> Bruce, > > I agree with your calculation and conclusion, as far as commercial > consumer > GPS receivers are concerned. > > A data sheet that was linked in a previous post for an aviation-grade > commercial GPS receiver indicated resistance to signals -30dBm at the > receiver input. That is quite considerable, and much better than the > hand-held consumer units (by 30dB?) I would be very surprised if "aviation-grade" receivers are better than your new average handheld Garmin. > I would expect planes and other potential "high value targets" to have > receivers of similar performance. I would expect the opposite, since important receivers - say for timing of power generation, timing of bankingsystems, etc typically runs forever often beeing 5 to ten years old before uppgrading. -- Björn

LORAN C represents a viable (albeit not often deployed) backup

I might add that there ARE some regional solutions to the timing

One is locking ATSC TV signals to Cs standards backed by GPS. I

And TV transmitters are LOUD compared to GPS and therefor not so

I don't imagine the cost of Cs locking a few TV signals is all

Obviously if one needs time of day to high precision one needs

I suppose this could substitute for Loran as a regional backup

On Sun, Nov 15, 2009 at 10:33:07PM -0500, David I. Emery wrote: > > LORAN C represents a viable (albeit not often deployed) backup > to time and frequency control and could be implemented in modern > hardware as a backup location service at reasonably low cost for those > applications where that is important enough. I might add that there ARE some regional solutions to the timing and frequency backup issue (time-nuts meat) that COULD be implemented pretty easily. One is locking ATSC TV signals to Cs standards backed by GPS. I am pretty sure that it would not take a lot of effort to adopt existing ATSC Tuner chip designs and maybe the actual current technology already available chips themselves to recover accurate time and frequency from a ATSC signal locked to a good standard. And TV transmitters are LOUD compared to GPS and therefor not so easily jammed on a wide area basis. I don't imagine the cost of Cs locking a few TV signals is all that high either... most of the gear can accept external frequency references and clocks... and already does to a considerable degree. Obviously if one needs time of day to high precision one needs to use a local GPS to determine the time offset of a TV signal as received at a particular site, but this should not change much provided the clock at the transmitter was really good. And the ATSC transport stream provides a rich channel for sending information about time offsets and other sync status on a real time basis... I suppose this could substitute for Loran as a regional backup for telecoms networks as protection against GPS denial... -- Dave Emery N1PRE/AE, die@dieconsulting.com DIE Consulting, Weston, Mass 02493 "An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either."

Various not so random notes: The power needed to jam GPS depends a lot on receiver state, during TTFF it takes virtually nothing. Therefore most "real" jammers will periodically blast at high power, to "dislodge" any locked receivers and then continue at low power to keep them off the signal. They are also built spectrum efficient, by emitting a signal designed specifically to interfere with GPS' spread-spectrum encoding, either by trying to lure the receivers to aim for the jammer (lowest power) or just by mimicking the worst kind of power for acquisition & tracking (higher power). Most receivers have hard-limiting inputs, so overload is a slightly more involved concept than for analog inputs, but it is still possible. The jammers which were quoted earlier are not "real jammers": they are just simple noise-sources, and long range is a negative sales parameter, because they are intended for "personal protection": a long range would increase the risk that they get detected. Their main customer base is drug-runners, fraudulent businessmen, infidel husbands and criminals sentenced to home-confinement with a GPS a ancle-bracelet. Many of those jammers does not work as well as advertised. Some of them are even "trojaned" and emit a signature signal for the benefit of law-enforcement. The infamous tv-preamp case was so efficient because it trippled the frequency of a local TV signal, due to instability, went into saturation/clipping and had a circular antenna with convenient dimensions to radiation of the resulting blanket of noise around the GPS frequency. Unfortunately, nobody tought about measuring its power-consumption or if they did, they didn't publish it. Given the kind of UHF transistors usually used in antenna-preamps, we are very likely talking no more than 1W. I an urban/hi-rise environment, havoc can be played with jammers that use glass facades as reflectors for the signal. The story about the "Mexican LORAN-C jammer" is instructive in how that complicates finding the trouble. GPS antennas on planes in the air do receive some help from being above it all, and pilots can still fly without GPS. The trouble starts once CATIII landings on GPS become routine. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.