Implausible Time Records

I’ve not heard back from Lxnav yet, but I had a private email from a very smart person that pointed out that both the October 2 and October 4 flights had the clock jump after takeoff but before soaring began. The flight this week had the clock jump a minute or so after soaring began, and thus is likely why OLC flagged this one and not the other two. It appears that the clock jump is 7 to 8 minutes after the unit is turned on. Since I generally do not turn the system on until just before takeoff, it is believed that the system is using an internal clock until it syncs with the gps satellites and then resets the clock which may be different by a couple of seconds. My plan is to turn the 9070 on a few minutes before launch to allow it to sync clocks to see if that helps. It is not clear to me if Dan’s issue is similar to mine or different.

MS

That's certainly worth a try.Â* My normal method is to saddle up, start
the engine, and turn on the CN2, the CN vario, and the PowerFlarm
portable (which does not have an ENL license).Â* Then I run checklists
and taxi out and, as soon as the temperatures are in the green, take
off.Â* I'll try turning on the computers first and take my time with
getting started and to the hold short line.Â* I'll shoot for 10 minutes
and see how that goes.

On 5/20/2019 2:42 PM, wrote:
I’ve not heard back from Lxnav yet, but I had a private email from a very smart person that pointed out that both the October 2 and October 4 flights had the clock jump after takeoff but before soaring began. The flight this week had the clock jump a minute or so after soaring began, and thus is likely why OLC flagged this one and not the other two. It appears that the clock jump is 7 to 8 minutes after the unit is turned on. Since I generally do not turn the system on until just before takeoff, it is believed that the system is using an internal clock until it syncs with the gps satellites and then resets the clock which may be different by a couple of seconds. My plan is to turn the 9070 on a few minutes before launch to allow it to sync clocks to see if that helps. It is not clear to me if Dan’s issue is similar to mine or different.

MS

--
Dan, 5J

On Monday, May 20, 2019 at 4:42:52 PM UTC-4, wrote:
I’ve not heard back from Lxnav yet, but I had a private email from a very smart person that pointed out that both the October 2 and October 4 flights had the clock jump after takeoff but before soaring began. The flight this week had the clock jump a minute or so after soaring began, and thus is likely why OLC flagged this one and not the other two. It appears that the clock jump is 7 to 8 minutes after the unit is turned on. Since I generally do not turn the system on until just before takeoff, it is believed that the system is using an internal clock until it syncs with the gps satellites and then resets the clock which may be different by a couple of seconds. My plan is to turn the 9070 on a few minutes before launch to allow it to sync clocks to see if that helps. It is not clear to me if Dan’s issue is similar to mine or different.

MS

We've established in another thread some months ago that the display of altitudes of a flight within the OLC web page is all messed up if OLC thinks that the flight "started" later, at some higher logged altitude, rather than at the altitude of the declared takeoff airport. (Typically due to the flight recorder not getting a GPS fix - or not being turned on at all - until after takeoff.) I am still of the opinion that this should be considered a bug in OLC, since it can "see" that the start location (lat/lon as well as altitude) is not at the airport. The altitudes inside the flight log file, even as downloaded from OLC, are still OK. Perhaps the time jump, too, makes OLC think it's a new "takeoff", and thus has similar consequences in its display (plus its "implausible" warning).

But I don't quite understand the conjecture that "the system is using an internal clock until it syncs with the gps satellites", because prior to getting a GPS fix the flight recorder shouldn't be recording locations into the flight log either, no?

But I don't quite understand the conjecture that "the system is using an internal clock until it syncs with the gps satellites", because prior to getting a GPS fix the flight recorder shouldn't be recording locations into the flight log either, no?

Yes, the system seems to have a fix on the satellites and the Lat/Lon/Alt seems to be consistent from one record to another. It certainly doesn’t take several minutes for the system to get a gps fix. The question is why does it record an incremental time for a few minutes and then resets the time by a couple of seconds. It would seem the system should record the time stamp directly from the gps. If it did that, how could sequential time records change? Is it possible the gps starts with an internal clock number initially and then after a few minutes makes a comparison check with the clock from the gps? It gets back to Kensell’s point about how the gps system is handling the data and or the firmware is using the info inside the computers. These seem like good questions for vendors which I hope to get soon.

