Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)
GPS receivers have considerable coding gain, which makes it possible for
them to detect such low signals in the first place. The jamming signal will
not benefit from the coding gain because it does not have the right coding
and will be uncorrelated to the actual spread-spectrum signals the receiver
is tracking.
This is probably 30 dB or more.
Also, you assume that your jammer has a similar antenna gain and radiation
efficiency as the GPS transmitter on the satellite, that is unlikely.
Bottom line, it is unlikely that a jammer running off a 9V battery, even on
a baloon will jam the entire US for weeks.
Can we go back to timing now? My 9V battery is running out...
Didier
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of J. Forster
Sent: Sunday, November 15, 2009 6:51 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re:
Reference oscillator accuracy)
See my earlier post. Briefly:
Antennas do not have an infinite front-to-back ratio. (<40 dB)
The path loss from a surface jammer to a plane (10 miles) is
many, many dB less than from plane to bird (15,000 miles).
-John
============
GPS receivers have considerable coding gain, which makes it possible for
them to detect such low signals in the first place. The jamming signal
will
not benefit from the coding gain because it does not have the right coding
and will be uncorrelated to the actual spread-spectrum signals the
receiver
is tracking.
This is probably 30 dB or more.
Also, you assume that your jammer has a similar antenna gain and radiation
efficiency as the GPS transmitter on the satellite, that is unlikely.
I don't assume ANY gain in the jammer. 0 dB.
Bottom line, it is unlikely that a jammer running off a 9V battery, even
on a balloon will jam the entire US for weeks.
I never implied that. Nothing close. But a few hundred to tens of
thousands, operating intermittantly, is another story.
-John
=========
The one the counts ARE! ;)
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator
accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage,
and that its radiation pattern is upward, it seems that a ground jammer
will
have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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Maybe, but what happens if the GPS in a large metropolitan area that is
overflown by a mix including General Aviation goes wonky? LA comes to mind
because there is a lot of north-south traffic directly over LAX.
-John
===============
The one the counts ARE! ;)
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator
accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage,
and that its radiation pattern is upward, it seems that a ground jammer
will
have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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Magnus Danielson wrote:
Well, then you have to consider what the potential gain would be from
such an attack. I fail to see the upside of that particular scenario. It
would draw the attention over to them and in a field I think they rather
stay calm about. Their ability to pull it off as such should not be
doubted, but their resoning for it. Rather inefficient actually.
The Chinese seem to have a difficulty worrying about the ethics of some
of the things they do towards others. Some simple examples would be the
lead in paints and plastics used in toys, melamine powder in milk, and pet
food, pork flavored cardboard in frozen dumplings, tainted raw ingredients
for heprin, the list goes on and on.
It may not be in the nation's political interests to jam our GPS signals
through toys, or other devices, but ... from what I have seen thus far,
you could easily find several dozen manufacturers in China that would be
willing to turn a blind eye to such a trojan payload in return for a few
bucks.
-Chuck Harris
Not all GPSs are on airplanes, but those I am really worried about are.
I don't care if the GPS receiver in my cell phone stops working in some
locations, it does not work everywhere to begin with (like in the house, I
can't use it to find the bathroom in the dark...), but I worry that the one
guiding a missile or a fighter plane might not work. I also worry that the
one abord a large passenger aircraft might not work. Most of those that are
stationary or move on the ground or on water, I am not too worried about.
For instance, the local garbage pickup people use GPS to locate the houses
of customers who have unusual (large or unsafe, or possibly contaminating)
garbage to pickup. I have had a dead TV on the side of my house for 3 weeks,
three times I called and 3 times they missed it because the GPS guided them
to my front door (my normal mailing address), but I live on a corner lot and
the garbage pickup is on the side of the house, by the garage. If that
particular GPS receiver stopped working and forced the employees to look
around, it would not bother me (the TV ended up in the trash can. After 3
times and 3 weeks, I assumed I had done a reasonable attempt at avoiding
putting too much lead in the garbage, and it had to go.)
Didier
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re:
Reference oscillator accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage, and that its radiation pattern is upward, it seems that a
ground jammer will have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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Contrary to popular belief, us pilots do know how to fly without GPS. I
have never seen an IFR aircraft with a GPS that didn't also have a VOR
receiver. Any VFR aircraft can be navigated using the Mk I eyeball.
IFR certified GPSes have integrity monitoring. So, if the signal gets
jammed or there is another system failure, the approach should be aborted
and the flight switched to another navigation means.
This LORAN debate is all well and good, but most of the general aviation
fleet does not have LORAN receivers. They haven't been manufactured in
years.
