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llondel
13th Nov 2007, 20:43
I don't know how hardened aviation GPS is, but the OfCom website has an interesting piece on loss of service, presumably only in the UK.

From http://www.ofcom.org.uk/static/subscribe/select_list.htm

GPS Jamming
The Ministry of Defence conduct occasional tests on military systems which may result in some loss of service to civilian users of the Global Positioning System (GPS) including in-car navigation devices and networks which rely on GPS signals. Ofcom seeks to provide citizens and consumers with information about possible interruptions to these services via these email updates giving advanced notification of these tests. It must be emphasised that this notification process only warns of future jamming excercises that are brought to the notice of Ofcom and may not cover all jamming exercises. It cannot be assumed that any loss of service is due to jamming exercises.

Shore Guy
13th Nov 2007, 21:00
Solar flare activity can disrupt GPS signals also, as it did last December over a good part of the U.S.

"More seriously, magnetic field disruptions corrupt signals between Global Positioning System satellites and receivers, creating measurement inaccuracies. According to NOAA scientists, a solar flare last December caused large numbers of receivers to stop tracking the GPS signal altogether.
"A small measurement mistake probably isn't crucial to somebody with GPS in their car," said Bogdan. "They will still know generally where they are. They're not going to drive into a ditch."

"But such disruptions "can be crucial for industries and activities that require precise positioning, something within a few meters or less," he added. That includes "everybody from farmers to offshore oil drillers to soldiers on the battlefield."
From this link:

http://www.bendweekly.com/Science/10278.html


That is why there will certainly have to be a position source backup when ADS-B is fully deployed….perhaps the leading candidate for that is Loran E.

Sunfish
13th Nov 2007, 21:26
According to reports, GPS is relatively easy to jam which is why in Australia you can be fined $500,000 and spend some time in jail if you are silly enough to try it. Certified IFR Aviation GPS receivers have RAIM.

alosaurus
13th Nov 2007, 21:32
True Sunfish...lot of concern from the experts that home made GPS jamming is far too easy/commonplace. The solar flares are predicted by NASA et al well in advance and military jamming activity is advised by NOTAM...but all you guys who trust GPS without raw data back upwatch out for Johny radio ham.

llondel
13th Nov 2007, 21:46
I certainly wouldn't trust GPS alone, but then I'm paranoid enough to require belt, braces and duct tape if keeping the trousers up is a safety issue. An aircraft is probably relatively immune to ground-based jamming if its antenna pattern only looks up and not down. As for radio hams jamming the signals, I'd say that was unlikely - you're looking more for a technical equivalent to the idiot who smashes up railway signalling gear or finds it funny to play with laser pointers under a flight path.

alosaurus
13th Nov 2007, 21:59
llondel...The fact is that whilst these jammers are "relatively" easy to make it could only be done by ,at least, a knowledgeable amateur (not a railway system vandal). "Johnny radio ham" was a figure of speech with no disrespect intended to our geeky couch potato friends:)

Granite Monolith
13th Nov 2007, 22:09
As for radio hams jamming the signals, I'd say that was unlikely - you're looking more for a technical equivalent to the idiot who smashes up railway signalling gear or finds it funny to play with laser pointers under a flight path


Indeed. For example, your bog standard radio ham won't have the kit, or money nor inclination to degrade that spectrum (which is generally proved here (http://www.iarums-r1.org/iarums/latest.pdf) )


However, the jammer is getting somewhere with this (http://www.nytimes.com/2007/11/04/technology/04jammer.html?ex=1351828800&en=e7b62041a51fdae5&ei=5090&partner=rssuserland&emc=rss)malarky. Although, that's maybe a good thing!:O

Solar Flare activity is currently nil - has been for months - so that's ruled out.

GPS jamming? It's a new one on me.

GPS freq's are between ( i think ) 1175-1575 Mhz...way high up the spectrum. Would take something sophisticated to drown out from there.

