[time-nuts] Truetime XLi sat signal levels (dBW to dBc/SNR conversion question)

[David I. Emery]

On Sat, Jun 15, 2019 at 05:21:17PM +0000, Mark Sims wrote:

> The 30 dBc refers to the signal level reported by the Ublox.   If the
> Ublox falls below around 30, the Truetime will be reporting -196 dBW.  
> That seems to be the cutoff threshold for its idea of a "good" signal...
> it does not have a user settable signal level mask     My conversion
> formula works very well over the full range of usable signals and
> matches what the Ublox is reporting within 1 dB.

	If I understand your conversion formula, it basically recognizes
that dB over some reported "zero" power in  dBW more or less precisely
tracks the usual GPS "dBc" one for one.

	I do believe that dBc as used in this context is more or less
our old friend Eb/No (and less clear to me whether that is plus or minus
a fudge factor or literally the same as the Eb/No of the 50 baud
despread BPSK GPS almanac).  As such one would expect it would track
one to one.

	Eb/No is widely used as a definition of the signal to noise
ratio of a digital signal of course, and deeply rooted in theory.   It
is the probably the most logical unit of CNR in which to report CNR of a
satellite GPS signal.  The other alternative being db margin over the
Eb/No that in theory provides less than some defined BER for the almanac 
(sometimes called the quasi error free point).
 
	I would expect that an actual GPS receiver has a threshold for
Eb/No below which it will not usefully lock to a GPS satellite signal...
and potentially  reports that as no signal... and maybe zero dBc (or
similar Eb/No) whether or not there is actually that CNR present. (This
might not match the Eb/No of the theoretical QEF point depending on
receiver implementation).

	It is possible to measure Eb/No by various signal processing
techniques from actual signal samples, one would suspect that that is
what is measured and converted to an equivalent signal power reported in
dBW relative to some kind of (defined) noise floor.  

	As I said before it is not readily possible to actually measure
real signal power without some kind of system calibration, the best one
can do is infer it from an assumed noise power and a measured Eb/No.


-- 
  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."