On Sun, Nov 15, 2009 at 8:33 PM, Didier Juges didier@cox.net wrote:
Not all GPSs are on airplanes, but those I am really worried about are.
I don't care if the GPS receiver in my cell phone stops working in some
locations, it does not work everywhere to begin with (like in the house, I
can't use it to find the bathroom in the dark...), but I worry that the one
guiding a missile or a fighter plane might not work. I also worry that the
one abord a large passenger aircraft might not work. Most of those that are
stationary or move on the ground or on water, I am not too worried about.
For instance, the local garbage pickup people use GPS to locate the houses
of customers who have unusual (large or unsafe, or possibly contaminating)
garbage to pickup. I have had a dead TV on the side of my house for 3
weeks,
three times I called and 3 times they missed it because the GPS guided them
to my front door (my normal mailing address), but I live on a corner lot
and
the garbage pickup is on the side of the house, by the garage. If that
particular GPS receiver stopped working and forced the employees to look
around, it would not bother me (the TV ended up in the trash can. After 3
times and 3 weeks, I assumed I had done a reasonable attempt at avoiding
putting too much lead in the garbage, and it had to go.)
Didier
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re:
Reference oscillator accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage, and that its radiation pattern is upward, it seems that a
ground jammer will have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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A much more effective and cheap strategy is a repeater jammer.. Receive
the signal and retransmit it: two antennas and an amplifier. The victim sees
the delayed retransmitted signal at a higher level than the direct one. It's
sort of like creating fake multipath interference. No need for PN
generators, oscillators, etc.
Granted, a smart receiver that understands the relationship between SV to
user geometry and doppler can beat it (because the carrier phase and PN
phase of the repeated signal won't be consistent with the geometry), but the
run of the mill PN tracking loop probably won't.
Most inexpensive receivers use a single bit sampler, so a suitable CW tone
could also probably jam it effectively, but might require some knowledge of
the victim receiver to pick an appropriate frequency (i.e. You'd need to
know the sampling rate.)
On 11/15/09 4:39 PM, "Magnus Danielson" magnus@rubidium.dyndns.org wrote:
Chuck Harris wrote:
Magnus Danielson wrote:
Chuck,
Chuck Harris wrote:
What makes you think it needs to be CW, and cannot be pulsed and
chirped?
May I roll in a noise jammer into the debate?
Absolutely! They can be extremely power efficient. Raise the noise
floor in the vicinity of the receiver, and it is all done.
Probably the easiest solution would be to take a PN source and use it
to drive a pulser that pulses a chirp oscillator. If you are feeling
really polite, you could put a bandpass filter on the thing to protect
other services.
The schematic out there is a PN source feeding an OCXO and then
amplified. Crystal loop to keep fairly centered. More or less the same
as personalized phone jammers do. Very simple design.
The aircrafts that really count should work without GPS!
The one the counts ARE! ;)
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator
accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage,
and that its radiation pattern is upward, it seems that a ground jammer
will
have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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GPS antennae are mounted top and bottom on tactical aircraft.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Francesco Ledda
Sent: 16 November 2009 00:37
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator
accuracy)
Considering that the GPS antenna in aircrafts is mounted on top of fuselage,
and that its radiation pattern is upward, it seems that a ground jammer will
have an uphill battle.
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of J. Forster
Sent: Sunday, November 15, 2009 6:28 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re: Reference
oscillator accuracy)
Or even into MP3 players, iPods, laptops, or cell phones. Then they'd
wander all over the place too. With the latter two hosts, they could even
be controlled remotely and even be fairly powerful. Would you notice
having to recharge your battery a bit more often?
-John
===========
Magnus Danielson wrote:
Chuck,
Chuck Harris wrote:
What makes you think it needs to be CW, and cannot be pulsed and
chirped?
May I roll in a noise jammer into the debate?
Absolutely! They can be extremely power efficient. Raise the noise
floor in the vicinity of the receiver, and it is all done.
Probably the easiest solution would be to take a PN source and use it
to drive a pulser that pulses a chirp oscillator. If you are feeling
really polite, you could put a bandpass filter on the thing to protect
other services.
All it has to do is confuse the receiver enough so that you can't
trust its readings.
Depends on the goal. For some strategies, blackout is the goal, for some
getting the readings go haywire every once in a while suffice.
Agreed!
My 9V battery suggestion was for a localized blackout device. You only
have to make the receiver question each satellite's signal often enough
for it to rule it out. No way is CW necessary, or even desirable.
As John suggested, someone (say the Chinese) could put these things in
battery operated stuffed animals, and set them up to jam a little bit
now and then. After Xmas, the GPS landscape would be littered with these
little stealth jammers, and willing supplicants to replace their
batteries.
-Chuck
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