[time-nuts] Speaking of Costas loops

[Bob Camp]

Hi

On Jul 3, 2013, at 3:51 PM, jmfranke at cox.net wrote:

> http://www.navipedia.net/index.php/WAAS_Signal_Structure
> 
> Doppler Shift: The Doppler shift, as perceived by a stationary user, on the signal broadcast by WAAS GEOs is less than 40 meters per second (≈210 Hz at L1)

So unless you can measure and correct for the doppler, you are at a few hundred Hz at 1.5 GHz. 150 Hz would be 0.1 ppm. That's not very accurate. 

> in the worst case (at the end of life of the GEOs). The Doppler shift is due to the relative motion of the GEO. 
> Carrier Frequency Stability: The short term stability of the carrier frequency (square root of the Allan Variance) at the input of the user´s receiver antenna will be better than 5x10-11 over 1 to 10 seconds, excluding the effects of the ionosphere and Doppler. 
> Polarization: The broadcast signal is right-handed circularly polarized. The ellipticity will be no worse than 2 dB for the angular range of ±9.1o from boresight. 
> 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). 

Once you are past 100 seconds, there's essentially no spec. One cycle per 100 sec is a lot, even at 1.5 GHz

> Correlation Loss: Correlation loss is defined as the ratio of output powers from a perfect correlator for two cases: 1) the actual receiver WAAS signal correlated against a perfect unfiltered PN reference, or 2) a perfect unfiltered PN signal normalized to the same total power as the WAAS signal in case 1. The correlation loss resulting from modulation imperfections and filtering inside the WAAS satellite payload is less than 1 dB. 

If you are only after the carrier, the code stuff pretty much does not matter.

Bob

> 
> John WA4WDL
> 
> ---- Bob Camp <lists at rtty.us> wrote: 
>> Hi
>> 
>> The pipe in this case is up on one frequency and down on another. The conversion oscillator on satellite that's the weak link, no matter how good the signal from the ground happens to be. 
>> 
>> Bob
>> 
>> On Jul 3, 2013, at 1:48 PM, Attila Kinali <attila at kinali.ch> wrote:
>> 
>>> On Wed, 3 Jul 2013 08:29:02 -0400
>>> Bob Camp <lists at rtty.us> wrote:
>>> 
>>>> There are two batches of GPS / WAAS sats up there:
>>>> 
>>>> 1) The ones with numbers above 100 that are geosync and that only do WAAS
>>>> 
>>>> 2) The ones with numbers <= 32 that do nav. These are not geosync. 
>>>> 
>>>> I believe the only ones with corrected / high stab clocks on board are
>>>> those in the second group. The stuff in the first group aren't dedicated
>>>> sats, just leased transponders on conventional multipurpose geosync birds. 
>>> 
>>> I don't know about WAAS, but AFAIK the EGNOS signals are generated on
>>> ground using Cs references and retransmitted by the satelites using
>>> a "bend pipe". Ie. the signals should be of time-nut quality even without
>>> high accuracy frequency standards in the birds themselves.
>>> 
>>> (Sorry, i'm not able to find where i read about that, so no references today)
>>> 
>>> 			Attila Kinali
>>> 
>>> -- 
>>> The people on 4chan are like brilliant psychologists
>>> who also happen to be insane and gross.
>>> 		-- unknown
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>