[teqc] helpful tip of week 1958

Lou Estey lou at unavco.org
Thu Jul 20 15:48:55 MDT 2017


This week's tip: ToC != ToE messages

Late in GPS week 1956 and then in GPS week 1957, I received a couple of emails from
teqc users who, when reading raw data files, were getting messages like:

! Warning ! NAVSTAR GPS SV G02 ephemeris ToC != ToE
	ToC= 2017 Jul 6 19:59:44.000; ToE= 2017 Jul 2 00:00:00.000
...
	ToC= 2017 Jul 6 21:59:44.000; ToE= 2017 Jul 2 00:00:00.000
...
	ToC= 2017 Jul 7 00:00:00.000; ToE= 2017 Jul 2 00:00:00.000

And it was noted that the same messages were showing up from files from different
receivers.

First, don't worry about a SV failure from messages like these.  This does not indicate
a failure.  But this particular set of messages from teqc for this GPS SV, and cases
like it, does come with some extra explanation.

Now some terminology: Any navigation message has many parts to it, two of which are
the ToE and ToC.  The ToE is the "time of ephemeris", i.e. when the Keplerian parameters
in the message of the SV's orbit apply. (And, of course, the position of the SV at
other times around the ToE can be found by applying the equations found in the GPS ICD.)
The ToC is the "time of clock" which refers to the SV's clock correction parameters
(which can also be estimated for other times around the ToC).

Note that in the above messages for G02 that teqc is reporting not only that the ToE and
ToC are not equal, but the difference in time between them is up to 5 days.

Now, there is nothing in the GPS ICD that says that the ToE and ToC have to be equal
for a given navigation message for an SV.  However, in practice, these two values _are_
the same for the overwhelming majority of GPS (and Galileo, Beidou, QZSS, and IRNSS)
navigation messages -- at least so far.

Why then does teqc bother to check and report this?  Here's the rub.  There have been
numerous cases where a receiver incorrectly records either the ToE or the ToC value,
usually due to bit errors.  (E.g. the TurboRogue receivers were prone to bit errors.)
Therefore, teqc eliminates any navigation message where the ToE is not identical to
the ToC.  This action has the side-effect of possibly eliminating a few valid navigation
messages where the ToE is not identical to the ToC, but I think, statistically (so far),
this outweighs the cases where a given receiver might have a glitch.

In this particular case for 6-7 July 2017, given that data from multiple receivers
were showing the same thing for the same SV, the navigation messages being eliminated
were probably "valid", i.e. what the receivers collected and stored were what the
SV transmitted.

Furthermore, if you look at the ToC epochs (various times late on 6 July 2017 to
early 7 July 2017, or 2017-187 to 2017-188), these match NANU 2017062 saying that
PRN 02 was forecast to be unusable for about 12 hours during those times:

http://celestrak.com/GPS/NANU/2017/nanu.2017062.txt

Given the NANU, the health parameters for the navigation messages in question
(i.e. those where ToE != ToC and maybe even a few others) for PRN 02 should have
been set to some unhealthy (non-zero) value.  If your receiver was set to not track
any unhealthy SV, it should not have collected any data for GPS PRN 02 between
17:55 UTC on July 6 and 05:55 UTC on July 7.

By the way, NANU 2017066, http://celestrak.com/GPS/NANU/2017/nanu.2017066.txt, summarizes
the GPS PRN 02 "unusable" period as being only from 18:41 UTC July 6 to 00:09 UTC July 7 --
less than 6 hours.

Happy teqc-ing!

cheers,
--lou

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Louis H. Estey, Ph.D.              office:  [+001] 303-381-7456
UNAVCO, 6350 Nautilus Drive           FAX:  [+001] 303-381-7451
Boulder, CO  80301-5554            e-mail:  lou  unavco.org

"If the universe is the answer, what is the question?"
                                                -- Leon Lederman
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Past helpful tips:

