[time-nuts] NUT4NT: Four-channel All-frequency GNSS RF-to-Bits

[Gregory Maxwell]

On Sat, Aug 13, 2016 at 11:53 PM, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>
> It’s not real clear what the second chip on the board does. If it is just a bit to Ethernet converter
> then you are dealing with 2 bit data out of each of the four channels. You aren’t just doing tracking
> in that case ….4 channels at 2 bits -> 8 bits per clock. Clock at 38 to 100 MHz. That could turn out
> to be a very crowded Ethernet connection.

I assume it's like the GNSS firehose (
http://pmonta.com/blog/2012/06/04/gnss-firehose/ ) or the other
existing GNSS SDR receivers: The device samples some bandpass around
the relevant GNSS frequency(/ies), sends a digitized signal for you to
despread and track.

2bits * 4ch * 20MHz * 2 (nyquist) = 320Mbit/sec, no big deal for
gigabit ethernet or USB3 (or even USB2 for that matter, at least
ignoring overheads).

>From there you can have arbitrarily complex processing on the host--
but from that setup you can't easily produce a PPS signal-- the host
will operate asynchronously with the signal, with lots of delay and
jitter.

I've long though a better design for a GPSDO is instead of having the
GPS produce a PPS, you have the GPS contain a TIC and feed it a PPS,
and capture those timestamps along with the gps observations.  The GPS
solution would then also include the observed offset.  Beyond
eliminating the extra complexity of sawtooth correction, this could
produce better results under some signal conditions because averaging
error signal produced from single fixes will not always produce the
same result as running a larger model over more observations (because
the errors can be multimodal). (besides-- for precise timing against
an local atomic standard... you probably don't want to perturb the
local clock, but instead track corrections to some reference time.)

In either case (PPS out or PPS in) I believe some actual hardware
support is needed to tie the PPS pulses to the GPS sampling clock, in
any of these SDR GPS approaches... though I think not much.