[time-nuts] PN sequence generation using GPS

Magnus Danielson magnus at rubidium.dyndns.org
Wed Feb 16 23:05:25 UTC 2011


Joe,

On 02/16/2011 10:13 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.
>
> 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.
>
> My question is: If I use a 1PPS derived reset interval, a 10 MHz clock
> and the PN sequence below, will the reset interval intrude on the
> sequence? If so, what reset interval or fraction of clock speed will be
> least disruptive?

A PN sequence often (but necessary always) having a length of l=2^n-1.
The GPS L1 C/A sequence is for instance 1023 which comes from the use of 
10 bit PRNG registers (two in parallel clocked at the same rate).

Consider that you use a 20 bit register at 10 MHz, then it would take 
1,048575 s to complete the sequence and a reset would interrupt the 
sequence if resetting it every second, but this form of shorted cycle is 
being used. One such example is the GPS P-sequence which is shortened 
down to only a week.

For GPS the PN sequence is being tracked by the receiver such that the 
carrier and code detection forms a PLL to maintain phase lock. I see no 
reason why you should not be able to use a similar lock mechanism for 
your system. It is well described and not too hard to implement.

For the receiver end you would only use GPS to bootstrap the lock-in as 
if receiver conditions is sufficient it will remain locked to the 
transmitter anyway. This boot-strap is assisted by having a code which 
repeats every second (but may repeat faster).

Your data you would modulate at a lower rate than the chip rate of the 
PN-sequence. That way you have the PN sequence lock maintained even if 
the polarity swaps due to data modulation (XORed sequence).

For instance, GPS L3 uses 93 chips for every bit even if the PN sequence 
is 1023 chips long.

Cheers,
Magnus



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