More than likely the Government bodies testing new comms development, which none of us will never really know about - then, out of the blue, you'll hear about another thwarted terrorist attack - c'est la vie nowadays.

alosaurus
14th Nov 2007, 07:01
Civil GPS operates 1574.42...the military use 1227.6.
Whilst I agree most would not have the inclination they are certainly well capable of producing cheap jammers which would disrupt our operations.
Full story here http://www.computerworld.com/securitytopics/security/story/0,10801,77702,00.html

HEATHROW DIRECTOR
14th Nov 2007, 07:37
<<but all you guys who trust GPS without raw data back upwatch out for Johny radio ham.>>

As a long-time licenced radio amateur I take exception to that and subsequent similar remarks.

Radio amateurs are responsible individuals, many of whom are at the forefront of radio technology. Radio amateurs can, and do, legally operate on frequencies well above those employed for GPS and I would be surprised if any of those experimenters caused problems.

It's certainly possible to obtain equipment which could causes problems because policing of the radio spectrum in thge UK is almost nil and there is little control over the sale of transmitting gear.

Temet_Nosce
14th Nov 2007, 09:22
Source (http://science.nasa.gov/headlines/y2007/13nov_africa.htm?list13577)


Strange Space Weather over Africa
11.13.2007


+ Play Audio (http://science.nasa.gov/headlines/y2007/images/africa/audio/story.m3u) | + Download Audio (http://science.nasa.gov/headlines/y2007/images/africa/audio/story.mp3) | + Email to a friend (http://science.nasa.gov/programs/mail/sendfriend.asp) | + Join mailing list (http://science.nasa.gov/headlines/y2006/news)
Nov. 13, 2007: Something strange is happening in the atmosphere above Africa and researchers have converged on Addis Ababa, Ethiopia, to discuss the phenomenon. The Africa Space Weather Workshop kicked off Nov. 12th with nearly 100 scientists and students in attendance.
The strange phenomenon that brings all these people together is the ion plume—"a newly discovered form of space weather," says University of Colorado atmospheric scientist and Workshop co-organizer Tim Fuller-Rowell.
Researchers liken the plumes to smoke billowing out of a factory smokestack—except instead of ordinary ash and dust, ion plumes are made of electrified gas floating so high above ground they come in contact with space itself. "The plumes appear during geomagnetic storms and they can interfere with satellite transmissions, airline navigation and radio communications," says Fuller-Rowell. Indeed, it is their effect on GPS signals that led to the discovery of plumes over North America just a few years ago.
A typical example is the plume of Nov. 20, 2003:
http://science.nasa.gov/headlines/y2007/images/africa/plume_strip.gif (http://science.nasa.gov/headlines/y2007/images/africa/plume.gif)
Above: A plume of excess electron density over North America on Nov. 20, 2003. The plume was discovered and mapped by its effect on GPS signals. Credit: Courtesy of Anthea Coster and John Foster of MIT.
Two days before this map was made, an explosion on the sun had hurled a cloud of magnetized gas—a CME—toward Earth. The plume formed when the CME hit, triggering a strong geomagnetic storm. The plume consists of ionized air at high altitude moving from Florida to Canada at a speed of 1 km/s (2200 mph).
"Okay, now we've seen the 'smoke,' but where is the smokestack?" asks Fuller-Rowell.
The search is leading researchers to Africa.
"Many believe the source of the plumes is near Earth's magnetic equator," explains NASA heliophysicist Lika Guhathakurta who is attending the Workshop. "Africa is a great place to check this possibility because the magnetic equator passes directly over the sub-Sahara."
Just one problem: "There aren't enough sensors in Africa to study the phenomenon," says Fuller-Rowell. The sensor of choice is the dual-frequency GPS receiver. "North America has an abundance of dual frequency GPS receivers—thousands of them in a network we use to monitor North American plumes. But Africa has only a few dozen."
Below: Dual frequency GPS receivers now in Africa. More are needed to investigate the plume phenomenon.
http://science.nasa.gov/headlines/y2007/images/africa/GPSdeployment_med2.gif (http://science.nasa.gov/headlines/y2007/images/africa/GPSdeployment.gif)The purpose of the Workshop is to familiarize African space scientists with the plume phenomenon and lay the groundwork for a continent-wide GPS network. "Within a few years we hope to deploy hundreds of receivers," he says.
Ion plumes inhabit a layer of Earth's atmosphere called the "ionosphere." It is a broad region 85 km to 600 km above ground level where ultraviolet radiation from the sun knocks electrons off atoms and molecules, creating a layer of ionized gas or "plasma" surrounding our entire planet. As ham radio operators have known for more than 100 years, the ionosphere can bend, distort, reflect and even absorb radio waves. Plumes amplify these effects.
How important is Africa to the study of this phenomenon? "Consider the list of organizations who have joined forces to sponsor the Africa Space Weather Workshop: NASA, NOAA, the National Science Foundation, the European Office of Aerospace Research and Development (EOARD), the International Center for Theoretical Physics (ICTP), and many others," says Guhathakurta. "It's widely understood that Africa is key to the puzzle."
At the moment only North America has a well-mapped ionosphere. NOAA posts new images every 15 minutes at this website (http://www.swpc.noaa.gov/ustec). "Five years from now," says Fuller-Rowell, "we hope to be making realtime maps of the ionosphere over Africa, too."
Africa is plasma incognita—but not for long. Stay tuned!
The Africa Space Weather Workshop is organized under the auspices of the 2007 International Heliophysical Year (IHY), continuing the tradition of international research and cooperation begun during the International Geophysical Year (IGY) of 1957. To learn more about the IHY on the web, visit http://ihy2007.org/ .

