Re: [time-nuts] PN sequence generation using GPS
lists@rtty.us said:
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.
Thanks, but I'm having trouble understanding that.
The problem is "not be reset while transmitting except by feedback". I
usually use reset to mean set the system to a specific state no matter what
the current state.
I think they are trying to say you can't use something like a PPS signal to
reset the LFSR. If so, I would have said something like "can't be reset.
The next state can only be influnced by feedback." (Which also rules out the
self-synchronizer type approaches which use the data bit rather than the LFSR
output.)
Are there any constraints on picking the initial state? (There has to be
some time when the transmitter starts.) Can I send packets? Can I start
each packet on a second boundary (and not reset if it goes over 1 second)?
What do you call a clump of voice when something like a PTT is used?
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.
That makes sense to me.
I think you have to figure out what to do if the transmission crosses
midnight. You can shut down and start over, or just keep going. In the
latter case, you have to agree (out of band) which day you started in, but
that only matters if you start near midnight.
--
These are my opinions, not necessarily my employer's. I hate spam.
On 2/17/11 2:46 PM, Hal Murray wrote:
lists@rtty.us said:
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.
Thanks, but I'm having trouble understanding that.
The problem is "not be reset while transmitting except by feedback". I
usually use reset to mean set the system to a specific state no matter what
the current state.
you could call the FCC and ask them...
Or, just build whatever, and wait for someone to complain, and say you
misinterpreted the rules. Unless you're a jerk, I suspect that they
won't fine you or anything else.
jimlux wrote:
you could call the FCC and ask them...
Or, just build whatever, and wait for someone to complain, and say you
misinterpreted the rules. Unless you're a jerk, I suspect that they
won't fine you or anything else.
Given the quality of the rank-and-file ham, and the fact that nobody, but
hams really care what happens on the ham bands, who would ever know?
I think you could run any form of SS you wanted to on the ham bands for
the rest of your life, and as long as you were semi-competent about it
(eg. didn't cause interference) you would remain undetected.
-Chuck Harris
Hi
If I can reset the sequence at any time I feel like while transmitting, I am modifying the sequence. The PN used to run my radio will not be the "official" PN they have legislated.
On a practical basis - I suspect they use autocorrelation to monitor you. As long as it's the right sequence, started where ever, their gear will work. The second issue would be that making gear that inter operated would be really tough if the sequence did strange things every so often. Manufacturer A would have a tough time talking to manufacturer B.
Bob
On Feb 17, 2011, at 5:46 PM, Hal Murray wrote:
lists@rtty.us said:
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.
Thanks, but I'm having trouble understanding that.
The problem is "not be reset while transmitting except by feedback". I
usually use reset to mean set the system to a specific state no matter what
the current state.
I think they are trying to say you can't use something like a PPS signal to
reset the LFSR. If so, I would have said something like "can't be reset.
The next state can only be influnced by feedback." (Which also rules out the
self-synchronizer type approaches which use the data bit rather than the LFSR
output.)
Are there any constraints on picking the initial state? (There has to be
some time when the transmitter starts.) Can I send packets? Can I start
each packet on a second boundary (and not reset if it goes over 1 second)?
What do you call a clump of voice when something like a PTT is used?
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.
That makes sense to me.
I think you have to figure out what to do if the transmission crosses
midnight. You can shut down and start over, or just keep going. In the
latter case, you have to agree (out of band) which day you started in, but
that only matters if you start near midnight.
--
These are my opinions, not necessarily my employer's. I hate spam.
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Speaking as a ham, if this is tried you will have hams complaining and DF'ing the offending signal.
Chuck, you are correct in that no one but the hams care but the reason behind this is that policing the ham bands has been delegated to the hams and specifically certified 'Official Observers'. Once a infraction and especially a.unlicensed infraction is noted penalties are pretty severe and swift.
If you want to do this get a 'experimental license'
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Chuck Harris cfharris@erols.com
Sender: time-nuts-bounces@febo.com
Date: Fri, 18 Feb 2011 01:49:36
To: Discussion of precise time and frequency measurementtime-nuts@febo.com
Reply-To: Discussion of precise time and frequency measurement
time-nuts@febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS
jimlux wrote:
you could call the FCC and ask them...
Or, just build whatever, and wait for someone to complain, and say you
misinterpreted the rules. Unless you're a jerk, I suspect that they
won't fine you or anything else.
