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

David I. Emery die at dieconsulting.com
Wed Jul 10 21:54:53 EDT 2013


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."



More information about the time-nuts mailing list