radeng
14th Nov 2007, 14:54
From memory (and it's 15 years since I designed GPS receivers), the processing gain of GPS is such that the jamming signal needs to be at least 5000 times as strong as the wanted signal. That's not quite so easy.....

llondel
14th Nov 2007, 16:51
radeng:

It's not that hard to generate a jamming signal of adequate strength on the ground, the hard part is upsetting an aircraft which isn't as sensitive to GPS signals from below due to antenna position. I've also done some GPS stuff in the past, including using a neat simulator that would happily replay a synthesised signal set for checking receiver performance. It wouldn't be hard to increase the power output and presumably confuse receivers over a wider range..

I've heard it described that the power of a GPS signal at the receiver is equivalent to being able to see a candle in New York from London (if they were line-of-sight), although it's probably better up in the clear where you get a decent number of satellites in view.

alosaurus
14th Nov 2007, 19:55
HD - Public apology to you here..was trying to get a message across to the guys who blindly rely on GPS that they should not. There is a bit more to it than that (not for discussion on a public forum...please check your PM).

HEATHROW DIRECTOR
15th Nov 2007, 07:21
alosaurus - Thank you, but no personal problem at all. Nothing in my PM but will check again later...

radeng
15th Nov 2007, 17:05
llondel,
From memory, there's something like 43dB of processing gain in GPS. As you say, an upwards looking antenna on an aeroplane is not going to necessarily see so much from a ground based jammer. In the days when I was involved, there were some cunning ideas to mitigate jamming. Direct Sequence Spread Spectrum is best jammed (in terms of total power) by a single carrier, but reasonable cunning in the signal processing can get rid of that. There were some clever correlation techniques designed (but I haven't a clue what happened to them) that meant that jamming had to be with a very GPS like signal. But it's over 15 years now since I was involved with that, having moved on to other things. I seem to remember suggestions of it being possible to get a jam/signal ratio of 60 or 70dB before things fell over.

Astral_Flyer
17th Nov 2007, 15:01
Radio amateurs are responsible individuals, many of whom are at the forefront of radio technology. Radio amateurs can, and do, legally operate on frequencies well above those employed for GPS and I would be surprised if any of those experimenters caused problems.
Thank you... Couldn't agree more.

I read the article presented by Temet_Nosce. Although I can see what they are getting at as to the effect the Ionosphere has on radio signals (as every advanced radio amateur knows) It is virtually unheard of for signals above 800 Mhz to be affected by the Ionosphere to any great extent. Areas of intense ionisation (Sporadic E propagation or Meteor trails) does occur and that can affect frequencies below the one I mentioned.

Another mode that can affect higher frequencies up to 10 Ghz or so is Tropospheric propagation. That might be something that people are more familiar with, as it uses as part of its method of travel the boundary layers between warm and cold air, and radio signals bounce between the boundaries. No doubt many of you have flown through one before, as it is exactly the same type of thing as a temperature inversion with its very marked boundary.

As to GPS signals being affected by such things. Yes I feel that they can be, but only signals that are traveling at a very shallow angle. I'm sure that somewhere there is research and various safeguards in place to ignore the erroneous signals .

Astral (ancient G8***)