[FOSS-GPS] High accuracy positioning with low cost GPS devices:a
eugenio.realini at gmail.com
Mon May 4 06:09:40 EDT 2009
Michael Tandy wrote:
> Something I've been wondering about is: the UBX-RXM-RAW message has a
> known jitter of a few milliseconds  and the time-tag is only
> precise to 1 millisecond, while some sources  suggest that time-tag
> errors between receivers need to be in the microsecond range. Was the
> imprecise time tag a problem for you, and if so how did you overcome
We have read the links about the time tag problems.
Concerning goGPS, at the moment these problems are not taken into
account. This means that we are considering master and rover perfectly
synchronized: all the de-synchronization is thrown in the noise. This
could be an explanation of the noise correlation.
A better solution would be achieved by changing the code and phase
observation equations adding some model to describe the time-tag errors
(maybe using other information coming from LEA-4T, such as UBX-TIM
messages). This would be more important for phase observations than for
code observations, because of their different accuracy. Let's consider
for example two receivers kept still and let's compute their phase
double difference over a certain period. In principle, phase double
differences should be equal to a constant value (without cycle-slips),
but actually, if at least one of the two receivers is an u-blox, it
comes out that double differences have a significant trend. As you
suggested, this could be attributed to time-tag errors; a time-tag error
model could be calibrated to reduce (or eliminate!) this trend.
Do you have any idea about how to handle these time-tag issue?
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