Given the quality of the rank-and-file ham, and the fact that nobody, but
hams really care what happens on the ham bands, who would ever know?
I think you could run any form of SS you wanted to on the ham bands for
the rest of your life, and as long as you were semi-competent about it
(eg. didn't cause interference) you would remain undetected.
-Chuck Harris
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Hi Scott,
Speaking as a ham, I can tell you that no one will even notice that
you are there. Frequency hop SS signals, when done correctly, only
land for a few milliseconds at seemingly random frequencies within
their band. Because their PN sequence makes them appear random, there
aren't any identifiable rhythms to their signals that would signal their
presence... just a slight increase in background noise. They really
aren't noticeable with the usual narrow bandwidth tuned rice boxes
that most hams use.
I have played extensively with frequency hoppers, like the PRC117, and
absent a wideband receiver, that covers their entire band, you would
never know they are there.
-Chuck Harris
scmcgrath@gmail.com wrote:
Speaking as a ham, if this is tried you will have hams complaining and DF'ing the offending signal.
Chuck, you are correct in that no one but the hams care but the reason behind this is that policing the ham bands has
been delegated to the hams and specifically certified 'Official Observers'. Once a infraction and especially
a.unlicensed infraction is noted penalties are pretty severe and swift.
If you want to do this get a 'experimental license' Sent from my Verizon Wireless BlackBerry
On 2/25/11 7:13 AM, scmcgrath@gmail.com wrote:
Speaking as a ham, if this is tried you will have hams complaining and DF'ing the offending signal.
Speaking as a ham, and as someone who used to build (and attempt to
detect and jam) systems like this for a living..
The odds that a OO would see and recognize the signal is vanishingly
small. As for DFing? It would be very difficult, considering that you
need a wideband receiver to see the signal, and there are usually plenty
of other signals in the band.
Yes.. if I were radiating a few watts, and you were my next door
neighbor, so my signal was really strong, and I used a slow hop rate,
and you had a fast sweep spectrum analyzer.. you "might" be able to see
it. IF you knew what to look for.
More likely, you'd be talking about it to your friends and various
mailing lists (like this one), and someone who knows someone who knows
someone who is "interested" in such things would find out, and THEY
would get out the fancy gear and they would track you down.
That's what leads to the guy in the government car sitting outside your
house one morning with "just a few questions".
Basically, it all comes down to the "rude rule" - if you're rude to
others, bad things happen. If you're fooling with a 10-20 hop/second
hopper on, say, 2 meters, and you happen to put the local repeater input
frequencies in your hop list, and you don't hop over the entire 4 MHz
band, so you're hammering the repeater every few seconds.. and you run
your link 24/7, yep.. you deserve the abuse you'll get.
But for most people.. the hopper's interference would barely be
noticeable. (4 MHz, 5 kHz channel spacing is 800 hop channels.. so at
10 hops/sec, you'd hit the same channel every 80 seconds.. And if you
use a hop sequence that is longer than 800 (no reason not to...), it's
not even like a repetitive interference.
Maybe a weak signal guy running JT65 or something might notice something
on their waterfall, but maybe not.
Chuck, you are correct in that no one but the hams care but the reason behind this is that policing the ham bands has been delegated to the hams and specifically certified 'Official Observers'. Once a infraction and especially a.unlicensed infraction is noted penalties are pretty severe and swift.
If you want to do this get a 'experimental license'
Well.. that would be a better solution, and the way to really go.
Hi
Remember that this started out running a sequence that was 127 hops long.
With something that short, it's pretty likely you will be rude to somebody.
Even if you are running a massive hop rate, I can likely walk around and
track you down within the average neighborhood. A diode detector behind a
bandpass filter and a small-ish directional antenna is about all I'd try to
use.
I suspect it would also work with one of the power detector chips. Range
wise, a lot would depend on just how good your local cable company is at
keeping their stuff running right. I'm not really sure the chip would add a
lot of range in a normal setting.
Bob
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of jimlux
Sent: Friday, February 25, 2011 10:57 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS
On 2/25/11 7:13 AM, scmcgrath@gmail.com wrote:
Speaking as a ham, if this is tried you will have hams complaining and
DF'ing the offending signal.
Speaking as a ham, and as someone who used to build (and attempt to
detect and jam) systems like this for a living..
