[time-nuts] GPS Modulation and 10 MHz Delay Lock (Jim Lux) (Jim Lux)

[johncroos at aol.com]

for Jim Lux --

Thanks for the comments -


Yes, if the receiver is linear (e.g. say you do a sliding code
correlator and slide until you get the peak, with the correlator using 
a
multiplier)...

That is how the system I did worked. It could defeat an on-frequency 
jammer that
was 20 db stronger than the signal while holding a 1e -6 BER. So 
considering the
BPSK demod required a 10 db CNR, the total AJ processing gain was about 
30 dB,
which is what was expected.

But since most (inexpensive) receivers have hard limiters/1 bit
quantizers in front of the correlator, what goes into the correlator is
a square wave at the jammer frequency, and you can slide your PN code
all day and not get a peak.

Yep - a hard limiter would kill any processing gain and simplify the 
design.

There's some analysis out there that tells you how many bits you need
for a given Jammer/Signal ratio, but in general, if the jammer is 20 dB
over the signal, you need 3-4 bits.

Back in the day, when bits were very expensive, that's why narrow band
excisers were popular.. basically you'd run something like a PLL to
recover the tone jammer, and subtract it out.

We were not that good in 1976. But going above 20 GHz and antennas with
a 2 degree beam width hid the signal and provided considerable immunity.

Anyhow - thanks for the information on more modern techniques!

-73 john k6iql