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battlefield Internet (was: Stryker/C-130 Pics)
(I'm not an electronic engineer, so I've cross-posted this to some
newsgroups which might be able to give informed comment on a number of points.) On 23 Sep 2003 05:51:41 -0700, Kevin Brooks wrote: (phil hunt) wrote in message ... [regarding battlefield internet] The signal must be such that the extended receiver can hear it. So others can too, in principle. (Though detecting the signal and knowing where it's from aren't the same thing). I'm not a radio engineer but I can imagine a few ways how direction-finding might work; for example place two (or 3) detectors a few meters apart and calculate the time delay between each one receiving the signal. No. Paul is correct, DF'ing a "frequency agile" (or "hopping") transmitter is no easy task. For example, the standard US SINCGARS radio changes frequencies about one hundred times per *second*, Bear in mind that I'm talking about automated electronic gear here, not manual intervention. Electronics works in time spans a lot quicker than 10 ms. over a pretty wide band of freq's (this is why synchronization of the radios on a time basis is critical to succesful operation of the net). So the frequency changes are pre-determined on a time basis? If there is a radio receiver, is it better able to detect/deceive a signal whgen it knows the frequency in advance? Or can it "sniff" for lots of frequencies at a time and pick out what looks interesting? If two receivers, placed say 10 m aparet, both pick up a signal, how accurately can the time difference between the repetion of both signals be calculated? Light moves 30 cm in 1 ns, so if time differences can be calculated to an accuracy of 0.1 ns, then direction could be resolved to an accuracy of 3 cm/10 m ~= 3 mrad. Alternately, would something like a pinhole camera work? What I mean here is: imagine a cubic metal box, 1 m on its side, with a vertical slit, about 1 cm wide down one of its vertical faces. On the opposite face, there are detectors for detecting radio waves. If the elevctromatnetic ratiation coming into the box can only go in through the slit, and goes in a straight line, then knowing which detectors are lit up would allow someone to tell where the radiation was coming from. It may be that, depending on the wavelength, the incoming radiation would be diffracted by the slit and would get spread all over the detectors. If this is the case, perehaps multiple slits could be used, and the diffraction pattern would differ dependent on the angle with which the radiation strikes the slitted face? (because the radation at each slit would be out-of-phase with the radiation at other slits). Has anything like this been tried? It is hard enough for the average "rest of the world" intel unit to DF an old fashioned non-hopping transmitter if the radio operator uses good RTO procedures--trying to pluck enough of these random fractional-second bursts out of the ether to determine a direction is more difficult by a few orders of magnitude. What methods are used to do DF? -- "It's easier to find people online who openly support the KKK than people who openly support the RIAA" -- comment on Wikipedia |
(I'm not an electronic engineer, so I've cross-posted this to some
newsgroups which might be able to give informed comment on a number of points.) On 23 Sep 2003 05:51:41 -0700, Kevin Brooks wrote: (phil hunt) wrote in message ... [regarding battlefield internet] The signal must be such that the extended receiver can hear it. So others can too, in principle. (Though detecting the signal and knowing where it's from aren't the same thing). I'm not a radio engineer but I can imagine a few ways how direction-finding might work; for example place two (or 3) detectors a few meters apart and calculate the time delay between each one receiving the signal. No. Paul is correct, DF'ing a "frequency agile" (or "hopping") transmitter is no easy task. For example, the standard US SINCGARS radio changes frequencies about one hundred times per *second*, Bear in mind that I'm talking about automated electronic gear here, not manual intervention. Electronics works in time spans a lot quicker than 10 ms. over a pretty wide band of freq's (this is why synchronization of the radios on a time basis is critical to succesful operation of the net). So the frequency changes are pre-determined on a time basis? If there is a radio receiver, is it better able to detect/deceive a signal whgen it knows the frequency in advance? Or can it "sniff" for lots of frequencies at a time and pick out what looks interesting? If two receivers, placed say 10 m aparet, both pick up a signal, how accurately can the time difference between the repetion of both signals be calculated? Light moves 30 cm in 1 ns, so if time differences can be calculated to an accuracy of 0.1 ns, then direction could be resolved to an accuracy of 3 cm/10 m ~= 3 mrad. Alternately, would something like a pinhole camera work? What I mean here is: imagine a cubic metal box, 1 m on its side, with a vertical slit, about 1 cm wide down one of its vertical faces. On the opposite face, there are detectors for detecting radio waves. If the elevctromatnetic ratiation coming into the box can only go in through the slit, and goes in a straight line, then knowing which detectors are lit up would allow someone to tell where the radiation was coming from. It may be that, depending on the wavelength, the incoming radiation would be diffracted by the slit and would get spread all over the detectors. If this is the case, perehaps multiple slits could be used, and the diffraction pattern would differ dependent on the angle with which the radiation strikes the slitted face? (because the radation at each slit would be out-of-phase with the radiation at other slits). Has anything like this been tried? It is hard enough for the average "rest of the world" intel unit to DF an old fashioned non-hopping transmitter if the radio operator uses good RTO procedures--trying to pluck enough of these random fractional-second bursts out of the ether to determine a direction is more difficult by a few orders of magnitude. What methods are used to do DF? -- "It's easier to find people online who openly support the KKK than people who openly support the RIAA" -- comment on Wikipedia |
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On 23 Sep 2003 20:00:32 -0700, Kevin Brooks wrote:
