[time-nuts] PN sequence generation using GPS

[Joe Leikhim]

I am trying to stay within the FCC Part 97 rules. The spreading or hopping will be of a narrowband (25 KHz BW) FM signal. I haven't decided on either the FHSS or DSSS approach. I had thought of a FH approach that exploited time of day to address a frequency look up table, but I think that is outside Part 97, although arguably it is a "net scheduler."

1) If I go with a FH approach running at around 10-20 hops/second will the 1 pps be sufficient?

2) Part 97 says the shift register cannot be reset by anything other than by itself during a transmission. Clearly many of the synchronization methods discussed in the ARRL SS Sourcebook utilize a synch method transmitted across the link or derived from a TV broadcast signal etc. I don't see the reason this rule exists?

3) If I block the 1 PPS during PTT, and the receiving end asserts 1 PPS reset, will synch be lost? If not how effective will the freewheeling be during a 30 second exchange with 10 MHz GPS derived clock reduced to 20 hops/second?

4) 20 hops per second is a 50 ms chip. Two radios 50 miles apart would be 270 us delayed. I don't think that should impair the analog FM. So don't envision needing any correlation adjustment. Comments?


    From: Bob Camp


    Hi

    The key item here is that the system is going to work via amateur
    radio here in the US. The FCC only lets you use three very specific
    PN sequences. The three are called out explicitly in the rules. The
    requirement that they not be reset while transmitting except by
    feedback is also called out explicitly in the rules. So no fancy
    stuff with multiple spreaders and the like.

    The fact that they work out to primes is not important other than
    there's not much use trying to factor them. Without any common
    factors, and with 1 pps as your reset, you get a pretty specific
    trigger point for reset. It's long enough to not be of much use.

    There's a much easier way to do it and keep everybody in sync. GPS
    gives you time of day along with the 1 pps signal. You agree that at
    this or that time, everybody starts in sync. It's just time math to
    figure out where you should be at any 1 pps point. If you aren't
    where you are supposed to be, you take action.

    I suspect you could do all the checking on a 50 cent processor.
    Latch the data at 1 pps and read the time string. Do the math to
    check it. Fire the alarm flag it's not right. Not a lot to get done
    in a second.

    Bob

-- 
Joe Leikhim

Leikhim and Associates
Communications Consultants
Oviedo, Florida

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