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Why was such a low power signal chosen for GPS?

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Why was such a low power signal chosen for GPS?

Old 18th Apr 2012, 23:39
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Why was such a low power signal chosen for GPS?

Hi,
I am a pilot instructor for FlightSafety International Teterboro. I cover GPS in my lectures. Why was such a low powered signal strength chosen for the operation of the system? This low signal power makes the probability of interference and jamming more likely. Was there a strategic reason for the choice of such a low powered signal? Why was no warning of jamming or interference activity not required to be built into GPS receeivers?
Thanks!
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Old 19th Apr 2012, 03:18
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Power requirements on the satellites?
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Old 19th Apr 2012, 09:45
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Yes. Solar power only - with the technology available when the system was conceived, it was the only option. Galileo has a higher transmission power, but I don't know by how much.

As for your second question, you are probably aware that GPS have three frequencies, X, Y and Z. Civilain units receive the X frequencies, military the Y. (Z is used for time co-ordiantion). Y is much less prone to spoofing as it is encripted and runs independently of X, so X can be tuned off or downgraded) to deny enemies of the USA the signals. However, all can be jammed as they are low powered. Commercial jammers can be bought on line for as little as US$20.

Also, there is an account (unsubstatiated) of an attempt to disrupt the system by retransmitting the time signal slightly later so defraud fiancial institutions who use GPS to maintein the 5 second window on currency trading transactions.

The vulnerability of Satnav systems to such attacks has prompted the renaissance of LORAN. New enhanged LORAN known as E LORAN has a much stronger signal and is less susceptable to jamming. Combined E LORAN/GNSS sets are available now, but I don't think certified for aviation use.
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Old 19th Apr 2012, 09:59
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At the heart of the problem is that a GPS signal that does not use the coded method would need a lot of power to overcome the normal background interference that would occur over such a long distance. This would require significant transmitter power and hence electrical generating capacity on each satelite. It also minimises the interference with other electronic transmissions.

A more detailed discussion of signal to noise ratio can be found here http://202.114.120.10/gnss/docs/AN101.pdf

Jamming and spoofing is possible of course but the transmitter power required to prevent this is truely phenominal and the electricity generation on board the satelite would need to be very impressive.
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Old 19th Apr 2012, 10:38
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As others have already pointed out you don't have much power available on a satellite. Unless you use something based on nuclear energy your only alternative is solar power (you don't want to shoot up e.g. enough oxygen and hydrogen to last for at least 10 years). And for solar power you need large solar panels for a few watts of power (if I remember correctly you get about 150 W of light per square meter on average, and the solar cells have rather likely an efficiency of not much more than 10 to 20%). Larger solar panels mean more mass and thus more cost for getting them into orbit. That's the one point. The other is that the power received from the satellite drops with the square of the distance. And those satellites are in an orbit roughly 20,000 km above the earths surface. So the signal received from a satellite directly above you is about 400 million times weaker than that from a source with the same output power but ony 1 km away. With that ratio you can't beat jammers even if you were to raise the satellite's output power by a factor of 10, 100 or 1000. I've read different numbers for the output power of a GPS satellite, varying between 50 and 500 W. So in order to beat a jammer with say just 1 W output power in 1 km distance you would have to raise the satellites output power by a factor of at least 1 million to about 500 MW - for that you would need a complete (nuclear) power plant.
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Old 20th Apr 2012, 03:45
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Why was no warning of jamming or interference activity not required to be built into GPS receeivers?
IFR certifcated aviation receivers have integrity monitoring as part of their specification. What is probably more remarkable is how little interference and/or jamming actually occurs worldwide - there are "hotspots" - but overall the system is pretty robust.
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Old 20th Apr 2012, 12:04
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Considering their importance, the GPS satellite signals are improbably weak: someone compared them to detecting a Christmas tree light bulb in New York, when viewed from Los Angeles. The reason is that in the 1970s USAF developers had classified technology that could hide them from adversary detection deep in the atmospheric “noise.”

