[time-nuts] Ublox Neo-6M time error.
Mike Cook
michael.cook at sfr.fr
Thu Jun 2 05:48:18 EDT 2016
Hi Mark,
> Le 2 juin 2016 à 10:28, Mark Sims <holrum at hotmail.com> a écrit :
>
> The Ublox receivers do not have a message that outputs GPS time directly. It's easiest to take the UTC time message and subtract the leapsecond offset to get GPS time. The itow value in the NAV_TIMEGPS message is milliseconds past midnight of the start of the GPS week... probably not something you want to be doing calculations from. I use the TIMEUTC message. I only use the leapsecond offset from TIMEGPS.
>
Ok makes sense. BTW my calculations on other TIMEGPS messages using iTOW value confirm the strange finding I saw before:
09:02:08 0000 B5 62 01 20 10 00 D0 0C 8A 16 5A 17 FD FF 6B 07
0010 11 07 0F 00 00 00 B3 4E
Header B5 62
ID 01 20
length 00 10 16
iTOW 16 8A 0C D0 378146000 ms
fTOW FF FD 17 5A -ve some value
week 07 6B 1:875
LeapS 11 17
flags 07 all bits valid
iAcc 00 00 00 0F
CK_A B3
CK_B 4E
Converting iTOW : the above hex byte values have been changed to big endian .
4days 9h 2m 26s from start of GPS week , that is Sunday midnight.
The seconds value make no sense in standard GPS time from the SV but as I saw before
can be MADE to make sense if you subtract current leap seconds.
4d 9h 2m 09s which is the NEXT second .
09:02:09 0000 24 47 50 52 4D 43 2C 30 39 30 32 30 39 2E 30 30 $GPRMC,090209.00
0010 2C 41 2C 34 38 34 37 2E 33 34 35 37 34 2C 4E 2C ,A,4847.34574,N,
0020 30 30 32 31 36 2E 32 39 37 38 32 2C 45 2C 30 2E 00216.29782,E,0.
0030 30 35 38 2C 2C 30 32 30 36 31 36 2C 2C 2C 41 2A 058,,020616,,,A*
0040 37 31 0D 0A 71
> It looks like Ublox is sending the time message AFTER the 1PPS pulse ("at the beep, the time was...") , and just about everybody else sends it before the pulse ("at the beep, the time is ...") ... Bastards! I wonder about the thought (or lack of it) process that decided that was the way to do it. And don't get me started on the idiot receivers that send the sawtooth correction message after the fact...
> ------------------
> Despite re-checking I am still doubtful that my sums are right. I’ll do a few more packets. Is this what you are seeing Mark?
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