[teqc] dumb clock questions
gblewitt at unr.edu
Mon Aug 29 23:06:23 MDT 2005
Lou - see response below
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.
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).
Dr Geoffrey Blewitt, Professor of Space Geodesy,
NV Bureau of Mines & Geology, and NV Seismo Lab,
University of Nevada, MS178, Reno, NV89557, USA.
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