[teqc] dumb clock questions

Geoff Blewitt gblewitt at unr.edu
Mon Aug 29 23:06:23 MDT 2005


Lou - see response below
   - Geoff

Lou Estey wrote:
> Jeff Freymueller wrote:
> 
>> Geoff's explanation (below) is basically correct. When clockprep finds 
>> an offset in the observables, it is almost never exactly 1 millisecond 
>> because of the (physical) change in range between epochs. In the case 
>> of this file, there will also be a component from the clock drift 
>> rate, as it appears that this file (from the clockprep output Andrew 
>> forwarded earlier) has 1 millisecond offsets about every 50 epochs, or 
>> about 5 times an hour. That is a very high rate of clock drift, but I 
>> have seen higher rates before. In this case, the clock's drift rate is 
>> ~0.02 milliseconds per epoch (~6 km), so I suspect that this effect 
>> dominates because clockprep calculates the offset by averaging across 
>> all the satellites, and some satellites will have range getting 
>> larger, others getting smaller so the average is usually closer to 
>> zero than the range change in any one satellite.
> 
> 
> That being the case, doing a normal clockprep on a RINEX with ms-jumpy time
> tags and smooth phase and pseudorange is _not_ the same as doing a new 
> `teqc +smtt`
> translation of the data which results in RINEX with smooth time tags and 
> ms-jumpy phase
> and pseudoranges.  `teqc +smtt` applies just an exact ms jump.

Lou,
    Sorry this wasn't explained:  clockprep *assumes* the clock is reset 
by exactly 1 msec (unless clockprep is invoked with a special option, 
which wasn't the case here).  So the jumpiness goes away.  I've verified 
that in almost all cases, the resulting rinex file is exactly the same 
as running 'teqc +smtt' (except for the problem that Jeff explained - 
the phase should show the same discontinuity as the phase, in which case 
clockprep has an option to handle it).

Geoff
________________________________________________
Dr Geoffrey Blewitt, Professor of Space Geodesy,
NV Bureau of Mines & Geology, and NV Seismo Lab,
University of Nevada, MS178, Reno, NV89557, USA.
http://www.nbmg.unr.edu/staff/geoff.htm

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