Re: [ntp:questions] NTP architecture recommendation
On Aug 18, 12:16 am, Harlan Stenn <[EMAIL PROTECTED]> wrote: > Depending on your budget you might want some sort of atomic clock in there > for the times when you are not getting a signal. You might also want to > look at a modem driver as a 2nd source. > > To my knowledge Meinberg has never been bitten by a software bug where their > clocks would report incorrect time. > > Regardless, I tend to suggest to folks that they peer all their top-tier > servers and use different vendors *and* different modalities. > > Finally, I believe President Bush said that in the time of certain > emergencies he would order the GPS satellites to broadcast incorrect time > (which would make GPS unreliable for ... navigation); this possibility is at > least something to be aware of. > > H Great comment, will consider it in my decision! thx ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Isn't GPS a four axis system were 3 axes are given in distance and the third axis in time? all are connected via c : 1ns ~= 1foot After selective availability had been discontinued the GPS receivers I had access to ( marine, non/semi professional) have errors and jitter in the range of less than 20m. uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
cray74 wrote: > On Aug 18, 12:16 am, Harlan Stenn <[EMAIL PROTECTED]> wrote: >>Finally, I believe President Bush said that in the time of certain >>emergencies he would order the GPS satellites to broadcast incorrect time >>(which would make GPS unreliable for ... navigation); this possibility is at >>least something to be aware of. >> >>H All the NeoCon Wunderwaffen depend on GPS ( afaik ) or/and at least a functioning space element. (SatCom, VideoLink, Networked Soldier, whatnot) One of the (IMHO the core) reasons Bush had it announced that any nation that is a potential risk to US hegemony will be denied space access. Shortly after that the Chinese produced a proof of concept that Bush may be sitting on thin air. > > > Great comment, will consider it in my decision! There is GLONAS, and there will be GALILEO. I have seen announcements of combi receivers for all three. Hail to a multipolar world. > > thx > uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
David J Taylor wrote: > Richard B. Gilbert wrote: > [] > >>Well, time is certainly one of the four variables that a four >>satellite fix gives you, the others being latitude, longitude, and >>elevation, but I don't see that time is in any way equal to position! >>UTC is the same in Paris, New York, and Los Angeles and those three >>positions are in no way equal! > > > Errors in the time delay of the signal from one satellite, whether > deliberate or accidental, will result in errors of position. Selective > Availability (SA) is a deliberate reduction in the accuracy of the time > measurement capability, resulting in a limitation on the accuracy of > position measurement. It would also make the UTC in Paris, Berlin and > Stockholm differ slightly were UTC measured purely by GPS observations. > > Cheers, > David > > True enough. But there are generally seven or eight satellites above the horizon and Motorola had a feature called "TRAIM" or Time Receiver Autonomous Integrity Monitoring which made use of additional satellites for cross checking. It makes sense to do something like that. In fact, ISTR at least one occasion in which a satellite "lost it's mind" and had to be shut down, for a time, by ground control. The rogue satellite was beaten into submission and was back on line in a day or so. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
In article <[EMAIL PROTECTED]>, Uwe Klein <[EMAIL PROTECTED]> wrote: > Isn't GPS a four axis system > were 3 axes are given in distance > and the third axis in time? The only information that the satellites transmit and must come from the satellites is time (i.e. the time at which they transmitted the signal. (The other information they transmit is their orbital elements, but that is just as a convenience to the receiver.) Position is deduced by a combination of the differential time of flight of the radio signals and the calculated positions of the satellites in orbit at the calculated launch time of the time signal. > all are connected via c : > 1ns ~= 1foot Although, the angles between the satellites can be small, which means a small time error can result in a large position solution error. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
David Woolley wrote: > Although, the angles between the satellites can be small, which means a small > time error can result in a large position solution error. > One reason why most chipsets nowadays have enough dsp Ohhmpf to track 12 or 14 sat signals simultaneously. uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
