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

Bob Camp lists at rtty.us
Thu Jul 11 06:34:55 EDT 2013


Hi

I read the patent and understand how you can get timing off of a WAAS sat. The carrier does not need to have fancy steering on it to enable that function. The thing that it does not show is doing carrier phase off of a WAAS sat.

Bob

On Jul 10, 2013, at 9:54 PM, David I. Emery <die at dieconsulting.com> wrote:

> On Wed, Jul 10, 2013 at 07:45:39PM -0400, Bob Camp wrote:
>> Hi
>> 
>> If the WAAS  birds are run in a fashion that gives a true GPS payload performance, why not assign them a SN 32 or below and use them? 
>> 
>> If the WAAS birds are not in the "right numbers", why bother to set them up and spend the bucks to make them behave like a nav sat? What's the payoff?
> 
> 
> 	The patent cited here recently explains... for fixed timing
> purposes and basic  anti jam a simple directional antenna pointed at the
> WAAS bird allows rejection of many interferers without elaborate and
> expensive active steered phased array nulling technology.   
> 
> 	And because - given a known fixed ground position - timing and
> frequency can work with only one bird visible, this allows
> timing/frequency using just the WAAS signal (or signals, they do provide
> more than one WAAS frequency).
> 
> 	And potentially if the timing accuracy via the hosted payload is
> respectable at least for the needs of many  fixed time/frequency users
> this might supply a solution MUCH less resistant to local (nearby)
> interferers than the usual more or less hemi pattern GPS antenna would -
> as fixed dishes with considerable gain toward the satellite could be
> used and in most places they would point well above the horizon and
> could be shielded by nearby structures to further reduce jamming
> susceptibility from jammers (intentional or unintentional) below or at
> the horizon for the site.   For timing/frequency users (certainly an
> important subset of the GPS user population) this provides some
> protection by antenna pattern that is hard to obtain otherwise (and
> users interested in higher precision or redundancy of timing could still
> just use another GPS timing system based on normal hemi GPS antennas as
> the primary - using the normal SVs - and rely on the dedicated dish
> pointed at the WAAS bird only as backup in the event of jamming).
> 
> 	The choice of using different spreading codes from the normal
> GPS set for WAAS or using a slightly different one is an overall system
> architecture decision... which I guess was made in favor of not tying
> up codes for regular SVs for the WAAS birds.   But AFAIK a receiver with
> suitable firmware could still extract pseudo ranges and use them.
> 
> 	I guess there is an issue in any frequency translation scheme
> with the relationship of carrier and code phase... a homodyne
> distortion... due to the random phase of the LO(s)...  but this too can
> be predistorted on the ground to come out right and that kept in line
> via closed loop tracking of the downlink from a ground site.
> 
> 	I do understand that this insight into a potential further use
> of WAAS beyond its use as a data channel and propagation beacon seems
> to have happened later and not initially.
> 
> -- 
>  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
> "An empty zombie mind with a forlorn barely readable weatherbeaten
> 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
> celebration of what could have been, but wasn't and is not to be now either."
> 
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