Re: [time-nuts] Epoch rollover ?
Bruce, I'm wondering whether there may be a glitch in the firmware supplied on that unit. The fact that it worked fine up until the other day, and then reverted its date seems rather odd. I don't believe it's anything to do with cross correlation as has been suggested. Unfortunately Odetics is now FEI-Zyfer, and those units are not supported any more, and the guy who designed them isn't there anymore either. Rob Kimberley -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of k4...@aol.com Sent: 19 October 2011 22:31 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Epoch rollover ? Not for another eight years or so Connected by DROID on Verizon Wireless -Original message- From: Bruce Lane To: time-nuts@febo.com Sent: Wed, Oct 19, 2011 21:19:01 GMT+00:00 Subject: [time-nuts] Epoch rollover? Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. Thanks much. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover ?
Connected by DROID on Verizon Wireless -Original message- From: Bruce Lane To: time-nuts@febo.com Sent: Fri, Oct 21, 2011 20:44:07 GMT+00:00 Subject: Re: [time-nuts] Epoch rollover ? Good day, *** REPLY SEPARATOR *** On 21-Oct-11 at 13:18 Dennis Ferguson wrote: On 21 Oct, 2011, at 11:53 , k4...@aol.com wrote: Bruce, the most common cause of a GPS receiver getting the date incorrect is due to cross-correlation. And cross-correlation is usually the result of too much gain in the GPS antenna's LNA. (snippage) I would be more likely to believe this if the setup had not been working just fine for the last four years. ;-) No, based on what I've read, I'm definitely inclined to believe it's a 1024-week rollover thing. (more snippage) Some of the heuristics I've heard of are these (or maybe combinations of these): - Assume the date must be more recent than when the firmware was compiled. By itself this leaves the device with a 1024 week rollover problem, but the 1024 weeks are counted from the date the firmware was compiled rather than from the GPS epoch. (still more...) This idea fascinates me, as it is, potentially, the easiest to correct AND the unit itself is vintage mid-90's. This would also explain why the thing stayed working for a while after the last 1024-week roll. I think I'll pull out the firmware, see if I can find the compile date in a dump of the EPROM, change said date, and put the whole thing back together. At worst, I'll end up with the same situation I have now. At best, it'll fix it. No matter what, I'll post the results. Dennis, thanks much! I hadn't even considered that line of thinking. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover ?
Good day, *** REPLY SEPARATOR *** On 21-Oct-11 at 13:18 Dennis Ferguson wrote: >On 21 Oct, 2011, at 11:53 , k4...@aol.com wrote: >> Bruce, the most common cause of a GPS receiver getting the date >incorrect is due to cross-correlation. And cross-correlation is usually >the result of too much gain in the GPS antenna's LNA. (snippage) I would be more likely to believe this if the setup had not been working just fine for the last four years. ;-) No, based on what I've read, I'm definitely inclined to believe it's a 1024-week rollover thing. (more snippage) >Some of the heuristics I've heard of are these (or maybe combinations of >these): > >- Assume the date must be more recent than when the firmware was compiled. > By itself this leaves the device with a 1024 week rollover problem, but > the 1024 weeks are counted from the date the firmware was compiled rather > than from the GPS epoch. (still more...) This idea fascinates me, as it is, potentially, the easiest to correct AND the unit itself is vintage mid-90's. This would also explain why the thing stayed working for a while after the last 1024-week roll. I think I'll pull out the firmware, see if I can find the compile date in a dump of the EPROM, change said date, and put the whole thing back together. At worst, I'll end up with the same situation I have now. At best, it'll fix it. No matter what, I'll post the results. Dennis, thanks much! I hadn't even considered that line of thinking. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover ?
