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Subject: How To Defeat GPS Jammers
SYSOP    5/29/2009 5:21:02 AM
 
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Whispering Death       5/29/2009 7:16:19 AM

I don't understand the whole GPS jamming and unencrypted thing.  Could someone explain why the system was made unencrypted in the first place?  It seems odd to have a system in place which would allow our enemies to use to hit us more accurately.  The second question is if it's unencrypted and anyone can buy a cell phone with the technology it stands to reason the wavelength that it operates on is well known.  Why couldn't an enemy just jam the frequency GPS transmits on around targets that GPS guided weapons would want to hit?
 
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Mike S       5/29/2009 3:06:51 PM
Presidnet Clinton decrypted the super accurate signal back in the 90s.  It turned out to be no big deal, companies had already designed antennaes to make the less precise signal more accurate (100m to 1m).  A jammer prevents the signal, encrypted or unencrypted, from getting to your receiver.  If the enemy used a big powerful jammer, we could locate it and blow it up.  If the used 100 low power jammers spread all over the place, then we might have a problem.  neither thwe nor the enemy has a monopoly on brains.  It is spy vs. spy, always thinking of a counter to the counter to the counter....
 
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ArtyEngineer       5/29/2009 3:23:34 PM

Presidnet Clinton decrypted the super accurate signal back in the 90s.  It turned out to be no big deal, companies had already designed antennaes to make the less precise signal more accurate (100m to 1m).  A jammer prevents the signal, encrypted or unencrypted, from getting to your receiver.  If the enemy used a big powerful jammer, we could locate it and blow it up.  If the used 100 low power jammers spread all over the place, then we might have a problem.  neither thwe nor the enemy has a monopoly on brains.  It is spy vs. spy, always thinking of a counter to the counter to the counter....

This needs a bit of correcting!!!!  What President Clinton did was to reduce the randomly fluctuating "Bias Error" inherent to the unencrypted civilian accessible signal to "Zero".  Making receivers using these signals "almost" as accurate as military receivers using the "Precise Positioning Signal" which was and still is "Encrypted".  As to why the military signal is 'Harder" to jam......well I cant truely explain that on an open forum.  But very simply its a function of how the information is 'Packaged" and placed on the carrier waveform.
But as other have pointed out on other threads dealing with jamming.  Jammers of any description have a life expectance measured in minutes if not less on todays battlefield!!!!!
 
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arodrig6       5/29/2009 6:05:35 PM


I don't understand the whole GPS jamming and unencrypted thing.  Could someone explain why the system was made unencrypted in the first place?



As to the 'why' - accurate GPS has been a huge boon to the Civilian economy. GPS navigation systems for cars and pedestrians are handy, and GPS is used in surveying, tectonics, and commercial transportation.  The GPS reciever market is about $30 billion in size and growing.
 
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jwilly48519       5/30/2009 4:24:01 PM
For the US, jamming is just one approach toward dealing with possible enemy use of GPS guided weapons. Another approach would be spoofing, especially on an agile/dynamic-battlespace-aware/automatic-responding context.
 
In a jammed-signal situation, most of our ordnance will fall back to inertial guidance. Presumably this will be true for future enemy ordnance that may be used against US forces as well. Thus jamming may not prevent such ordnance from being at least partially effective.
 
A better approach might be to maintain dynamic awareness of ordnance "in the air", and have electronic assets in place that can override, in a signal strength sense, the space-originated GPS signals with spoofed signals that are geographically synchronous when turned on, but then progressively shift the geographical reference frame at a rate that the ordnance cannot detect as spoofing-shift and instead perceives as just a need for flight correction, so that the effective aim point is now somewhere other than the originally intended target.
 
If the spoofing system is run in real time by someone with exact knowledge of the battlefield, a spoofing approach might even be used to re-target enemy ordnance on locations where it is harmful to the enemy.
 
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Herald12345    TMDI   5/30/2009 4:43:18 PM

For the US, jamming is just one approach toward dealing with possible enemy use of GPS guided weapons. Another approach would be spoofing, especially on an agile/dynamic-battlespace-aware/automatic-responding context.

 

In a jammed-signal situation, most of our ordnance will fall back to inertial guidance. Presumably this will be true for future enemy ordnance that may be used against US forces as well. Thus jamming may not prevent such ordnance from being at least partially effective.

 

A better approach might be to maintain dynamic awareness of ordnance "in the air", and have electronic assets in place that can override, in a signal strength sense, the space-originated GPS signals with spoofed signals that are geographically synchronous when turned on, but then progressively shift the geographical reference frame at a rate that the ordnance cannot detect as spoofing-shift and instead perceives as just a need for flight correction, so that the effective aim point is now somewhere other than the originally intended target.

 

If the spoofing system is run in real time by someone with exact knowledge of the battlefield, a spoofing approach might even be used to re-target enemy ordnance on locations where it is harmful to the enemy.


Herald
 
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warpig       5/30/2009 7:24:41 PM
MIJI-ing the GPS receiver by slowly building up a positional error like that is certainly a possibility when the enemy is using GPS for navagation, where the error can build over many minutes or even hours (depending on whether the enemy receiver is in the air, on the sea, or on the land).  I can't say it's not useful against weapon system guidance, but often in that case it is only a matter of just a couple minutes at most of exposure to being meaconed off course.  I think maybe introducing enough error sufficient to draw that quickly a 250kg (much less a 500kg or even 1500kg) bomb far enough off-course to save the intended target would be voted out as being a guidance failure, and the INS would take over?
 
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trenchsol       5/31/2009 11:54:24 AM
Couldn't GPS data be relayed over some other type of signal that is harder to jam ? As a backup, I mean. An aircraft could be used as relay. So, it would still be a GPS guidance in the end. Supposed aircraft would be much closer than a satellite, so there should be more  options available  to transmit  the data jam free.
 
DG
 
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HIPAR       6/1/2009 10:22:45 AM
Pseudolite -- This can be done to make the positioning more robust in a jamming or other difficult reception environment.
 
---  CHAS
 
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HIPAR       6/1/2009 10:47:50 AM
Many studies have been commissioned for development of adapted beam antennas.  One concept is to detect the direction of the jamming signal and cause the antenna to form a null to reject it.
 
The nature of the GPS military precision signal being sent with more spreading bits (chips) determining its power spectrum provides an effective power gain over the civil signal when the receiver locks on to it.  Also, the GPSII R(M) series transmit with somewhat more power than vintage satellites.
 
I've always thought a JDAM bomb with its antenna looking skyward will reject most of the signal power from a ground based jammer.
 
---  CHAS
 
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