week 1894: using teqc config files - http://postal.unavco.org/pipermail/teqc/2016/002067.html
week 1895: qc of high-rate data - http://postal.unavco.org/pipermail/teqc/2016/002071.html
week 1896: UNIX/Linux shells for Windows - http://postal.unavco.org/pipermail/teqc/2016/002072.html
week 1897: '-' vs. '+' teqc options - http://postal.unavco.org/pipermail/teqc/2016/002076.html
week 1898: auto-identification of formats - http://postal.unavco.org/pipermail/teqc/2016/002092.html
week 1899: auto-identification vs. format flags - http://postal.unavco.org/pipermail/teqc/2016/002096.html
week 1900: square brackets in options - http://postal.unavco.org/pipermail/teqc/2016/002105.html
week 1901: using option '+mds' - http://postal.unavco.org/pipermail/teqc/2016/002108.html
week 1902: qc results w/ problematic nav messages - http://postal.unavco.org/pipermail/teqc/2016/002113.html
week 1903: '-no_orb[it]' and '-no_pos[ition]' options - http://postal.unavco.org/pipermail/teqc/2016/002115.html
week 1904: '-week' option - http://postal.unavco.org/pipermail/teqc/2016/002117.html
week 1905: using '+bcf' for XYZ/geodetic conversion - http://postal.unavco.org/pipermail/teqc/2016/002126.html
week 1906: the '+v[erify]' option - http://postal.unavco.org/pipermail/teqc/2016/002128.html
week 1907: '+C2', '+L5', "+L6', '+L7', '+L8', '+all' options - http://postal.unavco.org/pipermail/teqc/2016/002130.html
week 1908: getting RINEX doppler and L2 - http://postal.unavco.org/pipermail/teqc/2016/002131.html
week 1909: using paths w/ file names - http://postal.unavco.org/pipermail/teqc/2016/002132.html
week 1910: the (un)importance of file names - http://postal.unavco.org/pipermail/teqc/2016/002133.html
week 1911: notices, warnings, and errors - http://postal.unavco.org/pipermail/teqc/2016/002134.html
week 1912: the '-max_rx_SVs' option - http://postal.unavco.org/pipermail/teqc/2016/002137.html
week 1913: the end of '++igs' and '+igs' - http://postal.unavco.org/pipermail/teqc/2016/002140.html
week 1914: splicing together RINEX files - http://postal.unavco.org/pipermail/teqc/2016/002144.html
week 1915: using '-O.int' and '-O.dec' - http://postal.unavco.org/pipermail/teqc/2016/002145.html
week 1916: '+doy' option - http://postal.unavco.org/pipermail/teqc/2016/002146.html
week 1917: '-tbin' and '-ast' options - http://postal.unavco.org/pipermail/teqc/2016/002152.html
week 1918: mp12 RMS before/after Oct 2013 - http://postal.unavco.org/pipermail/teqc/2016/002158.html
week 1919: the global windowing options - http://postal.unavco.org/pipermail/teqc/2016/002159.html
week 1920: '-M.dec' and '-N.dec' options - http://postal.unavco.org/pipermail/teqc/2016/002163.html
week 1921: combining time filtering options - http://postal.unavco.org/pipermail/teqc/2016/002176.html
week 1922: helping me (or someone else on the list) help you - http://postal.unavco.org/pipermail/teqc/2016/002187.html
week 1923: the "build" line - http://postal.unavco.org/pipermail/teqc/2016/002190.html
week 1924: the qc '-w[idth]' option - http://postal.unavco.org/pipermail/teqc/2016/002193.html
week 1925: qc with explicit time windowing - http://postal.unavco.org/pipermail/teqc/2016/002194.html
week 1926: the '+rx_state' option - http://postal.unavco.org/pipermail/teqc/2016/002200.html
week 1927: the '-O.sum' option - http://postal.unavco.org/pipermail/teqc/2016/002204.html
week 1928: the '+meta' and '+mds' options - http://postal.unavco.org/pipermail/teqc/2016/002206.html
week 1930: more on '-O.sum' - http://postal.unavco.org/pipermail/teqc/2017/002207.html
week 1931: the '-O.s[ystem]' option - http://postal.unavco.org/pipermail/teqc/2017/002208.html
week 1932: leap seconds - http://postal.unavco.org/pipermail/teqc/2017/002215.html
week 1936: what you can (and shouldn't) do in a RINEX obs file - http://postal.unavco.org/pipermail/teqc/2017/002229.html
week 1938: the '+psp' option - http://postal.unavco.org/pipermail/teqc/2017/002231.html
week 1939: the '+diag' option - http://postal.unavco.org/pipermail/teqc/2017/002235.html
week 1951: '-n_<system>' and SV filtering options - http://postal.unavco.org/pipermail/teqc/2017/002277.html
week 1953: more with '+diag' option - http://postal.unavco.org/pipermail/teqc/2017/002287.html
week 1954: using '+diag' output to split raw files - http://postal.unavco.org/pipermail/teqc/2017/002290.html
week 1955: current qc notation - http://postal.unavco.org/pipermail/teqc/2017/002302.html
week 1956: the '+qcq' option - http://postal.unavco.org/pipermail/teqc/2017/002304.html
week 1957: using Trimble formats - http://postal.unavco.org/pipermail/teqc/2017/002305.html

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