[time-nuts] Ublox Neo-6M time error.

[Mike Cook]

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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