No. Paul is correct, DF'ing a "frequency agile" (or "hopping") transmitter is no easy task. For example, the standard US SINCGARS radio changes frequencies about one hundred times per *second*, Bear in mind that I'm talking about automated electronic gear here, not manual intervention. Electronics works in time spans a lot quicker than 10 ms. So what? Unless you know the frequency hopping plan ahead of time (something that is rather closely guarded), you can't capture enough of the transmission to do you any good--they use a rather broad spectrum. OK, I now understand that DF generally relies on knowing the frequency in advance. BTW, when you say a rather broad spectrum, how broad? And divided into how many bands, roughly? Both radios have to be loaded with the same frequency hopping (FH) plan, and then they have to be synchronized by time. When SINGCARS first came out the time synch had to be done by having the net control station (NCS) perform periodic radio checks (each time your radio "talked" to the NCS, it resynchronized to the NCS time hack); failure to do this could result in the net "splitting", with some of your radios on one hack, and the rest on another, meaning the two could not talk to each other. I believe that the newer versions (known as SINCGARS EPLRS, for enhanced precision location system) may use GPS time data, ensuring that everyone is always on the same time scale. That would make sense. If two receivers, placed say 10 m aparet, both pick up a signal, how accurately can the time difference between the repetion of both signals be calculated? Light moves 30 cm in 1 ns, so if time differences can be calculated to an accuracy of 0.1 ns, then direction could be resolved to an accuracy of 3 cm/10 m ~= 3 mrad. The fact is that the direction finding (DF'ing) of frequency agile commo equipment is extremely difficult for the best of the world's intel folks, and darned near impossible for the rest (which is most of the rest of the world); that is why US radio procedures are a bit more relaxed than they used to be before the advent of FH, back when we tried to keep our transmissions to no more than five seconds at a time with lots of "breaks" in long messages to make DF'ing more difficult. So transmissions of 5 seconds tend to be hard to DF? Of course, with the battlefield internet, a text transmission will typically be a lot less than 5 s (assuming the same bandwidth as for a voice transmission, i.e. somewhere in the region of 20-60 kbit/s). transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. -- "It's easier to find people online who openly support the KKK than people who openly support the RIAA" -- comment on Wikipedia |
On 23 Sep 2003 20:00:32 -0700, Kevin Brooks wrote:
No. Paul is correct, DF'ing a "frequency agile" (or "hopping") transmitter is no easy task. For example, the standard US SINCGARS radio changes frequencies about one hundred times per *second*, Bear in mind that I'm talking about automated electronic gear here, not manual intervention. Electronics works in time spans a lot quicker than 10 ms. So what? Unless you know the frequency hopping plan ahead of time (something that is rather closely guarded), you can't capture enough of the transmission to do you any good--they use a rather broad spectrum. OK, I now understand that DF generally relies on knowing the frequency in advance. BTW, when you say a rather broad spectrum, how broad? And divided into how many bands, roughly? Both radios have to be loaded with the same frequency hopping (FH) plan, and then they have to be synchronized by time. When SINGCARS first came out the time synch had to be done by having the net control station (NCS) perform periodic radio checks (each time your radio "talked" to the NCS, it resynchronized to the NCS time hack); failure to do this could result in the net "splitting", with some of your radios on one hack, and the rest on another, meaning the two could not talk to each other. I believe that the newer versions (known as SINCGARS EPLRS, for enhanced precision location system) may use GPS time data, ensuring that everyone is always on the same time scale. That would make sense. If two receivers, placed say 10 m aparet, both pick up a signal, how accurately can the time difference between the repetion of both signals be calculated? Light moves 30 cm in 1 ns, so if time differences can be calculated to an accuracy of 0.1 ns, then direction could be resolved to an accuracy of 3 cm/10 m ~= 3 mrad. The fact is that the direction finding (DF'ing) of frequency agile commo equipment is extremely difficult for the best of the world's intel folks, and darned near impossible for the rest (which is most of the rest of the world); that is why US radio procedures are a bit more relaxed than they used to be before the advent of FH, back when we tried to keep our transmissions to no more than five seconds at a time with lots of "breaks" in long messages to make DF'ing more difficult. So transmissions of 5 seconds tend to be hard to DF? Of course, with the battlefield internet, a text transmission will typically be a lot less than 5 s (assuming the same bandwidth as for a voice transmission, i.e. somewhere in the region of 20-60 kbit/s). transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. -- "It's easier to find people online who openly support the KKK than people who openly support the RIAA" -- comment on Wikipedia |
"phil hunt" wrote in message . .. transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. Thank you Admiral Doenitz... |
"phil hunt" wrote in message . .. transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. Thank you Admiral Doenitz... |
L'acrobat wrote:
"phil hunt" wrote in message . .. transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. Thank you Admiral Doenitz... ------------ He's right. Major breaththrough of all possible barriers, the RSA algorithm. Uncrackable in the lifetime of the serious user, and crack is entirely predictable with improved computing power and can be lengthened to compensate. -Steve -- -Steve Walz ftp://ftp.armory.com/pub/user/rstevew Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!! http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public |
L'acrobat wrote:
"phil hunt" wrote in message . .. transmissions still very clear), and the use of FH combined with crypto key makes it darned near impossible for the bad guy to decypher it in any realistic timely manner. Modern crypto is good enough to withstand all cryptanalytic attacks. Thank you Admiral Doenitz... ------------ He's right. Major breaththrough of all possible barriers, the RSA algorithm. Uncrackable in the lifetime of the serious user, and crack is entirely predictable with improved computing power and can be lengthened to compensate. -Steve -- -Steve Walz ftp://ftp.armory.com/pub/user/rstevew Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!! http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public |
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