> There is GLONAS, and there will be GALILEO. > I have seen announcements of combi receivers for all three. > Hail to a multipolar world. GLONAS is barely working AFAIK, they just don't have the funds to keep enough working satellites up there, it is definitely not a reliable source for anyone to use. Pigs will fly before GALILEO does... It has been way over budget and nobody wants to sink any more money down that bottomless pit. Don't get me wrong I am not against either of the alternate systems, I'm all for multiple (and more accurate) systems, but at think point in time it just doesn't look promising for any other group to get something going. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Hello, The degradations of "Selective Availability" included a number of different possible effects, but the one that was generally used was a relatively slow dither of the clock on the satellite. The dither on each satellite was different. Since GPS receivers determine a position by measuring the distance (signal transit time) from each satellite in view to the receiver, the dither produced a corresponding fluctuation in the position solution. The magnitude of the dither was much smaller than 1 microsecond, so that it would generally not be observable with the usual NTP setup. The corresponding errors in position were less than hundreds of meters, but the exact value varied. As with many other things, your mileage would vary. It was easy to see the effect on the position solutions of any GPS receiver, and the effect on the time solution could be seen with a reasonably good oscillator -- even a good quartz oscillator could see the effect over periods of seconds or minutes. Since the clock dithering had a mean of 0 in the long term, the long-term average solution (either position or time) had no offset. The degradation was primarily directed towards real-time applications or those timing applications where the local clock had such poor stability that it was impossible to average the received signal for any appreciable time so as to take advantage of the fact that the long- term mean was 0. The whole business was turned off in 2000, and it will probably never be used again, but you never know. Best wishes, Judah Levine Time and Frequency Division NIST Boulder ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Uwe Klein wrote: > David Woolley wrote: > >> Although, the angles between the satellites can be small, which means >> a small >> time error can result in a large position solution error. >> > One reason why most chipsets nowadays have enough dsp Ohhmpf to > track 12 or 14 sat signals simultaneously. > > uwe I don't think that a 14 channel receiver would be useful! There simply are not that many satellites! The last I knew, there were 27 NavStar (GPS) satellites in orbit. Of these, about seven are usually above the horizon at any one time. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Richard B. Gilbert wrote: [] > I don't think that a 14 channel receiver would be useful! There > simply are not that many satellites! The last I knew, there were 27 > NavStar (GPS) satellites in orbit. Of these, about seven are usually > above the horizon at any one time. Richard, There can be more than that - here's an example with 13 visible. http://www.david-taylor.myby.co.uk/software/wxtrack-extras.htm According to this source, there are 31 active satellites: http://celestrak.com/NORAD/elements/gps-ops.txt More in orbit, I expect, now dead. What do they do with decommissioned GPS satellites? Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
David J Taylor wrote: > Richard B. Gilbert wrote: > [] > >>I don't think that a 14 channel receiver would be useful! There >>simply are not that many satellites! The last I knew, there were 27 >>NavStar (GPS) satellites in orbit. Of these, about seven are usually >>above the horizon at any one time. > > > Richard, > > There can be more than that - here's an example with 13 visible. > > http://www.david-taylor.myby.co.uk/software/wxtrack-extras.htm > > According to this source, there are 31 active satellites: > > http://celestrak.com/NORAD/elements/gps-ops.txt > > More in orbit, I expect, now dead. > What do they do with decommissioned GPS satellites? > > Cheers, > David > > I suspect there's not much they can do with decommissioned GPS satellites! You could probably launch 27 GPS satellites for the cost of one manned mission to retrieve/repair one. If you wait long enough, they will come down by themselves! It may take a few hundred years. . . . ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
In article <[EMAIL PROTECTED]>, "Richard B. Gilbert" <[EMAIL PROTECTED]> wrote: > David J Taylor wrote: > > Richard B. Gilbert wrote: > > [] > > > >>I don't think that a 14 channel receiver would be useful! There > >>simply are not that many satellites! The last I knew, there were 27 > >>NavStar (GPS) satellites in orbit. Of these, about seven are usually > >>above the horizon at any one time. > > > > > > Richard, > > > > There can be more than that - here's an example with 13 visible. > > > > http://www.david-taylor.myby.co.uk/software/wxtrack-extras.htm > > > > According to this source, there are 31 active satellites: > > > > http://celestrak.com/NORAD/elements/gps-ops.txt > > > > More in orbit, I expect, now dead. > > What do they do with decommissioned GPS satellites? > > > > Cheers, > > David > > > > > > I suspect there's not much they can do with decommissioned GPS satellites! > > You could probably launch 27 GPS satellites for the cost of one manned > mission to retrieve/repair one. > > If you wait long enough, they will come down by themselves! It may take > a few hundred years. . . . If I recall, they have a deorbiting system, which is a rocket that fires against their orbital motion, causing them to fall out of orbit rather more quickly than that. Of course, if the satellite has completely failed, the deorbit system won't be listening for commands, but usually things won't get that bad suddenly. Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