On 21 Oct, 2011, at 11:53 , k4...@aol.com wrote: > Bruce, the most common cause of a GPS receiver getting the date incorrect is > due to cross-correlation. And cross-correlation is usually the result of too > much gain in the GPS antenna's LNA. And depending on the make of the > receiver, some will clear the date information with a power cycling and > others require erasing the flash memory where those parameters are stored. > Do you have a way to see the relative C/No reading for your receiver? Most > receivers start experiencing cross-correlation when this reading exceeds 50. > Regards, Doug, K4CLE… That might be true in general, but seems exceedingly unlikely to be the problem when the date error has a magnitude of exactly 1024 weeks. The problem is that while GPS tells you the time within a 1024 week "era" it provides no information about which 1024 week era we're in so even perfectly received GPS signals don't tell the unit what is needed to fix that. Either the receiver needs a separate source of time information to determine the era, or it needs to implement a heuristic to guess at it from the data it does have. Some of the heuristics I've heard of are these (or maybe combinations of these): - Assume the date must be more recent than when the firmware was compiled. By itself this leaves the device with a 1024 week rollover problem, but the 1024 weeks are counted from the date the firmware was compiled rather than from the GPS epoch. - Assume the date must be more recent than the last date on which the unit saved restart information (ephemerides, etc.) in its flash memory, or whatever non-volatile storage it uses for this. This may fail if the unit's flash memory fails. - Guess at the "era" based on the leap second count (i.e. the UTC offset) informed by some expectation of how many leap seconds are likely to occur in each 1024 week era. This would be an excellent heuristic if the rate of leap second insertion was relatively predicable over long periods, but as it has turned out the relative dearth of leap seconds in the past dozen years might cause particular implementations of this guestimate to fail now. I don't know if any of this matters in the current case. The last (which I think was considered the "best practice", or at least the "best that could be made of a bad situation", at the time the GPS rollover occurred) is of particular interest right now since it suggests that ending UTC leap seconds might eventually have some unintended consequences for boat anchor GPS timing equipment. Dennis Ferguson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover ?
Bruce, the most common cause of a GPS receiver getting the date incorrect is due to cross-correlation. And cross-correlation is usually the result of too much gain in the GPS antenna's LNA. And depending on the make of the receiver, some will clear the date information with a power cycling and others require erasing the flash memory where those parameters are stored. Do you have a way to see the relative C/No reading for your receiver? Most receivers start experiencing cross-correlation when this reading exceeds 50. Regards, Doug, K4CLE... Connected by DROID on Verizon Wireless -Original message- From: Bruce Lane To: time-nuts@febo.com Sent: Wed, Oct 19, 2011 22:00:40 GMT+00:00 Subject: Re: [time-nuts] Epoch rollover? Heh... Well, that's annoying. This being the case, I have no idea why the date went whacko... Thanks. *** REPLY SEPARATOR *** On 19-Oct-11 at 17:45 Joseph M Gwinn wrote: The last rollover was in August 1999, just before Y2K, and the next rollover will be in April 2019. From: "Bruce Lane" To: time-nuts@febo.com Date: 10/19/2011 05:19 PM Subject: [time-nuts] Epoch rollover? Sent by:time-nuts-boun...@febo.com Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. Thanks much. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __ Information from ESET Smart Security, version of virus signature database 6558 (20111019) __ The message was checked by ESET Smart Security. http://www.eset.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __ Information from ESET Smart Security, version of virus signature database 6558 (20111019) __ The message was checked by ESET Smart Security. http://www.eset.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover?
On 19 Oct, 2011, at 14:18 , Bruce Lane wrote: > Did we just have another GPS epoch rollover? My trusty old Odetics 425 > seems to believe the date is March 4th, 1992. > > I could probably correct it in firmware, if I looked hard and long > enough, but the ToD is still correct and the frequency standard is > staying nicely locked. Not sure if recovering the correct Julian date is > worth the effort. That's neat. I understand that the typical heuristics used to determine the GPS epoch included one or more of: a) Assume the time must be more recent then the date the firmware was compiled; b) Assume the time must be more recent than the last time it dumped the ephemerides out to flash to speed reacquisition across a reboot (assuming it does that); or c) Take a guess based on the leap second count (i.e. the UTC offset) and some expectation of the number of leap seconds per epoch. If your unit does a) it may be that the units firmware just passed 1024 weeks of age. If it does b) it may be your flash memory (or whatever non-volatile storage it keeps that stuff in) has died. It it does c) it may be that the relative dearth of leap seconds in the past dozen years has fooled that algorithm. Or it could be something else entirely. It would be interesting to know which of those it is. Dennis Ferguson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover?