The odds that a OO would see and recognize the signal is vanishingly
small. As for DFing? It would be very difficult, considering that you
need a wideband receiver to see the signal, and there are usually plenty
of other signals in the band.
Yes.. if I were radiating a few watts, and you were my next door
neighbor, so my signal was really strong, and I used a slow hop rate,
and you had a fast sweep spectrum analyzer.. you "might" be able to see
it. IF you knew what to look for.
More likely, you'd be talking about it to your friends and various
mailing lists (like this one), and someone who knows someone who knows
someone who is "interested" in such things would find out, and THEY
would get out the fancy gear and they would track you down.
That's what leads to the guy in the government car sitting outside your
house one morning with "just a few questions".
Basically, it all comes down to the "rude rule" - if you're rude to
others, bad things happen. If you're fooling with a 10-20 hop/second
hopper on, say, 2 meters, and you happen to put the local repeater input
frequencies in your hop list, and you don't hop over the entire 4 MHz
band, so you're hammering the repeater every few seconds.. and you run
your link 24/7, yep.. you deserve the abuse you'll get.
But for most people.. the hopper's interference would barely be
noticeable. (4 MHz, 5 kHz channel spacing is 800 hop channels.. so at
10 hops/sec, you'd hit the same channel every 80 seconds.. And if you
use a hop sequence that is longer than 800 (no reason not to...), it's
not even like a repetitive interference.
Maybe a weak signal guy running JT65 or something might notice something
on their waterfall, but maybe not.
Chuck, you are correct in that no one but the hams care but the reason
behind this is that policing the ham bands has been delegated to the hams
and specifically certified 'Official Observers'. Once a infraction and
especially a.unlicensed infraction is noted penalties are pretty severe and
swift.
If you want to do this get a 'experimental license'
Well.. that would be a better solution, and the way to really go.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
I guess that is why I mentioned something about doing it competently.
The FCC so seriously winged the methods usable by hams as to render them
effectively useless.
A nice direct sequence spread spectrum system with a couple of MHz
spread would be well below the background noise of any narrow band
receiver. Sure, you could find it with a wide band detector if you were
close by, but how would you know that you weren't looking at some other
anomaly, like a bad insulator, or trash coming off of fluorescent lamps?
Done correctly, you could run spread spectrum just about anywhere you wanted
to, and remain undetected. Using direct sequence, you would be so low in
power density that it could easily be argued that you were operating within
the constraints of a part 15 device's leakage.
-Chuck Harris
Bob Camp wrote:
Hi
Remember that this started out running a sequence that was 127 hops long.
With something that short, it's pretty likely you will be rude to somebody.
Even if you are running a massive hop rate, I can likely walk around and
track you down within the average neighborhood. A diode detector behind a
bandpass filter and a small-ish directional antenna is about all I'd try to
use.
I suspect it would also work with one of the power detector chips. Range
wise, a lot would depend on just how good your local cable company is at
keeping their stuff running right. I'm not really sure the chip would add a
lot of range in a normal setting.
Bob
Hi
If you are putting out > 20 watts and I'm close enough to see your mailbox, I'll detect you with some pretty simple stuff.
Bob
On Feb 25, 2011, at 1:16 PM, Chuck Harris wrote:
I guess that is why I mentioned something about doing it competently.
The FCC so seriously winged the methods usable by hams as to render them
effectively useless.
A nice direct sequence spread spectrum system with a couple of MHz
spread would be well below the background noise of any narrow band
receiver. Sure, you could find it with a wide band detector if you were
close by, but how would you know that you weren't looking at some other
anomaly, like a bad insulator, or trash coming off of fluorescent lamps?
Done correctly, you could run spread spectrum just about anywhere you wanted
to, and remain undetected. Using direct sequence, you would be so low in
power density that it could easily be argued that you were operating within
the constraints of a part 15 device's leakage.
-Chuck Harris
Bob Camp wrote:
Hi
Remember that this started out running a sequence that was 127 hops long.
With something that short, it's pretty likely you will be rude to somebody.
Even if you are running a massive hop rate, I can likely walk around and
track you down within the average neighborhood. A diode detector behind a
bandpass filter and a small-ish directional antenna is about all I'd try to
use.
I suspect it would also work with one of the power detector chips. Range
wise, a lot would depend on just how good your local cable company is at
keeping their stuff running right. I'm not really sure the chip would add a
lot of range in a normal setting.
