I/We track down things that jam weather radars. Mostly WiFi access points misconfigured.
Which share many of the characteristics of GPS jammers 1 - small low powered 2 - one can ruin a pilots entire day 3 - distributed 4 - can literally be anywhere Stuff like this is why FCC blocked anyone but chip manufacturers from updating WiFi radio firmware. The overwhelming majority of the mods were attempts to improve performance. But the firmware hackers were unaware of the band sharing and also did not understand that signal characteristics need to be closely controlled and the limits the FCC applies are not there to ‘ruin my performance’ but to allow others to play nicely in that sandbox. So many things in life should have been learned in kindergarten. Sadly many have forgotten those lessons. On Aug 31, 2018, at 10:55 AM, Attila Kinali <[email protected]> wrote: On Thu, 30 Aug 2018 23:05:48 +0000 Gregory Maxwell <[email protected]> wrote: > Seeing some open source software implementing beam-forming was one of > the things I hoped to see result from the open hardware multi-band > GNSS receivers like the GNSS firehose project ( > http://pmonta.com/blog/2017/05/05/gnss-firehose-update/ ) since once > you're going through the trouble of running three coherent receivers > for three bands, stacking three more of them and locking them to the > same clock doesn't seem like a big engineering challenge... and the > rest is just DSP work. "Just DSP work" is a tad bit more than you think. You are dealing with sevaral 1Msps of data, even for a simple L1 C/A receiver. If you are going multi-band-multi-GNSS you are usually in the 50MHz BW at L1 and 80MHz BW at L2/L5 range, which means you are dealing with something in the order of 100Msps of data per channel (either as a single stream of sample or two streams of samples with half rate). Then you add to it that you will need at least 4bit ADCs to get somewhat jaming proof, probably even 10bit or more and suddenly you are dealing with 200-400Mbyte/s data per antenna. Constantly. To be able to do reasonable beam forming, you probably need a 4 by 4 grid at least, that makes 16 antennas which brings us into the 3Gbyte/s to 6Gbyte/s region. And that's just the raw _input_ datarate you have to handle.... A modern GNSS receiver has something in the order of 50-100 correlators per band, each of which needs to receive the full data rate mentioned above. So inside the chip, the data rate gets multiplied as well. Now take into considerations that beside running the correlators, after you phase shifted and weighted the inputs correctly, you have to run some fancy algorithms (on the raw data) to figure out what these phase shifts and weights are. For each satellite you are tracking individually. All this toghether means you have run some pretty heavy computation, that is very likely not going to fit into an FPGA, so you need to build a custom ASIC. > Even absent fancy beam forming, for GNSS timing with a surveyed > position except at high latitudes it should be possible to use a > relatively high gain antenna pointed straight up and by doing so blind > yourself to terrestrial jammers at a cost of fewer SVs being > available. But I've never tried it. You still need the lower satellites to survey your position accurately. Besides that you lose a lot of in terms of timing accuracy, if you have only a limited number of satellites. So you cannot decrease the lower elevation angles too much. Going below -10dB is probably not agood idea. Also, going from ~10 birds in view down to ~5 means that your PPS jitter just increased by a factor of 5 to 10. (source: experiment i've done here some time ago) Besides, a narrow band jammer trips up most of the commercial receivers badly (adds a correlation peak where it does not belong). And for that to be effective you only need to be 5-10dB above the noise level. Which is pretty easy to achieve, even if you have very directive antenna (the sidelobes are usually only 10-30dB down from the main lobe) Fortunately, narrow band jammers are also pretty easy to mask, given you have enough bits in your ADC. > As others have noted intermittent jamming is pretty benign to a GPSDO. > Spoofing, OTOH, can trivially mess up the timing. It's my view that > if you need timing for a security critical purpose there isn't really > any GNSS based solution commercially available to the general public > right now, the best bet is a local atomic reference with a GPSDO used > to monitor and initially set it. There is a reason why Microsemi is building more 5071 these days than ever before (rumors have it that they are are 3-4 devices per week). Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neal Stephenson _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
