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

Jim Lux jimlux at earthlink.net
Sat Jul 6 11:56:46 EDT 2013


On 7/6/13 8:10 AM, jmfranke 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) in the worst case (at the end of life of the GEOs).

That is more a requirement on the spacecraft.  Precompensation from the 
ground won't work... if the satellite is driving West, then users to the 
west see the frequency go up, and users to the east see it go down.

> 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.

that sounds like comparable to a decent OCXO (10811A, etc.)


> 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.

Antenna spec..

> 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?




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