Bob
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Of course. A simple field strength meter comes to mind. But how
are you going to know that you want to stand at my mailbox and look?
And with a direct sequence system, the power is going to be smeared so
thin over a wide band that you really won't have anything to see, other
than a bulk reading of power in my vicinity. I'm already doing that with
the switching supplies, compact fluorescent lamps, ordinary fluorescent
lamps, cell phones, wireless routers, microwave ovens, ... It's a literal
RF sewer over here!
-Chuck Harris
Bob Camp wrote:
Hi
If you are putting out> 20 watts and I'm close enough to see your mailbox, I'll detect you with some pretty simple
stuff.
Bob
If you want RF sewer - look at ethernet over power (e.g. Devolo) - it's horrid.
Regards,
David Partridge
Hi
Obviously I'd have to be in the neighborhood (as in well under a KM away). To be bothered by a fast system, I'd also have to be there already. With a poor system, I'd be bothered further out and could track it further out.
I would put a good RF filter in front of the detector to get rid of a lot of the clutter. Switchers aren't going to put out 10 watts of RF at 144 MHz. The biggest thing would be cable TV leakage that's "in band". If I had to put something together this weekend, I'd chop the front end out of a donor radio and keep it's RF stage intact. That would take care of a lot of the little loss issues that the filtering would bring to the party.
My real concern is more interference to GPS from some of these odd systems various commercial outfits come up with. I'd like to be able to keep running timing via GPS for a while. The same things that apply to tracking down a strange ham system apply equally to other spread spectrum systems. Simply figuring out who / what / where is going to get harder as time goes on.
Bob
On Feb 25, 2011, at 1:55 PM, Chuck Harris wrote:
Of course. A simple field strength meter comes to mind. But how
are you going to know that you want to stand at my mailbox and look?
And with a direct sequence system, the power is going to be smeared so
thin over a wide band that you really won't have anything to see, other
than a bulk reading of power in my vicinity. I'm already doing that with
the switching supplies, compact fluorescent lamps, ordinary fluorescent
lamps, cell phones, wireless routers, microwave ovens, ... It's a literal
RF sewer over here!
-Chuck Harris
Bob Camp wrote:
Hi
If you are putting out> 20 watts and I'm close enough to see your mailbox, I'll detect you with some pretty simple
stuff.
Bob
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Hi Chuck,
Serious contesters have directional antennas and most of the new contest quality rigs have FFT spectrum displays and the ability to record several Mhz of spectrum directly to disk for later analysis.
The old stereotype of unsophsticated home brewed gear is now a subculture of the Ham community.
These are the guys who will hear you and FIND you esp since most of these guys have north of 50k invested in their stations and anything which interferes with getting that last elusive multiplier will be tracked to the end of the earth.
and some of them like me are also time-nuts.
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Chuck Harris cfharris@erols.com
Sender: time-nuts-bounces@febo.com
Date: Fri, 25 Feb 2011 13:16:13
To: Discussion of precise time and frequency measurementtime-nuts@febo.com
Reply-To: Discussion of precise time and frequency measurement
time-nuts@febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS
I guess that is why I mentioned something about doing it competently.
The FCC so seriously winged the methods usable by hams as to render them
effectively useless.
A nice direct sequence spread spectrum system with a couple of MHz
spread would be well below the background noise of any narrow band
receiver. Sure, you could find it with a wide band detector if you were
close by, but how would you know that you weren't looking at some other
anomaly, like a bad insulator, or trash coming off of fluorescent lamps?
Done correctly, you could run spread spectrum just about anywhere you wanted
to, and remain undetected. Using direct sequence, you would be so low in
power density that it could easily be argued that you were operating within
the constraints of a part 15 device's leakage.
-Chuck Harris
Bob Camp wrote:
Hi
Remember that this started out running a sequence that was 127 hops long.
With something that short, it's pretty likely you will be rude to somebody.
Even if you are running a massive hop rate, I can likely walk around and
track you down within the average neighborhood. A diode detector behind a
bandpass filter and a small-ish directional antenna is about all I'd try to
use.
I suspect it would also work with one of the power detector chips. Range
wise, a lot would depend on just how good your local cable company is at
keeping their stuff running right. I'm not really sure the chip would add a
lot of range in a normal setting.
Bob
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi Chuck
I'd see you on my waterfall display. A flexradio is a wonderful thing. The Icom 7x00'es would also see the energy and display it.
