[teqc] dumb clock questions

Jeff Freymueller jfreymue at gi.alaska.edu
Tue Aug 30 14:21:46 MDT 2005


Lou,

I agree with you 100%. The danger with the rinex file that Andrew got 
(not the file from teqc, but the other one) is that the observables are 
not consistent.

I tried to use the word "normal" or "typical" as opposed to "correct". 
I think the reason for the old Bernese converter's behavior dates back 
to the 1980s when Bernese estimated polynomial clock corrections and 
could not handle the sawtooth clock pattern that results from teqc 
+smtt. There were some objections raised from GIPSY users at that time, 
but we lost the argument at the time, and Werner's program in effect 
defined the standard.

I would expect that any software ought to be able to handle the output 
of teqc +smtt, as long as it does not try to do any screening on the 
raw individual observables expecting them to be continuous. Any data 
screening that uses a clock-free linear combination, or that is done 
after an estimate of the clock error has been made and subtracted, 
should work on any flavor of RINEX file that has consistent 
observables.

Jeff

On Aug 30, 2005, at 11:45 AM, Lou Estey wrote:

> Jeff Freymueller wrote:
>
>> [...] (This is because the original rinex definition of the clock, 
>> unfortunately, is not compatible with GIPSY's definition of the 
>> clock, and never has been - GIPSY requires the time tag to be the 
>> nominal measurement time as a perfect external clock would record 
>> it).
>
> Maybe this is a misconception, i.e. we all think that "correct" RINEX
> uses the receiver time in the time tag -- probably because this is how
> Berne (i.e. Werner Gurtner) did it for the original RINEX translators
> for receivers with millisecond clock resets, i.e. Trimble .dat and
> Ashtech B-files -- and GIPSY uses GPS time.  But from the RINEX spec:
>
> "3. DEFINITION OF THE OBSERVABLES
>
> GPS observables include three fundamental quantities that need to be 
> defined:
> Time, Phase, and Range.
>
> TIME:
>
>   The time of the measurement is the receiver time of the received 
> signals.
>   It is identical for the phase and range measurements and is 
> identical for
>   all satellites observed at that epoch. It is expressed in _GPS time_ 
> (not
>   Universal Time)."
>
> Note the last line; the emphasis is mine.  (Of course, Werner was 
> really making
> the point that the time tag be GPS time instead of UTC which would be 
> jumpy
> due to insertion of leap seconds.)
>
> The main point is that there be consistency between time, phase, and 
> pseudoranges.
> Therefore either:
>
> rx ms clk jumps in time tags (== receiver time), smooth phase and 
> pseudorange
> (e.g. `teqc -smtt` output, Berne translation output)
>
> or
>
> smooth time tags (== GPS time), rx ms clk jumps in phase and 
> pseudorange
> (e.g. `teqc +smtt` output, clockprep output, GIPSY input)
>
> are equally valid representations of the observables in RINEX.  They 
> use different
> clocks though, which is why we can't use the observables in the former 
> representation
> as input to GIPSY which is expecting smooth GPS time.
>
> I'm pretty sure that Bernese processing deals equally well with either 
> RINEX
> presentation. What about Gamit, or the different manufacturers' 
> processing packages?
>
> --lou
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>
>

Dr. Jeffrey T. Freymueller         Office: 907-474-7286
Geophysical Institute              Fax:    907-474-7290
University of Alaska, Fairbanks    Home:   907-479-3550
PO Box 757320                      Cell:   907-322-7632
Fairbanks, AK 99775-7320           email: jeff at giseis.alaska.edu
URL: http://www.gps.alaska.edu/jeff/jeff.html

Download Alaska GPS data: ftp://gps.alaska.edu/pub/gpsdata/
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