From: "Paul F Austin" <paustin@digital.net>
Newsgroups: sci.military.naval
Subject: Re: GPS terminal Guidance Question
Date: Sun, 25 Apr 1999 21:42:10 -0400

Thomas Schoene wrote in message <01be8f6f$9388a3e0$f5754e0c@default>...
>John Warner <jewarner@home.com> wrote in article
><60MU2.3156$Ig1.440292@news1.rdc1.on.wave.home.com>...
>> I have noticed in several posts comments on using GPS for terminal
>> guidance especially with a small projectile.  There was a (several?)
>> post earlier this year in the comp.risks about how easy it is proving
>> to be to disrupt GPS with relatively low powered (<15 watts) and cheap
>> transmitters.
>>
>> Has anyone heard of these problems?
>
>Heard of them, and determined that they are highly overblown.  The
>main reason is that the military GPS signal is designed specifically
>for jam resistance, so a simple noise jammer won't necessarily do the
>trick.  Then realize that the antennas used for GPS-guided weapons
>are very well-designed, with small sidelobes and good noise
>rejection.  They also point upward, making it rather difficult for a
>terrestrial jammer to get into the main lobe of the receiver.
>Finally, GPS-guided weapons almost always have inertial back-ups, so
>even if you do jam the GPS, terminal accuracy will still be very
>good.
>


Not exactly. GPS power levels at the recieving antenna are very low. Both
the civil and military codes are carried through the same TWTA and have the
same power level. The civil codes give degraded accuracy and the military
codes have some anti-jam (AJ) characteristics (but not much). It's less AJ
than anti-spoofing. It still takes very little power in the main lobe to jam
any GPS receiver.

As you point out, JDAM's GPS antenna points up. My company makes the AJ
antenna. It come with a steerable null that can be place on the jammer. The
integral inertial navigation system (INS) lets the bomb fly through outages
but I understand that a GPS reciever takes a few seconds to reacquire.
During the relatively long flight of a bomb or missile, a few unjammed
intervals of (say) 2 seconds each are enough to update the INS so that it
can fly through jamming with relatively little degradation in accuracy.

The Air Force has a program to increase the jam resistance of the GPS
constellation. One option is to substantially increase power levels. GPS jam
vulnerability is important for civil applications since an augmented GPS is
envisioned as the One Great Navigation System for civil aviation, from start
of takeoff roll through enroute navigation to touchdown.

"people of means-decent folk-should be given more votes
than drifters, whores, criminals, degenerates, atheists
and indecent folks-people without means."

Paul F Austin
paustin@digital.net