David J Taylor wrote: > Richard B. Gilbert wrote: > [] > >>I don't think that a 14 channel receiver would be useful! There >>simply are not that many satellites! The last I knew, there were 27 >>NavStar (GPS) satellites in orbit. Of these, about seven are usually >>above the horizon at any one time. The Sirf Chipset forex supports 20 receiver chanels ( Regular GPS Sats and Egnos Signals ) With bi or trilingual Receivers you may see even more sources. > > > Richard, > > There can be more than that - here's an example with 13 visible. > > http://www.david-taylor.myby.co.uk/software/wxtrack-extras.htm > > According to this source, there are 31 active satellites: > > http://celestrak.com/NORAD/elements/gps-ops.txt > > More in orbit, I expect, now dead. > What do they do with decommissioned GPS satellites? afair they get a push into an orbit a bit outside the system "shell". 2km is a bit high for deorbit. low flying sats are sceduled for deorbit. > > Cheers, > David > uwe > ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
>accuracies are about 100m with SA enabled and 20m without. See also: That number only holds for inexpensive commercial units. You can do much better. Surveyors get to cm or mm accuracy. I'm not familiar with the details, but there are several tricks that I know about. Listening to L2 as well as L1 gives you a much better correction for the delay in the ionosphere. Using fancy antennas (multipath rejection) in good locations helps too. Another is differential GPS. You put another unit on a known location. Nearby units will have the same error pattern. If your known unit says it is off 100 ft to the north, you can correct nearby units by subtracting that from their reported position. Another way is to simply average lots of data. I think they go for several hours, maybe up to 24. There may be some complicated post processing they can do. Mumble. I don't understand that area. For timekeeping, GPS is used to compare atomic clocks around the world. Here is a fun read: Time Too Good to Be True Daniel Kleppner Physics Today, March 2006, page 10 http://www.physicstoday.org/vol-59/iss-3/p10.html -- These are my opinions, not necessarily my employer's. I hate spam. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Jason Rabel wrote: >> Finally, I believe President Bush said that in the time of certain >> emergencies he would order the GPS satellites to broadcast incorrect time >> (which would make GPS unreliable for ... navigation); this possibility is > at >> least something to be aware of. > > That would probably only be for areas of conflict or war. Unless we go to > war with Europe or the Asia-pacific I don't think he has much to worry > about. Not true. It would affect all satellites under US control. Danny ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP architecture recommendation
Harlan Stenn wrote: In article <[EMAIL PROTECTED]>, "Richard B. Gilbert" <[EMAIL PROTECTED]> writes: > > Richard> Harlan Stenn wrote: >> In article <[EMAIL PROTECTED]>, "Richard B. Gilbert" >> <[EMAIL PROTECTED]> writes: > Richard> I suspect that the precision desired was that of position rather > Richard> than time. >>> Have you read any of Judah Levine's posts on this subject? H > > ... > > Richard> Instead of cryptic questions, how about just saying what you mean? > > I recall what he said boils down to: time == position . > I very much doubt that Judah said any such thing. Time and Position are intertwined. Judah's Lab is way above sea level in Boulder, Colorado, US and he needs to adjust his servers to take account of his position above sea level. Danny ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
[ntp:questions] DNS server b.ntpns.org for pool.ntp.org domain is down
I have been having trouble resolve pool.ntp.org servers. It has been going on for about two weeks. I traced down the problem to that one of the DNS servers for pool.ntp.org domain "b.ntpns.org." is down. pool.ntp.org. 604800 IN NS b.ntpns.org. pool.ntp.org. 604800 IN NS c.ntpns.org. pool.ntp.org. 604800 IN NS d.ntpns.org. pool.ntp.org. 604800 IN NS e.ntpns.org. pool.ntp.org. 604800 IN NS a.ntpns.org. The problemil is illustrated below: # nslookup > server b.ntpns.org Default server: b.ntpns.org Address: 67.19.103.171#53 > 0.pool.ntp.org Server: b.ntpns.org Address:67.19.103.171#53 ** server can't find 0.pool.ntp.org: NXDOMAIN > server a.ntpns.org Default server: a.ntpns.org Address: 216.52.237.236#53 > 0.pool.ntp.org Server: a.ntpns.org Address:216.52.237.236#53 Name: 0.pool.ntp.org Address: 62.112.193.129 Name: 0.pool.ntp.org Address: 88.191.23.218 Name: 0.pool.ntp.org Address: 80.190.251.105 Name: 0.pool.ntp.org Address: 194.88.2.88 Name: 0.pool.ntp.org Address: 195.22.25.130 Name: 0.pool.ntp.org Address: 85.10.195.19 Name: 0.pool.ntp.org Address: 193.93.167.241 Name: 0.pool.ntp.org Address: 66.36.239.104 Name: 0.pool.ntp.org Address: 194.77.75.99 Name: 0.pool.ntp.org Address: 213.139.167.172 Name: 0.pool.ntp.org Address: 213.144.140.154 Name: 0.pool.ntp.org Address: 91.121.10.82 Name: 0.pool.ntp.org Address: 213.129.242.82 Name: 0.pool.ntp.org Address: 81.178.50.155 -- Michael Wang, http://www.unixlabplus.com/, /* for future expansion */ Email=$(echo [EMAIL PROTECTED] | sed "s:\(.\)\(.\)\(.\)\(.\):\4\3\2\1:g") Technical writings: http://www.unixlabplus.com/unix-prog/Publication.txt ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