On 10/19/2011 11:18 PM, Bruce Lane wrote: Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. Units can have their internal shifted rollovers. This is well illustrated using a GPS date calculator: http://csrc.ucsd.edu/scripts/convertDate.cgi?time=1992+03+04 http://csrc.ucsd.edu/scripts/convertDate.cgi?time=2011+10+19 If you look at GPS Week you see that it beleives it has GPS week 634 (which is what the GPS birds transmit since they do modulo 1024) while actual GPS week is 1658, which has the expected difference of 1024. A GPS receiver can operate correctly even if they do modulus 1024 errors, it is just annoying that the time is off. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. It would be a very limited part of the firmware in that case. Should be an offset date hidden in there somewhere. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover?
Heh... Well, that's annoying. This being the case, I have no idea why the date went whacko... Thanks. *** REPLY SEPARATOR *** On 19-Oct-11 at 17:45 Joseph M Gwinn wrote: >The last rollover was in August 1999, just before Y2K, and the next >rollover will be in April 2019. > > > > > > From: "Bruce Lane" > > > > To: time-nuts@febo.com > > > > Date: 10/19/2011 05:19 PM > > > > Subject:[time-nuts] Epoch rollover? > > > > Sent by:time-nuts-boun...@febo.com > > > > > > > > >Hi, gang, > >Did we just have another GPS epoch rollover? My trusty old >Odetics 425 >seems to believe the date is March 4th, 1992. > >I could probably correct it in firmware, if I looked hard and >long >enough, but the ToD is still correct and the frequency standard is >staying nicely locked. Not sure if recovering the correct Julian date is >worth the effort. > >Thanks much. > > >Bruce Lane, Owner & Head Hardware Heavy, >Blue Feather Technologies (http://www.bluefeathertech.com) >Assoc. member, AZA & AAZK for many moons. >"Salvadore Dali's computer has surreal ports..." > > >___ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. > > > >__ Information from ESET Smart Security, version of virus >signature database 6558 (20111019) __ > >The message was checked by ESET Smart Security. > >http://www.eset.com > > > > >___ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. > > >__ Information from ESET Smart Security, version of virus >signature database 6558 (20111019) __ > >The message was checked by ESET Smart Security. > >http://www.eset.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover?
The last rollover was in August 1999, just before Y2K, and the next rollover will be in April 2019. From: "Bruce Lane" To: time-nuts@febo.com Date: 10/19/2011 05:19 PM Subject:[time-nuts] Epoch rollover? Sent by:time-nuts-boun...@febo.com Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. Thanks much. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. <><>___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Epoch rollover ?
Not for another eight years or so Connected by DROID on Verizon Wireless -Original message- From: Bruce Lane To: time-nuts@febo.com Sent: Wed, Oct 19, 2011 21:19:01 GMT+00:00 Subject: [time-nuts] Epoch rollover? Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. Thanks much. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Epoch rollover?
Hi, gang, Did we just have another GPS epoch rollover? My trusty old Odetics 425 seems to believe the date is March 4th, 1992. I could probably correct it in firmware, if I looked hard and long enough, but the ToD is still correct and the frequency standard is staying nicely locked. Not sure if recovering the correct Julian date is worth the effort. Thanks much. Bruce Lane, Owner & Head Hardware Heavy, Blue Feather Technologies (http://www.bluefeathertech.com) Assoc. member, AZA & AAZK for many moons. "Salvadore Dali's computer has surreal ports..." ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
