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#51
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Implausible Time Records
If you're turning on the logger immediately before takeoff, you're risking losing
the early part of the flight entirely, not just having time problems. Good point. I’m not usually worried about the igc file much for casual flights which is what these all were. Or at least I wasn’t worried until the OLC problem. Of course for record, badge, or contest flights; I turn the computer and backup loggers on long before takeoff since tasks have to be entered and checked. You may be on the right track with the 2D vs 3D lock. I will try to look at the gps satellite status page before takeoff in the future. |
#53
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Implausible Time Records
Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes.
On Monday, May 20, 2019 at 6:38:44 PM UTC-6, kinsell wrote: You would think 8 satellites would be plenty. Looking at some discussion on the Ublox site, apparently it's more complicated than time valid vs not valid. There's flags that differentiate between low accuracy satellite time vs high accuracy. Probably of no significance in our application. I'll bet the FR code could be tweeked to use satellite time when they're recording x-y-z data. Or you could be more conservative in making sure the gps is fully locked up before takeoff, works for me. My recorders come on with the master, by the time I'm ready for takeoff they've had plenty of time and checking the displays confirms that. |
#54
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Implausible Time Records
Check the discussion he
https://portal.u-blox.com/s/question...g-not-asserted On Monday, May 20, 2019 at 8:34:07 PM UTC-6, Dave Leonard wrote: Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes. |
#55
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Implausible Time Records
Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes.
Dave, thanks for the info and link. I had no idea about the gps errors, inaccuracies, atomic clock drift, leap seconds, etc; until doing a few searches. Now I see why the clock time can change and have a greater appreciation for the difficulty GPS receiver manufacturers must deal with. It appears our real problem is that OLC does not know how to handle normal events within the log files. Other soaring analysis programs seem to know to expect such events. I had an email from lxnav that they noticed this issue a few weeks ago. While I updated on April 8; there is a new May 7 update that should help. |
#56
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Implausible Time Records
Well then, since I rig the wings in the hangar area and then tow the
glider to the apron for final prep before take off, I'll turn on the CN devices at that time.Â* Some times it's an hour or more before I take off.Â* That should give the hardware plenty of time to figure out what time it is. Griping hat on...Â* We're not planning a nuclear strike here, just getting bragging rights about a meaningless fun flight.Â* It's ridiculous that OLC acts like the bully on the play ground, enforcing unnecessary rules.Â* Griping hat off... On 5/20/2019 9:06 PM, Dave Leonard wrote: Check the discussion he https://portal.u-blox.com/s/question...g-not-asserted On Monday, May 20, 2019 at 8:34:07 PM UTC-6, Dave Leonard wrote: Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes. -- Dan, 5J |
#57
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Implausible Time Records
On 5/20/19 9:06 PM, Dave Leonard wrote:
Check the discussion he https://portal.u-blox.com/s/question...g-not-asserted On Monday, May 20, 2019 at 8:34:07 PM UTC-6, Dave Leonard wrote: Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes. Yes, but the number of leap seconds to apply changes so infrequently that it's silly to wait for that information to be downloaded at each powerup. GPS engines are free to store almanac and ephemeris data and shorten the initialization time considerably, many of them do. I would say that the Ublox behavior of waiting 10 minutes plus after achieving a 3D lock to do corrections to the time is out of the ordinary. So we have the perfect storm of a slow gps engine, pilots who don't think they need to wait for the gps to lock up before takeoff, and OLC doing stringent checking on the quality of the data. Of those three, I would say OLC is the least at fault in all this. |
#58
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Implausible Time Records
On 5/21/19 11:42 AM, kinsell wrote:
On 5/20/19 9:06 PM, Dave Leonard wrote: Check the discussion he https://portal.u-blox.com/s/question...g-not-asserted On Monday, May 20, 2019 at 8:34:07 PM UTC-6, Dave Leonard wrote: Maybe has to do with waiting to get the current almanac to know how many leap seconds to apply to received GPS time to get to UTC time used for the logs? Waiting for the whole message can take over 10 minutes. Yes, but the number of leap seconds to apply changes so infrequently that it's silly to wait for that information to be downloaded at each powerup. GPS engines are free to store almanac and ephemeris data and shorten the initialization time considerably, many of them do.Â* I would say that the Ublox behavior of waiting 10 minutes plus after achieving a 3D lock to do corrections to the time is out of the ordinary. So we have the perfect storm of a slow gps engine, pilots who don't think they need to wait for the gps to lock up before takeoff, and OLC doing stringent checking on the quality of the data.Â* Of those three, I would say OLC is the least at fault in all this. As a side note, I've used Ublox engines for Stratux boxes, was quite happy with them. They seemed sensitive, quick to lock up, and certainly were dirt cheap. Maybe they do cache the almanac data, but have some bug in them that explains the big delay in getting the time right. |
#59
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Implausible Time Records
I don't have time to catch up on this thread right now but good job on the leap second issue.
CN flight recorders update the UTC offset typically around five minutes after GPS lock. Short term work around is to power up more than five minutes before launch, use 4 second record interval (which should mask the problem if the offset change happens in flight). This is preliminary info... but seems to fit all the observed problems so far. Best regard, Evan Ludeman for CNi |
#60
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Implausible Time Records
Thanks Evan.
Out of town visitors and typical spring winds at Moriarty have kept me on the ground.Â* Maybe Sunday... On 5/23/2019 6:24 AM, Tango Eight wrote: I don't have time to catch up on this thread right now but good job on the leap second issue. CN flight recorders update the UTC offset typically around five minutes after GPS lock. Short term work around is to power up more than five minutes before launch, use 4 second record interval (which should mask the problem if the offset change happens in flight). This is preliminary info... but seems to fit all the observed problems so far. Best regard, Evan Ludeman for CNi -- Dan, 5J |
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