To extract a dedicated epoch interval from a RINEX-file you have to provide a Start-Epoch via -epo beg and the Duration -d or -duration in seconds. 

Here an example to extract the first hour of a daily input file.

 gfzrnx -finp pots0070.15o -fout pots007a.15o -epo_beg 2015-01-07_000000 -d 3600

 gfzrnx -finp pots0070.15o -fout pots007a.15o -epo_beg 2015007_000000 -d 3600

 gfzrnx -finp pots0070.15o -fout pots007a.15o -epo_beg 20150107_000000 -d 3600

更改采样率

gfzrnx -finp ${_site}${doy}0.${_yr}o -smp ${_smp} > ../Sample_${_smp}s/${_site}${doy}0.${_yr}o