is purpose specific and single channel, then making it immune to
>>> the local oscillator is straightforward.
>> 
>> I worked on a proposal for the original WAAS system.  The WAAS signal
>> is not a timing signal in the sense that GPS signals from space are
>> timing signals.  WAAS instead sends out a stream of correction data
>> that allows one to greatly improve the accuracy and reliability of GPS
>> signals.
>> 
>> So, unless things have changed greatly, the geostationary satellite
>> that broadcasts the WAAS signal need not have an atomic clock.
> 
> This is naturally still true, but we are into the level of "there's a 
> signal here, what can we use it for?". Doing a much simplified receiver 
> could serve some well enough, without going the full monty. It's like 
> taking the color-carrier of analog TV broadcasts.

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.  

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.

Joe Gwinn