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

[Magnus Danielson]

On 07/10/2013 11:08 PM, David I. Emery wrote:
> On Wed, Jul 10, 2013 at 08:10:45PM +0200, Magnus Danielson wrote:
>> On 07/09/2013 04:25 AM, Joseph Gwinn wrote:
>>> 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.
>>
>> A bent pipe is more generic, but there are limits to how much you can
>> alter the output frequency too.
>
> 	It seems completely inconceivable to me that either the antenna
> system (particularly feeds) or transponder RF hardware on any commercial
> Ku or C or Ka or X band satellite could possibly be frequency agile
> enough to tune to 1575.42 MHz unless it was purpose designed to radiate
> on that frequency from the start.
>
> 	So any hosted WAAS payload is completely application specific.

I was thinking along the same lines, but I have too little experience in 
RF design for birds. There are several potential other uses for L-band 
transmission if tweaking a little up or down from L1 is feasible, 
otherwise it's pretty application specific.

WAAS links primarily provides an information channel, so it doesn't have 
to be very accurate. However, as you devote a channel to it, you might 
as well use it to produce pseudo-ranges, but it seems like they didn't 
care too much on the carrier-phase part compared to the code-phase, but 
10 years back not many receivers used the code phase for nav at all, but 
carrier smoothed code should at least be common now, so for those it may 
not fully meet the needs. The added precision for the other channels 
compensate thought.

Cheers,
Magnus