***SPAM*** How hard is it to detect a GPS Jammer?
It does no good to prohibit clients from using GPS on land because unless
you keep them blindfolded the entire time they will see and photograph
their surroundings. People with some training can find location to within
about 25 feet with no GPS even in a flat dessert. I've hiked out to find
a tent stake that was placed there by an instructor in open dessert. We
photo the stake then hike back. THose who don't know the tricks can
certainly find a location to within a 1/2 mile.
The only place this makes sense is on the ocean when you are not within
sight of land. There ARE some fishing boat captains who ask clients not to
use the GPS. But people cheat and turn on the GPS and leave it packed in
a bag and let it run until the battery dies. This way they are never seen
using it. This is rare because usually fish move and yesterday's good
spot is of no use. Many of the boats hire a spotter who works from an
airplane. But then "everyone" uses one of only two spotters who fly in the
area.
On Mon, Oct 7, 2013 at 10:37 PM, David I. Emery die@dieconsulting.comwrote:
On Mon, Oct 07, 2013 at 11:30:57PM -0500, Bill Hawkins wrote:
In general, we expect a jammer to be involved in criminal activity.
What about a wilderness guide whose reputation is built on finding
the best spots to view Nature's wonders. Should he or she be happy
to let people in the guided group save the coordinates of those
spots in order to compete with the guide or avoid guide's services
in the future?
Or would that be a justifiable use of jamming?
No.
Jamming can potentially impact a much wider area and safety of
life critical uses of GPS, and very few wilderness guides are expert
enough to understand, control and mitigate this risk. And even if one
particular one was, how is this to be reliably ensured ? Should he or
she be licensed and certified and regulated as to how and when he might
jam safely - we don't current issue GPS denial licenses (except maybe to
LightSquared) ?
And even with someone suitably trained and licensed and
certified there is a balance between the slight risk that his jamming
harm something important or critical that society relies upon and his
own personal interest in denying GPS to his customers that I am not sure
we as a society have quite figured out. Should private businesses be
allowed to deny GPS (or for that matter cellphone access) to folks
nearby or their customers - is this a legitimate interference with a
public resource ?
Obviously if the answer is yes, businesses should be allowed to
jam cellphones or GPS - then what are the limits - eventually all those
private jammers will make such services MUCH less useful - as they will
be unpredictably unreliable at apparently random times and places which
makes it very hard to trust them for anything important. We will in
other words have allowed the private interests of certain folks to
destroy a commons, something we are getting increasingly good at BTW.
I suppose it is SOMEWHAT justified for a guide to search his
customers for GPSes, or have a strict policy of dropping them off at the
nearest road if they are found to be using one, or even to use a GPS
detector to ID GPSes in use... but an active attack seems wrong given
the risks involved.
And anything more nuanced in a policy on such invites various
low life scumbags to push the limits and use blatantly excessive power,
dangerous antenna locations, and generally horrid engineering that puts
important uses and users of the technology at serious risk for very
selfish personal reasons - perhaps in some cases just buying some cheapo
jammer instead of a proper licensed and limited one managed and
installed by someone who knows what he is doing.
Finally of course, who is to say that I cannot use my cellphone
for important messages - perhaps life critical (my wife is a doctor and
does this from time to time when she is on call)... or my GPS to locate
a store somewhere... perhaps if the jamming ONLY worked in a completely
private space owned or controlled by the jammer and not at all outside
of it would this begin to be marginally acceptable but it certainly
isn't in public spaces. And I believe there should be mandatory readily
visible notices saying that jamming is use... allowing someone who
truly needs access to know what is happening.
As for the wilderness case specifically there are any number of
strategies to defeat short range jamming of that sort - just announce
you have to "go" as the group leaves the scenic knoll and go back and
take a bearing (maybe automatically from a concealed GPS you already
have in you backpack) while peeing on the special rock... Inverse
square law applies here of course... hard to radiate enough power to
deny over a long enough distance without being completely unsafe for
other users.
--
Dave Emery N1PRE/AE, die@dieconsulting.com DIE Consulting, Weston,
Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole -
in
celebration of what could have been, but wasn't and is not to be now
either."
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Chris Albertson
Redondo Beach, California
On 10/07/2013 01:36 PM, Bob Camp wrote:
Hi
Anything that will receive up there should be able to tell you when a jammer comes by. The issue is that not a lot of gear is made for that band (other than GPS receivers). The easy approach would be to use a modern GPS module that puts out noise level / jamming information.
Turns out they are not that good. AGC detection based works fairly well.
Cheers,
Magnus
Hi
Well finding a +10 dbm 1.5 GHz transmitter isn't very hard to do at all. I've got several of those….
Bob
On Oct 8, 2013, at 9:42 AM, Jim Lux jimlux@earthlink.net wrote:
On 10/8/13 4:17 AM, Bob Camp wrote:
Hi
But there's obviously something wrong with the 400 KM number.
- If +10 dbm is good enough to burry a useful signal at that distance, it should be good enough to communicate at that distance. That's pretty impressive QRP without high gain / directional antennas involved.
- The radios (at least the modern ones) do have CW signal immunity. Weather that's 60 db or something else probably varies with the make / model of the GPS. How well that works with a VCO jammer - again, a that depends sort of thing.
- There's a (maybe) 40 db variation in GPS signals. To deny service you need to take out the strong ones, not just the weak ones.
- Even without specific anti-jam in the GPS, the code it's self does have some immunity to a jammer.
Of course you don't have to look very far into the archives to find wonderful examples of slipped decimal points in my posts….
You're right, I forgot the process gain of the despreading.
Assuming you're going from 1 Mchip/sec for the C/A code to 50 bps for the nav message, that's 43 dB
That alone gets you to 2km jamming range from 400km, but that also assumes that the jamming signal doesn't inhibit acquiring the code and despreading.
As Dixon's book on Spread Spectrum says, "acquisition is the hard part"; because you don't have the process gain yet. I suppose with a parallel acquisition strategy, you're basically trying all codes, and that might be able to work.
But even so, those sorts of process gain arguments don't necessarily work if the receiver has a hard limiter or quantizer in it.
I don't know that modern consumer GPSes have CW immunity. If they're using a 1 or 2 bit quantizer, a strong CW signal pretty much captures the front end.
Easy to try. Let me just fire up my kilowatt 1.5 GHz transmitter here<grin>
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