[aprssig] Terrestrial Threat to GPS Has Now Hit The Manstream Media

[Alex Carver]

> From: "Rahn Abbott" 
> 
> Ummmm, you do know that that US Military GPS units and your
> Garmin Etrex are
> not listening to the same radio signals right? You do know
> military and US
> government GPS runs on a completely different system? 


No, the military listens to both L1 at 1.57542 GHz (the "civilian" GPS signal) and L2 at 1.22760 GHz, the military-only GPS signal.  They use both signals in order to determine delays caused by the ionosphere which improves the accuracy of the military GPS receivers.  If interference is caused to the L1 signal it will affect both civilian and military users.

A military GPS receiver and a civilian GPS receiver both listen to L1 but they each use a different chipping code (civilian uses unencrypted C/A and military uses both unencrypted C/A and encrypted P).  Only the military GPS receiver listens to L2 at the same time.

Later on there will be new codes (L1C and M) as new satellites are put in orbit but they will still have both civilian and military codes sitting on L1.  In addition, civilians will soon have access to L2 in the form of a civilian code on L2 (L2C) which will allow for some basic ionospheric corrections to improve accuracy and reduce drift.  It won't be as accurate as the military receivers but better than the current L1-only receivers.