73 - Scott N1JIN
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Chuck Harris cfharris@erols.com
Sender: time-nuts-bounces@febo.com
Date: Fri, 25 Feb 2011 10:31:32
To: Discussion of precise time and frequency measurementtime-nuts@febo.com
Reply-To: Discussion of precise time and frequency measurement
time-nuts@febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS
Hi Scott,
Speaking as a ham, I can tell you that no one will even notice that
you are there. Frequency hop SS signals, when done correctly, only
land for a few milliseconds at seemingly random frequencies within
their band. Because their PN sequence makes them appear random, there
aren't any identifiable rhythms to their signals that would signal their
presence... just a slight increase in background noise. They really
aren't noticeable with the usual narrow bandwidth tuned rice boxes
that most hams use.
I have played extensively with frequency hoppers, like the PRC117, and
absent a wideband receiver, that covers their entire band, you would
never know they are there.
-Chuck Harris
scmcgrath@gmail.com wrote:
Speaking as a ham, if this is tried you will have hams complaining and DF'ing the offending signal.
Chuck, you are correct in that no one but the hams care but the reason behind this is that policing the ham bands has
been delegated to the hams and specifically certified 'Official Observers'. Once a infraction and especially
a.unlicensed infraction is noted penalties are pretty severe and swift.
If you want to do this get a 'experimental license' Sent from my Verizon Wireless BlackBerry
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Spread-spectrum and similar schmes were a little kind of secret and
secure communication around 1970 where the mathematics where done in the
years before, beginning with the classic Shannon paper about information
theory. Many papers were classified to help protect the knowledge their.
Especially for the military it was interesting because the average
knowledge level was low for this and the machinery to crack the code on
air was way more expensive the average bad 'terroris't would effort.
As it became common to have FFT-based receiver equipment 'for cheap'
this doctrine wen't useless. Useless the same as thinking atomic bombs
will secure freedom.
If one looks in the spectrum in a very fine granular structure the data
transferred by SS will be seen on every single spread-code bit!! All
needed is a high-enough S/N and a lot of computing power. On the analog
side the receiver must be very strong signal capable so other
'intruders' will not be mixed-in in the wanted signal.
A FFT ist nothing more than an integration on every small frequency step.
- Henry
scmcgrath@gmail.com schrieb:
Hi Chuck,
Serious contesters have directional antennas and most of the new contest quality rigs have FFT spectrum displays and the ability to record several Mhz of spectrum directly to disk for later analysis.
The old stereotype of unsophsticated home brewed gear is now a subculture of the Ham community.
These are the guys who will hear you and FIND you esp since most of these guys have north of 50k invested in their stations and anything which interferes with getting that last elusive multiplier will be tracked to the end of the earth.
and some of them like me are also time-nuts.
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Chuck Harris cfharris@erols.com
Sender: time-nuts-bounces@febo.com
Date: Fri, 25 Feb 2011 13:16:13
To: Discussion of precise time and frequency measurementtime-nuts@febo.com
Reply-To: Discussion of precise time and frequency measurement
time-nuts@febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS
I guess that is why I mentioned something about doing it competently.
The FCC so seriously winged the methods usable by hams as to render them
effectively useless.
A nice direct sequence spread spectrum system with a couple of MHz
spread would be well below the background noise of any narrow band
receiver. Sure, you could find it with a wide band detector if you were
close by, but how would you know that you weren't looking at some other
anomaly, like a bad insulator, or trash coming off of fluorescent lamps?
Done correctly, you could run spread spectrum just about anywhere you wanted
to, and remain undetected. Using direct sequence, you would be so low in
power density that it could easily be argued that you were operating within
the constraints of a part 15 device's leakage.
--
ehydra.dyndns.info
On 2/26/11 4:58 AM, scmcgrath@gmail.com wrote:
Hi Chuck
I'd see you on my waterfall display. A flexradio is a wonderful thing. The Icom 7x00'es would also see the energy and display it.
Maybe, maybe not.
Depends on what other activity there is in the band and what the noise
properties are. In the land mobile VHF bands, there's a lot more
ongoing activity, and your hops blend in with everything else. On HF,
since you have noise bursts from thunderstorms, etc.
