Re: time-nuts Digest, Vol 108, Issue 36 On Sat, 06 Jul 2013 13:59:26 -0400, time-nuts-requ...@febo.com wrote:
> > Message: 5 > Date: Sat, 06 Jul 2013 18:57:05 +0200 > From: Magnus Danielson <mag...@rubidium.dyndns.org> > To: time-nuts@febo.com > Subject: Re: [time-nuts] Speaking of Costas loops (WAAS) > Message-ID: <51d84c61.3070...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
