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

[Joseph Gwinn]

Re: time-nuts Digest, Vol 108, Issue 36
On Sat, 06 Jul 2013 13:59:26 -0400, time-nuts-request at febo.com wrote:

> 
> Message: 5
> Date: Sat, 06 Jul 2013 18:57:05 +0200
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Speaking of Costas loops (WAAS)
> Message-ID: <51D84C61.3070503 at rubidium.dyndns.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> On 07/06/2013 06:29 PM, Joseph Gwinn wrote:
>>>> Code/Carrier Frequency Coherence: The lack of coherence between the
>>>> broadcast carrier phase and the code phase shall be limited. The short
>>>> term (<10sec) fractional frequency difference between the code phase
>>>> rate and the carrier frequency shall be less than 5x10-11 (one sigma).
>>>> Over the long term (<100 sec), the difference between the change in the
>>>> broadcast code phase (convert to carrier cycles) and the change in the
>>>> broadcast carrier phase shall be within one carrier cycle (one sigma).
>>> 
>>> 
>>> This is interesting. Does it imply that they regenerate the code on board?
>> 
>> Very unlikely, because then the bird would have to understand every
>> possible code, including those not invented when the bird was launched.
> 
> If it is within the Gold codes being used for GPS and WAAS, they only 
> need to alter the 10 bit reset-value of the G2 PRN code. See the WAAS 
> specification, as this method is being recommended for receivers.
> 
> Within that limit, it is relatively cheap to provide code tunability.

Yes, of course, but I don't think I explained very well.  The issue was 
more economic than technical.   

There isn't much spare space, weight, or power in the birds, technology 
moves rapidly, and the satellite companies don't want to have expensive 
satellites that no longer generate rental income because something 
became obsolete.  So they ruthlessly simplify.  A bent pipe will handle 
any possible band-limited modulation, no matter if currently known or 
not, and so is the safest solution.

Now WAAS may have become important enough to command dedicated 
hardware, but that came later, to the degree it came at all.

Joe Gwinn