Your flex only sees, at most, 192 kHz of BW. If you're hopping over a 4
MHz band, only 5% of the hops will be in your detection bandwidth. If
the band were quiet, those might be noticeable. Of course, you'd also
have to be looking when the hopper is transmitting.
then, you have to look at the resolution of your FFTs.. if you're
running 512 or 1024 sample buffers, you do a transform every 5-10
milliseconds, so a 10 Hz hopper would be fairly easy to see, depending
on what the modulation looks like.
And this is why nobody who does this "for real" uses 10 Hz hop rates and
4 MHz hop bands. Run at 1000 hops/sec or 10,000 hops a sec and the
"detection game" becomes quite a bit more challenging. Back in
"retrofit land mobile radio" days, 10 hops/sec was easy, and quite a few
commercial hoppers (e.g. the Racal Jaguar V) hopped in the 100 hop/sec
range, because they had to wait for the PLL to settle. Today, with a
DDS, phase continuous hopping at 10,000 hops/sec would be trivial.
And with GPS to give you a good sync, a decent crypto algorithm to turn
time of day into sequence seed, it would be easy to build something that
is quite literally undetectable.
Let's for sake of argument, assume we're sending digitized voice at 16
kbps. The strategy will be to use FSK, with one bit per hop, and the
two FSK tones being chosen randomly for each hop.
16khop/sec is 62.5 microsecond hops. Taking your flex example...Let's
say 1000 bins in the FFT at 192kHz sample rate, so your bins are 192 Hz
wide, and you do a transform every 10 milliseconds. There are 160 hops
during that 10 ms. Only 20% of the hops are in the receiver bandwidth,
so 8 of the bins will see some power, and it will be randomly
distributed across your band. (it looks like snow on analog TV). Don't
forget that the signal's only in a given bin for 62.5 microseconds out
of the millisecond, so it's down by 12 dB.. If the signal were 30dB
above the noise floor, unhopped, then you'd see it 18 dB above the
noise. You can't average transforms to smooth out the noise, of course,
so that's above the grass which is probably 10dB high.
If the transmission pulses on and off (like a typical QSO), you might
notice the snow coming and going.
But this all presumes that you're actually looking for it. If you were,
say, on 144.200 trying to make contacts, and the ham boneheadedly didn't
exclude that from their hop list.. I think we calculated 800 channels
before, so at 16 khop/sec, you hit a given channel, on average, at 100
times/second.
That would probably be hard to distinguish from just increased
background noise. Yes, if you had a directional antenna, and you went
hunting for this source of noise, eventually you'd find it.
Such is the quandary for a spread spectrum designer looking for low
probability of detection. The game is to spread yourself wide enough
that a total power detector (radiometer) won't see you, because your
increment in total power over thermal noise is small enough that it's
not statistically detectable over the normal variations in noise power.
Standoff range is a problem.. if you're trying to communicate with
someone 1000km away, and the guy looking for you is 1km away, that's a
problem.
On 2/26/11 5:23 AM, ehydra wrote:
If one looks in the spectrum in a very fine granular structure the data
transferred by SS will be seen on every single spread-code bit!! All
needed is a high-enough S/N and a lot of computing power. On the analog
side the receiver must be very strong signal capable so other
'intruders' will not be mixed-in in the wanted signal.
For the types of system time-nuts might have, aside from the "total
power radiometer" (aka "crystal video") approach (which requires good
knowledge of the expected noise power, so you can see the increase)..
100 percent probability of intercept receivers (of which an FFT is one)
have a dynamic range challenge. either you have a narrow band, with good
dynamic range (slow ADC, lots of bits), and miss a lot of the energy, or
you have a wideband system, with less dynamic range (so the noise floor
comes up from quantization noise).
As Henry points out, if there's any other signals in the band you're
looking at, you need to run your ADC loading so that they aren't
clipped, making the guy you're looking for that much closer to the grass.
It's all about Time-Bandwidth product and detection statistics. You
drag out your copy of the Radar Handbook, etc.
If the transmitter happens to have some obvious vulnerabilities (a new
hop starts exactly as the previous hop ends, and the hop rate is
constant) you can leverage that.
But this is getting pretty far afield from time nuts..
I was intrigued to think about the ubiquity of time/frequency
distribution these days, as a way to more easily get sync. That was
always the challenge back in the 80s. Today, getting a 10 Mchip/s
system synced would be no problem with even a fairly grungy 50ns
uncertainty 1pps from GPS to get close, and FFTs to do parallel acquisition.
The folks doing this for real must be doing something far more exotic,
then.. one can only wonder.