One UK report stated that over a six-month period in 2011, twenty dedicated GPS signal monitors spread across the country had recorded between 50 and 450 deliberate interference events every day. Of these, almost all were attributed to small, low-powered devices selling for around $50 on the Internet since, as in the U.S., GPS jammers are illegal in the UK. Nevertheless, several thousand are reportedly in use in Britain and more than 100,000 in the U.S., their main purpose being to defeat GPS tracking systems installed in trucks by their company owners.

Last year the ground-based augmentation system (GBAS) at Newark, suffered random and unpredictable shutdowns that were eventually found to be caused by jammers in trucks travelling along the nearby New Jersey Turnpike. The only cure for the problem was to move the four GBAS antennas farther infield to a point out of range of the jammers.

Rise of GPS Interference Raises Concerns for NextGen | Aviation International News
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Old 20th Apr 2012, 14:08
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Nevertheless, several thousand are reportedly in use in Britain and more than 100,000 in the U.S., their main purpose being to defeat GPS tracking systems installed in trucks by their company owners.
Being active devices, these things must give themselves away to the right detection equipment.

In the US, the FCC should set up along highways and reel in the drivers who think they are being cute by using jammers to defeat their company's tracking devices. The better the enforcement and the higher the fines, the better the dis-incentive for using GPS jammers.
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Old 20th Apr 2012, 14:40
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Quote:
Nevertheless, several thousand are reportedly in use in Britain and more than 100,000 in the U.S., their main purpose being to defeat GPS tracking systems installed in trucks by their company owners.
Being active devices, these things must give themselves away to the right detection equipment
.

In the US, the FCC should set up along highways and reel in the drivers who think they are being cute by using jammers to defeat their company's tracking devices. The better the enforcement and the higher the fines, the better the dis-incentive for using GPS jammers.
Would also seem that the truck owners would investigate GPS tracking records that showed a suspicioulsy high amount of "no data" time and fire those caught with the (illegal - a firing offence on it own) devices.

That said I don't know how the truckers use the jammers, would be harder to detect if they were used for short burst of time to mask speeding or un-approved shortcuts etc.
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Old 20th Apr 2012, 20:19
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Many interesting comments in regards to jamming. Years ago, cell phone jammers were legal, and were in many places such as concert halls, Courtrooms, and other facilities. In the US, the FCC ruled that jamming any legal broadcast was illegal, so the jammers were then removed.
The business continued with people wanting their whereabouts to remain untracked, such as the truckers (who didnt want their companies knowing they were picking up and delivering loads on their own) The jammers have a very limited range, and are only needed to jam the GPS on the vehicle, and perhaps the cell phone, with most having a range of only 50 meters.

With GBAS, the system is, by definition a ground based augmentation system. In the aircraft, the GPS system Reference Receiver, (RR) is receiving the sat data, (the Kalman Filter has some value in reducing interference) comparing the ellipsoid data from the individual satellite and the timing signal, and constantly applying correction factors.
The GBAS system on the ground computes an average of the corrections from the multiple RR's and broadcasts this average correction to the user. This improves the accuracy of the correction over that of a single RR.
The GBAS system is sending the aircraft a hyper accurate correction factor, along with other approach data, not re-broadcasting the GPS signal. Hence the GBAS system requires constant feed from Sats, and cannot act as its own Sat.
At Newark, the freeways run all along the runways, on the runway ends, and are very close, hence the jamming signals caused enough short term disruption to the antennae, to affect the integrity of the system.

The US, as a result of the issues at Newark, has a major enforcement campaign, beginning with the sellers of the equipment. The jammers are very difficult to track, with a very limited range, but with the limited range, the effect should be very minimal at best.
Unfortunately, a company called LightSquared, now owns the frequencies right up to the GPS bands, (sold to them by the US FCC) and was slated to begin 4G cell phone broadcast at those frequencies (at a million times the power) GPS manufacturers showed complete loss of GPS signal up to 5 miles from a cell antennae...
think of how many cell towers there are around an airport, and that part where they make you turn off devices before approach or departure, may become very real.
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