On 5/20/19 2:42 PM, wrote:
I’ve not heard back from Lxnav yet, but I had a private email from a very smart person that pointed out that both the October 2 and October 4 flights had the clock jump after takeoff but before soaring began. The flight this week had the clock jump a minute or so after soaring began, and thus is likely why OLC flagged this one and not the other two. It appears that the clock jump is 7 to 8 minutes after the unit is turned on. Since I generally do not turn the system on until just before takeoff, it is believed that the system is using an internal clock until it syncs with the gps satellites and then resets the clock which may be different by a couple of seconds. My plan is to turn the 9070 on a few minutes before launch to allow it to sync clocks to see if that helps. It is not clear to me if Dan’s issue is similar to mine or different.

MS


They do have clocks to help achieve lock on the satellites, but I've
seen the reported time jump immediately after lock was achieved. Don't
know why the 9070 would wait minutes after achieving lock before
correcting the time. Maybe an issue of 2D vs 3D lock. 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.

Dan was having lots of gps altitude dropouts during flights, that might
just be jamming. He did have a time jump just at liftoff, so that fits
in with switching the clocks.

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.

At 21:27 20 May 2019, kinsell wrote:
On 5/20/19 2:42 PM, wrote:
I’ve not heard back from Lxnav yet, but I had a private email from a
very smart person that pointed out that both the October 2 and October 4
flights had the clock jump after takeoff but before soaring began. The
flight this week had the clock jump a minute or so after soaring began,
and
thus is likely why OLC flagged this one and not the other two. It
appears
that the clock jump is 7 to 8 minutes after the unit is turned on. Since
I
generally do not turn the system on until just before takeoff, it is
believed that the system is using an internal clock until it syncs with
the
gps satellites and then resets the clock which may be different by a
couple
of seconds. My plan is to turn the 9070 on a few minutes before launch
to
allow it to sync clocks to see if that helps. It is not clear to me if
Dan’s issue is similar to mine or different.

MS


They do have clocks to help achieve lock on the satellites, but I've
seen the reported time jump immediately after lock was achieved. Don't
know why the 9070 would wait minutes after achieving lock before
correcting the time. Maybe an issue of 2D vs 3D lock. 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.

Dan was having lots of gps altitude dropouts during flights, that might
just be jamming. He did have a time jump just at liftoff, so that fits
in with switching the clocks.

I've looked at Dan's trace, and the dropouts are in the latter part of the
flight,
nowhere near the time when the clock problem occurs, so probably not
jamming. Micheal's flight shows no GPs dropouts at all, but still
experienced
the problem.

So...Â* Does having a 3D fix with, say 8 satellites, constitute good GPS
time?Â* I would think so.

On 5/20/2019 3:27 PM, kinsell wrote:
On 5/20/19 2:42 PM, wrote:
I’ve not heard back from Lxnav yet, but I had a private email from a
very smart person that pointed out that both the October 2 and
October 4 flights had the clock jump after takeoff but before soaring
began.Â* The flight this week had the clock jump a minute or so after
soaring began, and thus is likely why OLC flagged this one and not
the other two.Â* It appears that the clock jump is 7 to 8 minutes
after the unit is turned on.Â* Since I generally do not turn the
system on until just before takeoff, it is believed that the system
is using an internal clock until it syncs with the gps satellites and
then resets the clock which may be different by a couple of seconds.
My plan is to turn the 9070 on a few minutes before launch to allow
it to sync clocks to see if that helps.Â* It is not clear to me if
Dan’s issue is similar to mine or different.

MS


They do have clocks to help achieve lock on the satellites, but I've
seen the reported time jump immediately after lock was achieved.Â*
Don't know why the 9070 would wait minutes after achieving lock before
correcting the time.Â* Maybe an issue of 2D vs 3D lock.Â* 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.

Dan was having lots of gps altitude dropouts during flights, that
might just be jamming.Â* He did have a time jump just at liftoff, so
that fits in with switching the clocks.

--
Dan, 5J