[FOSS-GPS] RTKPos processing options

[António Pestana]

Thank you, once again. Now it’s time for a careful reading of appendix E.

 

Regards

 

Antonio

 

De: foss-gps-bounces at lists.osgeo.org [mailto:foss-gps-bounces at lists.osgeo.org] Em nome de Felipe G. Nievinski
Enviada: sábado, 14 de Fevereiro de 2015 14:40
Para: foss-gps at lists.osgeo.org
Assunto: Re: [FOSS-GPS] RTKPos processing options

 

One more question:  integer ambiguity is estimated and resolved by
epoch‐by‐epoch basis. So, if I understand correctly each epoch we have to
deal with one unknown ambiguity for each observation. So the number of
unknowns equals the number of the available observation equations plus the
position unknowns plus etc... How can we manage to get an unique solution
to the problem?

 

It's only in instantaneous AR that ambiguities are reset every epoch.  Even then, there are always two observations per satellite signal: one carrier-phase and one pseudorange.  So if there are, say, 7 satellites visible from a single-frequency receiver, there are at least 7 x 2 = 14 observations and 7 + 4 = 11 unknowns (7 ambiguities, 3 position coordinates, and 1 receiver clock).  As 14 > 11, it's still doable.   Furthermore, one can track multiple signals broadcast by a given satellite, which increases redundancy.

 

If using fixed positioning mode, there would be only 7 + 1 = 8 unknowns per epoch.  And if using static mode, eventually the position coordinates will be well known, so the filter starts having 11 unknowns but ends with effectively only 8 unknowns per epoch; and if you run the filter backwards (then later combine it with the forward run, as RTKPOST does), the start of the session is covered as well.

 

Now, that was for instantaneous AR, and I don't think you'd ever want to use it, except perhaps if you're investigating the quality of your data, or into some research about the various biases.  Continuous AR carries over the previous epoch's ambiguities estimate and their uncertainty into the next epoch, so that eventually the ambiguities will have converged.

 

After ambiguities are estimated as "float" (floating-point values), RTKPLOT will also try to resolve double-differenced ambiguities as integer multiples of the wavelength, as they're supposed to be (that's a physical constraint). Finally "fix and hold" means that once an ambiguity is successfully resolved an an integer, it's assumed exactly known afterwards, which improves the precision of the position coordinate estimates subsequently.

 

Hope this helps.

-Felipe.

 

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