Hi Martin,
On 01/27/2016 05:49 PM, Martin Burnicki wrote:
Magnus Danielson wrote:
Hi,
You know I want more details. ;-)
I did see the report you guys sent to the USCG, good work there and good
report.
Thanks.
After we had received the first reports about GPS receiver showing a 13
us time offset one of my colleagues who maintains the firmware of our
GPS receivers started to investigate. Looking at the data sets decoded
from the satellites he saw that temporarily the UTC correction parameter
jumped from one or 2 nanoseconds to more than 13 microseconds.
So he added some debug code which let the GPS receiver sent the
satellite number plus the raw subframe words 7 and 8 as hex numbers to a
monitor program which logged them to a file.
Several GPS receivers have support to output the raw data, and this is
one very good reason. Being able to pull the interpreted data, the
covariance matrix etc. and other state like raw pseudo-ranges and
doppler data all helps. Being able to output RINEX is always very nice
for many reasons. If I only had the time...
Then we looked at the raw data, decoded the bits manually and found that
some satellites were suspicious UTC correction data with a valid checksum.
Here are some subframe words 7 and 8 captured starting around 2016-01-26
12:00 UTC from subframe 4 page 18, and the decoded parameters
according to Figure 20-1, sheet 8 of IS-GPS-200H (printed page 83, PDF
page 96)
http://www.gps.gov/technical/icwg/IS-GPS-200H.pdf
sv sfw7 sfw8 wnt|tot a0 bits a0[us]
09 0x3FFFF1B3 0x23800017 --> 00|000000: 0xFFFFC68E -13.696 *
07 0x3FFFFFEA 0x3FD3967B --> 89|319488: 0xFFFFFFFF -0.001
02 0x3FFFFFD5 0x3FD39644 --> 89|319488: 0xFFFFFFFF -0.001
06 0x3FFFF18C 0x23800028 --> 00|000000: 0xFFFFC68E -13.696 *
23 0x3FFFF18C 0x23800028 --> 00|000000: 0xFFFFC68E -13.696 *
30 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
05 0x0000003F 0x0013965B --> 89|319488: 0x00000000 +0.000
16 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
26 0x3FFFF18C 0x23800028 --> 00|000000: 0xFFFFC68E -13.696 *
07 0x3FFFFFEA 0x3FD3967B --> 89|319488: 0xFFFFFFFF -0.001
09 0x3FFFF1B3 0x23800017 --> 00|000000: 0xFFFFC68E -13.696 *
02 0x3FFFFFD5 0x3FD39644 --> 89|319488: 0xFFFFFFFF -0.001
30 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
05 0x0000003F 0x0013965B --> 89|319488: 0x00000000 +0.000
06 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
23 0x3FFFF18C 0x23800028 --> 00|000000: 0xFFFFC68E -13.696 *
16 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
07 0x3FFFFFEA 0x3FD3967B --> 89|319488: 0xFFFFFFFF -0.001
09 0x00000000 0x0098D642 --> 89|405504: 0x00000002 +0.002
30 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
02 0x3FFFFFD5 0x3FD39644 --> 89|319488: 0xFFFFFFFF -0.001
05 0x0000003F 0x0013965B --> 89|319488: 0x00000000 +0.000
23 0x3FFFF18C 0x23800028 --> 00|000000: 0xFFFFC68E -13.696 *
06 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
16 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
07 0x3FFFFFEA 0x3FD3967B --> 89|319488: 0xFFFFFFFF -0.001
09 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
30 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
05 0x0000003F 0x0013965B --> 89|319488: 0x00000000 +0.000
02 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
06 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
23 0x0000003F 0x0098D67D --> 89|405504: 0x00000002 +0.002
16 0x00000000 0x00139664 --> 89|319488: 0x00000000 +0.000
It's interesting to see how consistent these errors are.
On the other hand, it is interesting to see how it varies even for the
same PRN. Look at how PRN 09 varies between +0.002 us and -13.696 us.
You see that SVs 09, 06, 23, and 26 sent a tot time 0 and a correction
parameter A0 of -13.696 microseconds, while the A0 parameter received
from healthy satellites was only 1 or 2 nanoseconds, as usual, and the
week number wnt matches the current week number, truncated to 8 bit.
Eventually there were even more satellites sending faulty data, but
these were the ones we could track at our location.
You can also see that in the last lines the A0 value sent by SVs 6, 9,
and 23 was "normal" again, so we happily just captured a few of the last
wrong data sets.
All data captured after this was OK again. Please find the full set of
debug output in the attached text file.
Unfortunately there was no time stamp in the debug output, but we know
the satellites repeat the same page 4 once every 12 minutes.
Many thanks for that wealth of data.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
