[FOSS-GPS] Question of reference satellite selection

[Shaocheng Zhang]

Dear GPS Developers

I am a Ph.D student working on real-time GPS algorithms. Currently I have a
problem about the reference satellite selection.

As we know, most of the RTK algorithm choose the highest elevation satellite
as reference satellite, but when the baseline is quite long(eg.
network-rtk), it may take quite long time (may be more than half hours) to
get the ambiguity initialized, and during the initialization we don't want
the reference satellite changes, otherwise we may need to restart the
initialization.

So if at the very beginning, we could find the longest session satellite as
the reference satellite, it will help us to guarantee the reference
satellite will not change before the initialization finished. I read GPStk's
source code of "vecsol", it give a solution of project the satellite's
velocity "V" to the U direction of the local NEU coordinate "Vu", and take
the biggest "Vu" satellite as the reference satellite. I check this method
and find it indeed give us a longer session satellite, however, when I check
this method for all the available satellite at that epoch, It doesn't
conclude that "the bigger the Vu, the longer the following observed
session".

I am very interested in this method because It's easy to implement and can
be use in real-time algorithm. So I am wondering if someone can explain this
method in a mathematical way, or give a better solution.

Thanks in advance.

Regards.

Shaocheng Zhang
School of Geodesy and Geomatics, Wuhan University, P.R.China
Email: gnss.zhangsc at gmail.com
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