[time-nuts] PN sequence generation using GPS

[jimlux]

On 2/16/11 1:13 PM, Joe Leikhim wrote:
> For clarification;
> I am investigating an experiment using GPS to create a FHSS or DSSS
> project similar to those of AMRAD and described in the ARRL Spread
> Spectrum Sourcebook. In those experiments, a specific shift register
> sequence was used (see below), the clock was free running and a reset
> tone was transmitted on irregular intervals (whenever synch was believed
> to be lost) over the radio circuit.

You can guesstimate how long it will be til you're out of sync.
if your chip rate is 1 MHz, and your oscillators are the same within 1 
ppm, then you'll drift one 1 chip in a second, so your 1pps from the GPS 
won't be frequent enough.

OTOH, if you're using a disciplined 10MHz from a GPSDO which is good to 
1E-10 or so, then in a second, you'll only be 1E-4 chip out of sync (for 
a 1MHz chip rate).

For a DS system, being half a chip out of sync is a 3dB loss. A lot of 
systems (particularly designed in the 70s) which used sync circuits that 
could only track within 1/2 chip were happy to have that kind of 
performance. Hence, GPS C/A code provided accuracy on the order of 500ns 
(half a chip) which corresponds to 150ish meters, which is not 
sufficient to destroy a hardened missile silo.  Remember that back in 
the 70s, tracking a code at 10 Mchip/s to a fraction of a chip (a no 
brainer these days) was very challenging.

Depending on what you're sending over the DS link, 1/2 chip sync might 
be good enough.  For instance, if you're spreading an AM signal..

>
> In my experiment I would like to derive the clock directly from a GPS at
> each radio and use the GPS to periodically reset the shift register
> without causing a glitch or disrupting the sequence. There is mention in
> the sourcebook of a relationship between clock speed, reset interval and
> shift register stage length. It is this area that I am confused.
>

Your challenge will not be in the reset.. it will be in determining the 
code phase offset between your two stations (unless you're doing 
something with low chip rates and close distances).