How To Defeat GPS Jammers

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Subject: How To Defeat GPS Jammers
SYSOP 5/29/2009 5:21:02 AM

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WarNerd 6/1/2009 6:04:03 PM

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

Yes, but all the sales demonstrations (which are what counts) take place on the ground.

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Question -- Which is better, a system that your enemy cannot possibly jam or one where he is encouraged to waste money on inherently ineffective jamming technologies?

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HIPAR 6/2/2009 10:03:00 AM

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

Yes, but all the sales demonstrations (which are what counts) take place on the ground.

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Question -- Which is better, a system that your enemy cannot possibly jam or one where he is encouraged to waste money on inherently ineffective jamming technologies?

I doubt GPS was fielded to spend an enemy into oblivion.

--- CHAS

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WarNerd 6/2/2009 8:54:35 PM

Question -- Which is better, a system that your enemy cannot possibly jam or one where he is encouraged to waste money on inherently ineffective jamming technologies?

I doubt GPS was fielded to spend an enemy into oblivion.

--- CHAS

I agree, but it looks like it is starting to have that effect. So I guess the question would be do we want to encourage it?"

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HIPAR 6/3/2009 12:41:43 PM

Any technical system, tactic or doctrine that 'encourages' an adversary to divert financial resources in response provides that secondary benefit.

--- CHAS

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forester 6/3/2009 1:55:04 PM

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.

GPS doesn't work that way. GPS determines your position by noting the time it takes for the time-coded signals to travel from the GPS satellites to your receiver. The longer the distance, the longer it takes for a signal to reach your receiver (we're talking very precise time measurements here because the signal is traveling close to the speed of light). By comparing the distances from multiple satellites, your position is triangulated. A relayed signal would just add greater time for you to recieve it, which in turn would throw off the math involved in triangulating your position.

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WarNerd correction 6/4/2009 3:18:05 AM

GPS doesn't work that way. GPS determines your position by noting the time it takes for the time-coded signals to travel from the GPS satellites to your receiver. The longer the distance, the longer it takes for a signal to reach your receiver (we're talking very precise time measurements here because the signal is traveling at ~~close to~~ the speed of light). By comparing the distances from multiple satellites, your position is triangulated. A relayed signal would just add greater time for you to recieve it, which in turn would throw off the math involved in triangulating your position.

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forester Corrected correction 6/8/2009 4:37:51 PM

No, I was correct the first time. ;) In a vacuum, the signal travels at the speed of light, but once it enters the atmosphere it is refracted slightly by water vapor, etc so that it is very slightly affected. This is why differential correction (either real time or post processing) improves the quality of the GPS fix; the base station provides the correction for atmospheric distortion.

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WarNerd 6/9/2009 7:12:06 AM

The velocity of light in any media is a function of the index of refraction. The velocity of light in dry air is greater than in damp air, but both are slower the velocity of light in a vaccuum. The velocity of light is also greater at high altitudes (lower pressure) than at ground level.

And before anyone asks, no there is not a substance where the speed of light is greater than in a vacuum.

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ArtyEngineer This has turned into a Physics lesson ;) 6/9/2009 7:57:43 PM

And before anyone asks, no there is not a substance where the speed of light is greater than in a vacuum.

Here is what I remember:

in any other medium the value of c changes as a function of the Permissitivity and Permeability of teh medium. (I might have those two words wrong but they are both perm something) I thinks its c = 1/(SQRT(Perma x Permiss))

The refractive index is a measure of how much the direction of propogation is changed by moving from one medium to another. given by (sine(angle of incidence))/(sine(angle of exidence)).

Thats all I can remember without googling a bunch of stuff and then coming back and pretending I actually remember it all perfectly ;)

Regards

Arty

(Once upon a time I seriously considered becoming a physics teacher!!! Then I realised I would have to deal with people like me and that I would not be allowed to beat teh crap out of them so I gave up on that idea)

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WarNerd 6/10/2009 4:20:31 AM

The speed of propogation of ANY electromagnetic wave (of which visible light is just one portion) is simply the product of frequency and wavelength and is represented as "c". Its one of the universal constants. which in a vacume is 3.0 x10^8 m/s. in any other medium the value of c changes as a function of the Permissitivity and Permeability of teh medium. (I might have those two words wrong but they are both perm something) I thinks its c = 1/(SQRT(Perma x Permiss))

The refractive index is a measure of how much the direction of propogation is changed by moving from one medium to another. given by (sine(angle of incidence))/(sine(angle of exidence)).

Thats all I can remember without googling a bunch of stuff and then coming back and pretending I actually remember it all perfectly ;)

Regards

Arty

I looked it up. The Refractive Index = SQRT(Perma x Permiss), so we are in complete agreement. It is just a difference in notation.

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