[time-nuts] Speaking of Costas loops

[Dennis Ferguson]

On 3 Jul, 2013, at 10:48 , 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)

I have also read that WAAS satellites can be usefully included in the GPS
solution, so they aren't necessarily inferior, but I also don't have
a reference.  There is this:

    http://tf.boulder.nist.gov/general/pdf/2299.pdf

The clocks are indeed ground based and good quality.  The advantage of using
them as an alternative to GPS CV (which is what the paper is about) is that
they transmit unencrypted code on two frequencies to allow computing ionospheric
corrections and they don't move (much) so you can track them continuously with
a dish to get a big signal-to-noise improvement and multipath insensitivity.  The
last bit seems like a mixed blessing, though, since the dish means you depend on
only the one satellite it is pointed at and hence suffer from whatever bad things
happen to it.  The paper notes events that it characterises as an "increasing problem
with the broadcast WAAS ephemeris", followed by an outage and clock jump, which I
interpret as maybe being an adjustment made to the satellite orbit which can't
be represented properly in the ephemeris.  I assume that could happen with regular GPS
satellites too, but if you are tracking a lot of them at once it is easy to detect
and toss out a solution outlier.

Dennis Ferguson