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March 25, 2003
Jammers Not So Insignificant
A recent post suggested that GPS jammers do not matter. The military has said that the jammers have not affected our campaign. As someone with RF Engineering experience I believe the second statement to be true but not the first. An obvious question: if the GPS jammers did not matter, why would the military use precious resources to bomb them? To answer this, one needs to understand some technical details. GPS satellites transmit very low power signals around 1.5 GHz. These signals can thus be overwhelmed with relatively weak signals. For example, a 4 watt jammer can block civilian GPS for many tens of miles. It appears that the jammers in Iraq were more powerful. One could easily imagine powers to 1000 Watts or more (radars often use peak power of 100,000 to 1,000,000 watts). This means the jammer may be putting out 10,000,000,000,000,000,000,000 (190 dB) times as much power as the valid signal received by the weapon or many times the power of the satellites that are at least 12,000 miles away! Now lets consider how the precision weapons used so far work. Two weapons systems are most prominent: JDAM equipped bombs, and TLAM cruise missiles. JDAM is a kit that converts an ordinary iron bomb to a guided bomb. It uses two guidance systems: GPS and inertial. The latter is quite accurate, but not as good as the GPS. From a jamming standpoint, the antenna of the JDAM is critical. When dropped from high altitude, the bomb has two advantages over the jammer: it is far away above it, and the antenna is pointed receive signals from above, "blocking" those from below - the jammer. As the bomb falls, it's GPS receiver may eventually be overwhelmed by the jammer. By this time, the bomb is much lower and its inertial guidance system (constantly updated from the GPS, presumably) can provide great accuracy for the rest of its fall. The cruise missile also has at least inertial and GPS guidance. Because of its greater size and cost, it can have a more sophisticated GPS antenna(it uses a steered antenna) and more precise inertial guidance On the other hand, the cruise missile typically flies at low altitude and thus may be in effective range of the jammer for a longer portion of its flight. Again, however, it will be highly accurate because of its inertial guidance system which is presumably updated accurately from the GPS. Thus both of these systems can work well, if not perfectly, in a jammed environment. However, there are other combat systems that are very susceptible to jamming. These are the GPS receivers used by ground troops and low level aircraft (such as Apaches) when used in the vicinity of the jammers. Today, GPS is used for navigation, deconfliction, indirect fires targetting and air support. I believe that the military destroyed some high powered jammers because they were protecting the vicinity of Baghdad and the positions of the Republican Guard around Baghdad, and thus threatening the operations of forces about to enter those areas. Technical notes: Definition of dB = 10*log10(power ratio) Comments
The raw EMF output is irrelevant. It is obvious that you don't know squat about EW. See Donald Sensing's blog. Gideon makes a most unuseful post telling me that I don't know squat about EW. Gideon, I suggest you attack specific points in my post rather than making broad insults ("...you don't know squat about EW..." - btw, in addition to doing RF engineering, I used to operate an EW station on a P-3, but that's not really relevant) and arguing by authority (I read Sensing's blog the other day, and his references). As far as "raw EMF output" - perhaps you meant "raw output power" or "raw ERP" or something, because "raw EMF output" is not a meaningful term in RF engineering. And it is true that raw power cannot be considered in a vacuum. Issues such as antenna gain (in the direction of the jammer and the direction of the satellite), processing gain (GPS is spread spectrum), and a zillion other factors impact jamming effectiveness. BI didn't set out in my main post to write a technical treatise, but rather something that is understandable by most readers. So, if you wish to do something other than spew insults, why don't *you* answer two questions: 1) Why did the US military bomb the jammers, especially at this phase of the campaign? 2) What specifically is wrong with my assertion that the jammers are likely to be harmful to ground forces? And by specifically, I mean a decent technical argument, not some handwaving or resort to authority. Posted by: John Moore (Useful Fools at March 26, 2003 01:34 AMWell, stone the (old) Crows. Just a suggestion, but let's stick to generalities shall we. It would be prudent IMHO. Posted by: Alan E Brain at March 26, 2003 02:57 AMDespite the first post, John makes a good point. I was going to write a simlilar post until I read his. The jamming is largely worthless against bombs. However, it hinders (or makes useless) GPS receivers at ground level. This is not useful to the US military as there are many, many uses for ground-based GPS. This is why the US destroyed the jammers. On the bright side, jammers are easily located as their signal is a beacon. If such bombs do not already exists, I am sure the military will develop weapons that home in on GPS jammers in the future. Posted by: Admiral Quixote at March 26, 2003 01:26 PMHARM missiles can be tuned to the appropriate frequency. I assume they can be set to home on noise or home on other jammer signals if the jammers are not using noise. It looks like in this case the transmitters were identified by DF aircraft and were then destroyed by precision guided munitions. Posted by: John Moore (Useful Fools at March 26, 2003 01:38 PMPost a comment
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